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  • An XEvent a Day (22 of 31) – The Future – fn_dblog() No More? Tracking Transaction Log Activity in Denali

    - by Jonathan Kehayias
    I bet that made you look didn’t it?  Worry not, fn_dblog() still exists in SQL Server Denali, and I plan on using it to validate the information being returned by a new Event in SQL Server Denali CTP1, sqlerver.transaction_log, which brings with it the ability to correlate specific transaction log entries to the operations that actually caused them to occur. There is no greater source of information about the transaction log in SQL Server than Paul Randal’s blog category Transaction Log . ...(read more)

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  • An XEvent a Day (23 of 31) – How it Works – Multiple Transaction Log Files

    - by Jonathan Kehayias
    While working on yesterday’s blog post The Future – fn_dblog() No More? Tracking Transaction Log Activity in Denali I did a quick Google search to find a specific blog post by Paul Randal to use it as a reference, and in the results returned another blog post titled, Investigating Multiple Transaction Log Files in SQL Server caught my eye so I opened it in a new tab in IE and went about finishing the blog post.  It probably wouldn’t have gotten my attention if it hadn’t been on the SqlServerPedia...(read more)

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  • An XEvent a Day (25 of 31) – The Twelve Days of Christmas

    - by Jonathan Kehayias
    In the spirit of today’s holiday, a couple of people have been posting SQL related renditions of holiday songs.  Tim Mitchell posted his 12 days of SQL Christmas , and Paul Randal and Kimberly Tripp went as far as to record themselves sing SQL Carols on their blog post Our Christmas Gift To You: Paul and Kimberly Singing!   For today’s post on Extended Events I give you the 12 days of Christmas, Extended Events style (all of these are based on true facts about Extended Events in SQL Server)....(read more)

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  • An XEvent a Day (28 of 31) – Tracking Page Compression Operations

    - by Jonathan Kehayias
    The Database Compression feature in SQL Server 2008 Enterprise Edition can provide some significant reductions in storage requirements for SQL Server databases, and in the right implementations and scenarios performance improvements as well.  There isn’t really a whole lot of information about the operations of database compression that is documented as being available in the DMV’s or SQL Trace.  Paul Randal pointed out on Twitter today that sys.dm_db_index_operational_stats() provides...(read more)

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  • London User Group Meetings this week (19th/20th May); 26th May-Agile Data Warehousing; 17th June-Kim

    - by tonyrogerson
    Got two user group meetings in London for you, we've also started the Cuppa Corner sessions - the first 3 are up on the site - A trip to First Normal Form, Lookup and Cache Transform in SSIS and Pipeline Limiter in SSIS - we are aiming for at least one per week. WhereScape are doing a breakfast meeting on Agile techniques to Data Warehousing and Kimberly Tripp and Paul Randal are over in June for a 1 day master class. Finally a 3 day performance and monitoring workshop on 22- 24th June in London...(read more)

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  • Tagged: 5 things SQL Server should drop

    - by AaronBertrand
    I was tagged by Paul Randal ( blog | twitter ) last night in his latest blog post, entitled, " What 5 things should SQL Server get rid of? " His top 5 pretty much coincide with my top 5, so I'll have to dig a little deeper. In no particular order: Syntax inconsistencies This isn't really a specific thing that Microsoft should get rid of, but rather an attitude and overall approach to SQL Server's long-term development. Every time they add a feature or option to SQL Server, it seems to be implemented...(read more)

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  • ***Master Class competition extended***

    - by Testas
     We have acquired two additional tickets to attend the SQL Server Master Class with Paul Randal and Kimberly Tripp  For a chance to win these coveted tickets In the subject line type MasterClass and email [email protected] before 9pm on Sunday night  The winners will be announced Monday Morning  Don’t worry if you have already purchased a ticket, should you be win, your ticket cost will be reimbursed  

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  • The pros and cons of learning

    - by AaronBertrand
    This week I am at a training course put on by Paul Randal ( blog | twitter ) and Kimberly Tripp ( blog | twitter ) entitled " SQL Immersion ." It is essentially a 5-day deep dive into some of the more important bowels of the Microsoft SQL Server 2008 Internals book they co-wrote with Kalen Delaney, Adam Machanic and Conor Cunningham - with a lot of extra information, insight, experience and interactivity thrown in. I am seeing a lot of benefits from this already. The additional insight around what's...(read more)

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  • SQL SERVER – PAGELATCH_DT, PAGELATCH_EX, PAGELATCH_KP, PAGELATCH_SH, PAGELATCH_UP – Wait Type – Day 12 of 28

    - by pinaldave
    This is another common wait type. However, I still frequently see people getting confused with PAGEIOLATCH_X and PAGELATCH_X wait types. Actually, there is a big difference between the two. PAGEIOLATCH is related to IO issues, while PAGELATCH is not related to IO issues but is oftentimes linked to a buffer issue. Before we delve deeper in this interesting topic, first let us understand what Latch is. Latches are internal SQL Server locks which can be described as very lightweight and short-term synchronization objects. Latches are not primarily to protect pages being read from disk into memory. It’s a synchronization object for any in-memory access to any portion of a log or data file.[Updated based on comment of Paul Randal] The difference between locks and latches is that locks seal all the involved resources throughout the duration of the transactions (and other processes will have no access to the object), whereas latches locks the resources during the time when the data is changed. This way, a latch is able to maintain the integrity of the data between storage engine and data cache. A latch is a short-living lock that is put on resources on buffer cache and in the physical disk when data is moved in either directions. As soon as the data is moved, the latch is released. Now, let us understand the wait stat type  related to latches. From Book On-Line: PAGELATCH_DT Occurs when a task is waiting on a latch for a buffer that is not in an I/O request. The latch request is in Destroy mode. PAGELATCH_EX Occurs when a task is waiting on a latch for a buffer that is not in an I/O request. The latch request is in Exclusive mode. PAGELATCH_KP Occurs when a task is waiting on a latch for a buffer that is not in an I/O request. The latch request is in Keep mode. PAGELATCH_SH Occurs when a task is waiting on a latch for a buffer that is not in an I/O request. The latch request is in Shared mode. PAGELATCH_UP Occurs when a task is waiting on a latch for a buffer that is not in an I/O request. The latch request is in Update mode. PAGELATCH_X Explanation: When there is a contention of access of the in-memory pages, this wait type shows up. It is quite possible that some of the pages in the memory are of very high demand. For the SQL Server to access them and put a latch on the pages, it will have to wait. This wait type is usually created at the same time. Additionally, it is commonly visible when the TempDB has higher contention as well. If there are indexes that are heavily used, contention can be created as well, leading to this wait type. Reducing PAGELATCH_X wait: The following counters are useful to understand the status of the PAGELATCH: Average Latch Wait Time (ms): The wait time for latch requests that have to wait. Latch Waits/sec: This is the number of latch requests that could not be granted immediately. Total Latch Wait Time (ms): This is the total latch wait time for latch requests in the last second. If there is TempDB contention, I suggest that you read the blog post of Robert Davis right away. He has written an excellent blog post regarding how to find out TempDB contention. The same blog post explains the terms in the allocation of GAM, SGAM and PFS. If there was a TempDB contention, Paul Randal explains the optimal settings for the TempDB in his misconceptions series. Trace Flag 1118 can be useful but use it very carefully. I totally understand that this blog post is not as clear as my other blog posts. I suggest if this wait stats is on one of your higher wait type. Do leave a comment or send me an email and I will get back to you with my solution for your situation. May the looking at all other wait stats and types together become effective as this wait type can help suggest proper bottleneck in your system. Read all the post in the Wait Types and Queue series. Note: The information presented here is from my experience and there is no way that I claim it to be accurate. I suggest reading Book OnLine for further clarification. All the discussions of Wait Stats in this blog are generic and vary from system to system. It is recommended that you test this on a development server before implementing it to a production server. Reference: Pinal Dave (http://blog.SQLAuthority.com)   Filed under: Pinal Dave, PostADay, SQL, SQL Authority, SQL Query, SQL Scripts, SQL Server, SQL Tips and Tricks, SQL Wait Stats, SQL Wait Types, T SQL, Technology

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  • Paul and Kimberly are coming the UK

    - by simonsabin
    Are you working with SQL, if so then attending a Paul Randal and Kimberly Tripp seminar is a must. The amount these guys know about SQL is just scary. Their life is so SQL that the last time I was having dinner at theirs they were arguing the severity levels of certain IO error codes. Talk about extreme. Paul and Kimberly are running a Master class on the 17th June at the Radisson Edwardian Heathrow. The normal price is a bargain at £249 + VAT. I’ve managed to negotiate a discount of £100 for my...(read more)

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  • PASS Summit 2011 &ndash; Part III

    - by Tara Kizer
    Well we’re about a month past PASS Summit 2011, and yet I haven’t finished blogging my notes! Between work and home life, I haven’t been able to come up for air in a bit.  Now on to my notes… On Thursday of the PASS Summit 2011, I attended Klaus Aschenbrenner’s (blog|twitter) “Advanced SQL Server 2008 Troubleshooting”, Joe Webb’s (blog|twitter) “SQL Server Locking & Blocking Made Simple”, Kalen Delaney’s (blog|twitter) “What Happened? Exploring the Plan Cache”, and Paul Randal’s (blog|twitter) “More DBA Mythbusters”.  I think my head grew two times in size from the Thursday sessions.  Just WOW! I took a ton of notes in Klaus' session.  He took a deep dive into how to troubleshoot performance problems.  Here is how he goes about solving a performance problem: Start by checking the wait stats DMV System health Memory issues I/O issues I normally start with blocking and then hit the wait stats.  Here’s the wait stat query (Paul Randal’s) that I use when working on a performance problem.  He highlighted a few waits to be aware of such as WRITELOG (indicates IO subsystem problem), SOS_SCHEDULER_YIELD (indicates CPU problem), and PAGEIOLATCH_XX (indicates an IO subsystem problem or a buffer pool problem).  Regarding memory issues, Klaus recommended that as a bare minimum, one should set the “max server memory (MB)” in sp_configure to 2GB or 10% reserved for the OS (whichever comes first).  This is just a starting point though! Regarding I/O issues, Klaus talked about disk partition alignment, which can improve SQL I/O performance by up to 100%.  You should use 64kb for NTFS cluster, and it’s automatic in Windows 2008 R2. Joe’s locking and blocking presentation was a good session to really clear up the fog in my mind about locking.  One takeaway that I had no idea could be done was that you can set a timeout in T-SQL code view LOCK_TIMEOUT.  If you do this via the application, you should trap error 1222. Kalen’s session went into execution plans.  The minimum size of a plan is 24k.  This adds up fast especially if you have a lot of plans that don’t get reused much.  You can use sys.dm_exec_cached_plans to check how often a plan is being reused by checking the usecounts column.  She said that we can use DBCC FLUSHPROCINDB to clear out the stored procedure cache for a specific database.  I didn’t know we had this available, so this was great to hear.  This will be less intrusive when an emergency comes up where I’ve needed to run DBCC FREEPROCCACHE. Kalen said one should enable “optimize for ad hoc workloads” if you have an adhoc loc.  This stores only a 300-byte stub of the first plan, and if it gets run again, it’ll store the whole thing.  This helps with plan cache bloat.  I have a lot of systems that use prepared statements, and Kalen says we simulate those calls by using sp_executesql.  Cool! Paul did a series of posts last year to debunk various myths and misconceptions around SQL Server.  He continues to debunk things via “DBA Mythbusters”.  You can get a PDF of a bunch of these here.  One of the myths he went over is the number of tempdb data files that you should have.  Back in 2000, the recommendation was to have as many tempdb data files as there are CPU cores on your server.  This no longer holds true due to the numerous cores we have on our servers.  Paul says you should start out with 1/4 to 1/2 the number of cores and work your way up from there.  BUT!  Paul likes what Bob Ward (twitter) says on this topic: 8 or less cores –> set number of files equal to the number of cores Greater than 8 cores –> start with 8 files and increase in blocks of 4 One common myth out there is to set your MAXDOP to 1 for an OLTP workload with high CXPACKET waits.  Instead of that, dig deeper first.  Look for missing indexes, out-of-date statistics, increase the “cost threshold for parallelism” setting, and perhaps set MAXDOP at the query level.  Paul stressed that you should not plan a backup strategy but instead plan a restore strategy.  What are your recoverability requirements?  Once you know that, now plan out your backups. As Paul always does, he talked about DBCC CHECKDB.  He said how fabulous it is.  I didn’t want to interrupt the presentation, so after his session had ended, I asked Paul about the need to run DBCC CHECKDB on your mirror systems.  You could have data corruption occur at the mirror and not at the principal server.  If you aren’t checking for data corruption on your mirror systems, you could be failing over to a corrupt database in the case of a disaster or even a planned failover.  You can’t run DBCC CHECKDB against the mirrored database, but you can run it against a snapshot off the mirrored database.

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  • Mythbusters &ndash; SQL Edition

    - by AjarnMark
    I love the Mythbusters television show.  That has to be one of the coolest jobs in the world…it involves investigation, problem solving, science, trial & error, searching for the truth, robotics and remote controls, and in the end, you usually get to blow stuff up.  How great is that?!  I know I’ll never forget the episode where they blew up a cement truck.  That was truly awesome. Well, perhaps not quite made for TV, but pretty cool nonetheless, Paul Randal (@PaulRandal) has been doing some SQL Server myth busting here in the month of April with his DBA Myth a Day series.  It starts with In-Flight Transactions Continue After a Failover.  Check it out!

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  • Why are so many DBCC commands undocumented?

    - by DBA
    Paul Randal of SQLskills.com does a great job of answering the question of why there are so many undocumented DBCC commands in his post Why are so many DBCC commands undocumented? I would like to go on to say that not only does this apply to the DBCC commands but is some respect to all parts of SQL, other Servers, IDE's, Operating Systems, just about everywhere. There is always something that just does not make it into the official documentation. And as Paul points out probably never will make it. That could be why there are so many "Tips & Tricks" types of books, blog post, etc. everywhere you look. And I also agree with Janos's comments on Paul's post, which was "I'm fine with them undocumented. All of us who need to use these commands know where to find "documentation" and whom to ask ". Till later,

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  • 5 Things SQL Server should get rid of

    - by Jonathan Kehayias
    Paul Randal ( blog | twitter ) started a new meme last night with his blog post " What 5 things should SQL Server get rid of? " A few bloggers have posted their top 5 lists, so here is mine. Creating Foreign Keys without mentioning Indexes This is probably a performance tuning consultants favorite.  I know that Greg Low has blogged about this in the past (see Indexing Foreign Keys - should SQL Server do that automatically? ) and back then, and now I still do think this should be an...(read more)

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  • The DBA Team tackles data corruption

    Paul Randal joins the team in this instalment of the DBA Team saga. In this episode, Monte Bank is trying to cover up insider trading - using data corruption to eliminate the evidence, and a patsy DBA to take the blame. It's a great story with useful advice on how to perform thorough data recovery tasks. "A real time saver" Andy Doyle, Head of IT ServicesAndy and his team saved time by automating backup and restores with SQL Backup Pro. Find out how much time you could save. Download a free trial now.

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  • SQL SERVER – Shrinking Database is Bad – Increases Fragmentation – Reduces Performance

    - by pinaldave
    Earlier, I had written two articles related to Shrinking Database. I wrote about why Shrinking Database is not good. SQL SERVER – SHRINKDATABASE For Every Database in the SQL Server SQL SERVER – What the Business Says Is Not What the Business Wants I received many comments on Why Database Shrinking is bad. Today we will go over a very interesting example that I have created for the same. Here are the quick steps of the example. Create a test database Create two tables and populate with data Check the size of both the tables Size of database is very low Check the Fragmentation of one table Fragmentation will be very low Truncate another table Check the size of the table Check the fragmentation of the one table Fragmentation will be very low SHRINK Database Check the size of the table Check the fragmentation of the one table Fragmentation will be very HIGH REBUILD index on one table Check the size of the table Size of database is very HIGH Check the fragmentation of the one table Fragmentation will be very low Here is the script for the same. USE MASTER GO CREATE DATABASE ShrinkIsBed GO USE ShrinkIsBed GO -- Name of the Database and Size SELECT name, (size*8) Size_KB FROM sys.database_files GO -- Create FirstTable CREATE TABLE FirstTable (ID INT, FirstName VARCHAR(100), LastName VARCHAR(100), City VARCHAR(100)) GO -- Create Clustered Index on ID CREATE CLUSTERED INDEX [IX_FirstTable_ID] ON FirstTable ( [ID] ASC ) ON [PRIMARY] GO -- Create SecondTable CREATE TABLE SecondTable (ID INT, FirstName VARCHAR(100), LastName VARCHAR(100), City VARCHAR(100)) GO -- Create Clustered Index on ID CREATE CLUSTERED INDEX [IX_SecondTable_ID] ON SecondTable ( [ID] ASC ) ON [PRIMARY] GO -- Insert One Hundred Thousand Records INSERT INTO FirstTable (ID,FirstName,LastName,City) SELECT TOP 100000 ROW_NUMBER() OVER (ORDER BY a.name) RowID, 'Bob', CASE WHEN ROW_NUMBER() OVER (ORDER BY a.name)%2 = 1 THEN 'Smith' ELSE 'Brown' END, CASE WHEN ROW_NUMBER() OVER (ORDER BY a.name)%10 = 1 THEN 'New York' WHEN ROW_NUMBER() OVER (ORDER BY a.name)%10 = 5 THEN 'San Marino' WHEN ROW_NUMBER() OVER (ORDER BY a.name)%10 = 3 THEN 'Los Angeles' ELSE 'Houston' END FROM sys.all_objects a CROSS JOIN sys.all_objects b GO -- Name of the Database and Size SELECT name, (size*8) Size_KB FROM sys.database_files GO -- Insert One Hundred Thousand Records INSERT INTO SecondTable (ID,FirstName,LastName,City) SELECT TOP 100000 ROW_NUMBER() OVER (ORDER BY a.name) RowID, 'Bob', CASE WHEN ROW_NUMBER() OVER (ORDER BY a.name)%2 = 1 THEN 'Smith' ELSE 'Brown' END, CASE WHEN ROW_NUMBER() OVER (ORDER BY a.name)%10 = 1 THEN 'New York' WHEN ROW_NUMBER() OVER (ORDER BY a.name)%10 = 5 THEN 'San Marino' WHEN ROW_NUMBER() OVER (ORDER BY a.name)%10 = 3 THEN 'Los Angeles' ELSE 'Houston' END FROM sys.all_objects a CROSS JOIN sys.all_objects b GO -- Name of the Database and Size SELECT name, (size*8) Size_KB FROM sys.database_files GO -- Check Fragmentations in the database SELECT avg_fragmentation_in_percent, fragment_count FROM sys.dm_db_index_physical_stats (DB_ID(), OBJECT_ID('SecondTable'), NULL, NULL, 'LIMITED') GO Let us check the table size and fragmentation. Now let us TRUNCATE the table and check the size and Fragmentation. USE MASTER GO CREATE DATABASE ShrinkIsBed GO USE ShrinkIsBed GO -- Name of the Database and Size SELECT name, (size*8) Size_KB FROM sys.database_files GO -- Create FirstTable CREATE TABLE FirstTable (ID INT, FirstName VARCHAR(100), LastName VARCHAR(100), City VARCHAR(100)) GO -- Create Clustered Index on ID CREATE CLUSTERED INDEX [IX_FirstTable_ID] ON FirstTable ( [ID] ASC ) ON [PRIMARY] GO -- Create SecondTable CREATE TABLE SecondTable (ID INT, FirstName VARCHAR(100), LastName VARCHAR(100), City VARCHAR(100)) GO -- Create Clustered Index on ID CREATE CLUSTERED INDEX [IX_SecondTable_ID] ON SecondTable ( [ID] ASC ) ON [PRIMARY] GO -- Insert One Hundred Thousand Records INSERT INTO FirstTable (ID,FirstName,LastName,City) SELECT TOP 100000 ROW_NUMBER() OVER (ORDER BY a.name) RowID, 'Bob', CASE WHEN ROW_NUMBER() OVER (ORDER BY a.name)%2 = 1 THEN 'Smith' ELSE 'Brown' END, CASE WHEN ROW_NUMBER() OVER (ORDER BY a.name)%10 = 1 THEN 'New York' WHEN ROW_NUMBER() OVER (ORDER BY a.name)%10 = 5 THEN 'San Marino' WHEN ROW_NUMBER() OVER (ORDER BY a.name)%10 = 3 THEN 'Los Angeles' ELSE 'Houston' END FROM sys.all_objects a CROSS JOIN sys.all_objects b GO -- Name of the Database and Size SELECT name, (size*8) Size_KB FROM sys.database_files GO -- Insert One Hundred Thousand Records INSERT INTO SecondTable (ID,FirstName,LastName,City) SELECT TOP 100000 ROW_NUMBER() OVER (ORDER BY a.name) RowID, 'Bob', CASE WHEN ROW_NUMBER() OVER (ORDER BY a.name)%2 = 1 THEN 'Smith' ELSE 'Brown' END, CASE WHEN ROW_NUMBER() OVER (ORDER BY a.name)%10 = 1 THEN 'New York' WHEN ROW_NUMBER() OVER (ORDER BY a.name)%10 = 5 THEN 'San Marino' WHEN ROW_NUMBER() OVER (ORDER BY a.name)%10 = 3 THEN 'Los Angeles' ELSE 'Houston' END FROM sys.all_objects a CROSS JOIN sys.all_objects b GO -- Name of the Database and Size SELECT name, (size*8) Size_KB FROM sys.database_files GO -- Check Fragmentations in the database SELECT avg_fragmentation_in_percent, fragment_count FROM sys.dm_db_index_physical_stats (DB_ID(), OBJECT_ID('SecondTable'), NULL, NULL, 'LIMITED') GO You can clearly see that after TRUNCATE, the size of the database is not reduced and it is still the same as before TRUNCATE operation. After the Shrinking database operation, we were able to reduce the size of the database. If you notice the fragmentation, it is considerably high. The major problem with the Shrink operation is that it increases fragmentation of the database to very high value. Higher fragmentation reduces the performance of the database as reading from that particular table becomes very expensive. One of the ways to reduce the fragmentation is to rebuild index on the database. Let us rebuild the index and observe fragmentation and database size. -- Rebuild Index on FirstTable ALTER INDEX IX_SecondTable_ID ON SecondTable REBUILD GO -- Name of the Database and Size SELECT name, (size*8) Size_KB FROM sys.database_files GO -- Check Fragmentations in the database SELECT avg_fragmentation_in_percent, fragment_count FROM sys.dm_db_index_physical_stats (DB_ID(), OBJECT_ID('SecondTable'), NULL, NULL, 'LIMITED') GO You can notice that after rebuilding, Fragmentation reduces to a very low value (almost same to original value); however the database size increases way higher than the original. Before rebuilding, the size of the database was 5 MB, and after rebuilding, it is around 20 MB. Regular rebuilding the index is rebuild in the same user database where the index is placed. This usually increases the size of the database. Look at irony of the Shrinking database. One person shrinks the database to gain space (thinking it will help performance), which leads to increase in fragmentation (reducing performance). To reduce the fragmentation, one rebuilds index, which leads to size of the database to increase way more than the original size of the database (before shrinking). Well, by Shrinking, one did not gain what he was looking for usually. Rebuild indexing is not the best suggestion as that will create database grow again. I have always remembered the excellent post from Paul Randal regarding Shrinking the database is bad. I suggest every one to read that for accuracy and interesting conversation. Let us run following script where we Shrink the database and REORGANIZE. -- Name of the Database and Size SELECT name, (size*8) Size_KB FROM sys.database_files GO -- Check Fragmentations in the database SELECT avg_fragmentation_in_percent, fragment_count FROM sys.dm_db_index_physical_stats (DB_ID(), OBJECT_ID('SecondTable'), NULL, NULL, 'LIMITED') GO -- Shrink the Database DBCC SHRINKDATABASE (ShrinkIsBed); GO -- Name of the Database and Size SELECT name, (size*8) Size_KB FROM sys.database_files GO -- Check Fragmentations in the database SELECT avg_fragmentation_in_percent, fragment_count FROM sys.dm_db_index_physical_stats (DB_ID(), OBJECT_ID('SecondTable'), NULL, NULL, 'LIMITED') GO -- Rebuild Index on FirstTable ALTER INDEX IX_SecondTable_ID ON SecondTable REORGANIZE GO -- Name of the Database and Size SELECT name, (size*8) Size_KB FROM sys.database_files GO -- Check Fragmentations in the database SELECT avg_fragmentation_in_percent, fragment_count FROM sys.dm_db_index_physical_stats (DB_ID(), OBJECT_ID('SecondTable'), NULL, NULL, 'LIMITED') GO You can see that REORGANIZE does not increase the size of the database or remove the fragmentation. Again, I no way suggest that REORGANIZE is the solution over here. This is purely observation using demo. Read the blog post of Paul Randal. Following script will clean up the database -- Clean up USE MASTER GO ALTER DATABASE ShrinkIsBed SET SINGLE_USER WITH ROLLBACK IMMEDIATE GO DROP DATABASE ShrinkIsBed GO There are few valid cases of the Shrinking database as well, but that is not covered in this blog post. We will cover that area some other time in future. Additionally, one can rebuild index in the tempdb as well, and we will also talk about the same in future. Brent has written a good summary blog post as well. Are you Shrinking your database? Well, when are you going to stop Shrinking it? Reference: Pinal Dave (http://blog.SQLAuthority.com) Filed under: Pinal Dave, PostADay, SQL, SQL Authority, SQL Index, SQL Performance, SQL Query, SQL Scripts, SQL Server, SQL Tips and Tricks, SQLServer, T SQL, Technology

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  • DBCC CHECKDB on VVLDB and latches (Or: My Pain is Your Gain)

    - by Argenis
      Does your CHECKDB hurt, Argenis? There is a classic blog series by Paul Randal [blog|twitter] called “CHECKDB From Every Angle” which is pretty much mandatory reading for anybody who’s even remotely considering going for the MCM certification, or its replacement (the Microsoft Certified Solutions Master: Data Platform – makes my fingers hurt just from typing it). Of particular interest is the post “Consistency Options for a VLDB” – on it, Paul provides solid, timeless advice (I use the word “timeless” because it was written in 2007, and it all applies today!) on how to perform checks on very large databases. Well, here I was trying to figure out how to make CHECKDB run faster on a restored copy of one of our databases, which happens to exceed 7TB in size. The whole thing was taking several days on multiple systems, regardless of the storage used – SAS, SATA or even SSD…and I actually didn’t pay much attention to how long it was taking, or even bothered to look at the reasons why - as long as it was finishing okay and found no consistency errors. Yes – I know. That was a huge mistake, as corruption found in a database several days after taking place could only allow for further spread of the corruption – and potentially large data loss. In the last two weeks I increased my attention towards this problem, as we noticed that CHECKDB was taking EVEN LONGER on brand new all-flash storage in the SAN! I couldn’t really explain it, and were almost ready to blame the storage vendor. The vendor told us that they could initially see the server driving decent I/O – around 450Mb/sec, and then it would settle at a very slow rate of 10Mb/sec or so. “Hum”, I thought – “CHECKDB is just not pushing the I/O subsystem hard enough”. Perfmon confirmed the vendor’s observations. Dreaded @BlobEater What was CHECKDB doing all the time while doing so little I/O? Eating Blobs. It turns out that CHECKDB was taking an extremely long time on one of our frankentables, which happens to be have 35 billion rows (yup, with a b) and sucks up several terabytes of space in the database. We do have a project ongoing to purge/split/partition this table, so it’s just a matter of time before we deal with it. But the reality today is that CHECKDB is coming to a screeching halt in performance when dealing with this particular table. Checking sys.dm_os_waiting_tasks and sys.dm_os_latch_stats showed that LATCH_EX (DBCC_OBJECT_METADATA) was by far the top wait type. I remembered hearing recently about that wait from another post that Paul Randal made, but that was related to computed-column indexes, and in fact, Paul himself reminded me of his article via twitter. But alas, our pathologic table had no non-clustered indexes on computed columns. I knew that latches are used by the database engine to do internal synchronization – but how could I help speed this up? After all, this is stuff that doesn’t have a lot of knobs to tweak. (There’s a fantastic level 500 talk by Bob Ward from Microsoft CSS [blog|twitter] called “Inside SQL Server Latches” given at PASS 2010 – and you can check it out here. DISCLAIMER: I assume no responsibility for any brain melting that might ensue from watching Bob’s talk!) Failed Hypotheses Earlier on this week I flew down to Palo Alto, CA, to visit our Headquarters – and after having a great time with my Monkey peers, I was relaxing on the plane back to Seattle watching a great talk by SQL Server MVP and fellow MCM Maciej Pilecki [twitter] called “Masterclass: A Day in the Life of a Database Transaction” where he discusses many different topics related to transaction management inside SQL Server. Very good stuff, and when I got home it was a little late – that slow DBCC CHECKDB that I had been dealing with was way in the back of my head. As I was looking at the problem at hand earlier on this week, I thought “How about I set the database to read-only?” I remembered one of the things Maciej had (jokingly) said in his talk: “if you don’t want locking and blocking, set the database to read-only” (or something to that effect, pardon my loose memory). I immediately killed the CHECKDB which had been running painfully for days, and set the database to read-only mode. Then I ran DBCC CHECKDB against it. It started going really fast (even a bit faster than before), and then throttled down again to around 10Mb/sec. All sorts of expletives went through my head at the time. Sure enough, the same latching scenario was present. Oh well. I even spent some time trying to figure out if NUMA was hurting performance. Folks on Twitter made suggestions in this regard (thanks, Lonny! [twitter]) …Eureka? This past Friday I was still scratching my head about the whole thing; I was ready to start profiling with XPERF to see if I could figure out which part of the engine was to blame and then get Microsoft to look at the evidence. After getting a bunch of good news I’ll blog about separately, I sat down for a figurative smack down with CHECKDB before the weekend. And then the light bulb went on. A sparse column. I thought that I couldn’t possibly be experiencing the same scenario that Paul blogged about back in March showing extreme latching with non-clustered indexes on computed columns. Did I even have a non-clustered index on my sparse column? As it turns out, I did. I had one filtered non-clustered index – with the sparse column as the index key (and only column). To prove that this was the problem, I went and setup a test. Yup, that'll do it The repro is very simple for this issue: I tested it on the latest public builds of SQL Server 2008 R2 SP2 (CU6) and SQL Server 2012 SP1 (CU4). First, create a test database and a test table, which only needs to contain a sparse column: CREATE DATABASE SparseColTest; GO USE SparseColTest; GO CREATE TABLE testTable (testCol smalldatetime SPARSE NULL); GO INSERT INTO testTable (testCol) VALUES (NULL); GO 1000000 That’s 1 million rows, and even though you’re inserting NULLs, that’s going to take a while. In my laptop, it took 3 minutes and 31 seconds. Next, we run DBCC CHECKDB against the database: DBCC CHECKDB('SparseColTest') WITH NO_INFOMSGS, ALL_ERRORMSGS; This runs extremely fast, as least on my test rig – 198 milliseconds. Now let’s create a filtered non-clustered index on the sparse column: CREATE NONCLUSTERED INDEX [badBadIndex] ON testTable (testCol) WHERE testCol IS NOT NULL; With the index in place now, let’s run DBCC CHECKDB one more time: DBCC CHECKDB('SparseColTest') WITH NO_INFOMSGS, ALL_ERRORMSGS; In my test system this statement completed in 11433 milliseconds. 11.43 full seconds. Quite the jump from 198 milliseconds. I went ahead and dropped the filtered non-clustered indexes on the restored copy of our production database, and ran CHECKDB against that. We went down from 7+ days to 19 hours and 20 minutes. Cue the “Argenis is not impressed” meme, please, Mr. LaRock. My pain is your gain, folks. Go check to see if you have any of such indexes – they’re likely causing your consistency checks to run very, very slow. Happy CHECKDBing, -Argenis ps: I plan to file a Connect item for this issue – I consider it a pretty serious bug in the engine. After all, filtered indexes were invented BECAUSE of the sparse column feature – and it makes a lot of sense to use them together. Watch this space and my twitter timeline for a link.

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  • DENY select on sys.dm_db_index_physical_stats

    - by steveh99999
    Technorati Tags: security,DMV,permission,sys.dm_db_index_physical_stats I recently saw an interesting blog article by Paul Randal about the performance overhead of querying the sys.dm_db_index_physical_stats. So I was thinking, would it be possible to let non-sysadmin users query DMVs on a SQL server but stop them querying this I/O intensive DMV ? Yes it is, here’s how… 1. Create a new login for test purposes, with permissions to access AdventureWorks database only … CREATE LOGIN [test] WITH PASSWORD='xxxx', DEFAULT_DATABASE=[AdventureWorks] GO USE [AdventureWorks] GO CREATE USER [test] FOR LOGIN [test] WITH DEFAULT_SCHEMA=[dbo] GO 2.login as user test and issue command SELECT  * FROM sys.dm_db_index_physical_stats(DB_ID('AdventureWorks'),NULL,NULL,NULL,'DETAILED') gets error :-  Msg 297, Level 16, State 12, Line 1 The user does not have permission to perform this action. 3.As a sysadmin, issue command :- USE AdventureWorks GRANT VIEW DATABASE STATE TO [test] or GRANT VIEW SERVER STATE TO [test] if all databases can be queried via DMV. 4. Try again as user test to issue command SELECT * FROM sys.dm_db_index_physical_stats(DB_ID('AdventureWorks '),NULL,NULL,NULL,'DETAILED') -- now produces valid results from the DMV.. 5 now create the test user in master database, public role only USE master CREATE USER [test] FOR LOGIN [test] 6 issue command :- USE master DENY SELECT ON sys.dm_db_index_physical_stats TO [test] 7 Now go back to AdventureWorks using test login and try SELECT * FROM sys.dm_db_index_physical_stats(DB_ID('AdventureWorks’),NULL,NULL,NULL,’DETAILED') Now gets error... Msg 229, Level 14, State 5, Line 1 The SELECT permission was denied on the object 'dm_db_index_physical_stats', database 'mssqlsystemresource', schema 'sys'. but the user is still able to query all other non-IO-intensive DMVs. If the user attempts to view the index physical stats via a builtin management studio report  – see recent blog post by Pinal Dave they get an error also

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  • SQL SERVER – WRITELOG – Wait Type – Day 17 of 28

    - by pinaldave
    WRITELOG is one of the most interesting wait types. So far we have seen a lot of different wait types, but this log type is associated with log file which makes it interesting to deal with. From Book On-Line: WRITELOG Occurs while waiting for a log flush to complete. Common operations that cause log flushes are checkpoints and transaction commits. WRITELOG Explanation: This wait type is usually seen in the heavy transactional database. When data is modified, it is written both on the log cache and buffer cache. This wait type occurs when data in the log cache is flushing to the disk. During this time, the session has to wait due to WRITELOG. I have recently seen this wait type’s persistence at my client’s place, where one of the long-running transactions was stopped by the user causing it to roll back. In the future, I will see if I could re-create this situation once again on my machine to validate the relation. Reducing WRITELOG wait: There are several suggestions to reduce this wait stats: Move Transaction Log to Separate Disk from mdf and other files. Avoid cursor-like coding methodology and frequent committing of statements. Find the most active file based on IO stall time based on the script written over here. You can also use fn_virtualfilestats to find IO-related issues using the script mentioned over here. Check the IO-related counters (PhysicalDisk:Avg.Disk Queue Length, PhysicalDisk:Disk Read Bytes/sec and PhysicalDisk :Disk Write Bytes/sec) for additional details. Read about them over here. There are two excellent resources by Paul Randal, I suggest you understand the subject from those videos. The links to videos are here and here. Note: The information presented here is from my experience and there is no way that I claim it to be accurate. I suggest reading Book OnLine for further clarification. All the discussion of Wait Stats in this blog is generic and varies from system to system. It is recommended that you test this on a development server before implementing it to a production server. Reference: Pinal Dave (http://blog.SQLAuthority.com) Filed under: Pinal Dave, PostADay, SQL, SQL Authority, SQL Query, SQL Scripts, SQL Server, SQL Tips and Tricks, SQL Wait Stats, SQL Wait Types, T SQL, Technology

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  • Ghost Records, Backups, and Database Compression…With a Pinch of Security Considerations

    - by Argenis
      Today Jeffrey Langdon (@jlangdon) posed on #SQLHelp the following questions: So I set to answer his question, and I said to myself: “Hey, I haven’t blogged in a while, how about I blog about this particular topic?”. Thus, this post was born. (If you have never heard of Ghost Records and/or the Ghost Cleanup Task, go see this blog post by Paul Randal) 1) Do ghost records get copied over in a backup? If you guessed yes, you guessed right. The backup process in SQL Server takes all data as it is on disk – it doesn’t crack the pages open to selectively pick which slots have actual data and which ones do not. The whole page is backed up, regardless of its contents. Even if ghost cleanup has run and processed the ghost records, the slots are not overwritten immediately, but rather until another DML operation comes along and uses them. As a matter of fact, all of the allocated space for a database will be included in a full backup. So, this poses a bit of a security/compliance problem for some of you DBA folk: if you want to take a full backup of a database after you’ve purged sensitive data, you should rebuild all of your indexes (with FILLFACTOR set to 100%). But the empty space on your data file(s) might still contain sensitive data! A SHRINKFILE might help get rid of that (not so) empty space, but that might not be the end of your troubles. You might _STILL_ have (not so) empty space on your files! One approach that you can follow is to export all of the data on your database to another SQL Server instance that does NOT have Instant File Initialization enabled. This can be a tedious and time-consuming process, though. So you have to weigh in your options and see what makes sense for you. Snapshot Replication is another idea that comes to mind. 2) Does Compression get rid of ghost records (2008)? The answer to this is no. The Ghost Records/Ghost Cleanup Task mechanism is alive and well on compressed tables and indexes. You can prove this running a simple script: CREATE DATABASE GhostRecordsTest GO USE GhostRecordsTest GO CREATE TABLE myTable (myPrimaryKey int IDENTITY(1,1) PRIMARY KEY CLUSTERED,                       myWideColumn varchar(1000) NOT NULL DEFAULT 'Default string value')                         ALTER TABLE myTable REBUILD PARTITION = ALL WITH (DATA_COMPRESSION = PAGE) GO INSERT INTO myTable DEFAULT VALUES GO 10 DELETE myTable WHERE myPrimaryKey % 2 = 0 DBCC TRACEON(2514) DBCC CHECKTABLE(myTable) TraceFlag 2514 will make DBCC CHECKTABLE give you an extra tidbit of information on its output. For the above script: “Ghost Record count = 5” Until next time,   -Argenis

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  • Ghost Records, Backups, and Database Compression…With a Pinch of Security Considerations

    - by Argenis
      Today Jeffrey Langdon (@jlangdon) posed on #SQLHelp the following questions: So I set to answer his question, and I said to myself: “Hey, I haven’t blogged in a while, how about I blog about this particular topic?”. Thus, this post was born. (If you have never heard of Ghost Records and/or the Ghost Cleanup Task, go see this blog post by Paul Randal) 1) Do ghost records get copied over in a backup? If you guessed yes, you guessed right. The backup process in SQL Server takes all data as it is on disk – it doesn’t crack the pages open to selectively pick which slots have actual data and which ones do not. The whole page is backed up, regardless of its contents. Even if ghost cleanup has run and processed the ghost records, the slots are not overwritten immediately, but rather until another DML operation comes along and uses them. As a matter of fact, all of the allocated space for a database will be included in a full backup. So, this poses a bit of a security/compliance problem for some of you DBA folk: if you want to take a full backup of a database after you’ve purged sensitive data, you should rebuild all of your indexes (with FILLFACTOR set to 100%). But the empty space on your data file(s) might still contain sensitive data! A SHRINKFILE might help get rid of that (not so) empty space, but that might not be the end of your troubles. You might _STILL_ have (not so) empty space on your files! One approach that you can follow is to export all of the data on your database to another SQL Server instance that does NOT have Instant File Initialization enabled. This can be a tedious and time-consuming process, though. So you have to weigh in your options and see what makes sense for you. Snapshot Replication is another idea that comes to mind. 2) Does Compression get rid of ghost records (2008)? The answer to this is no. The Ghost Records/Ghost Cleanup Task mechanism is alive and well on compressed tables and indexes. You can prove this running a simple script: CREATE DATABASE GhostRecordsTest GO USE GhostRecordsTest GO CREATE TABLE myTable (myPrimaryKey int IDENTITY(1,1) PRIMARY KEY CLUSTERED,                       myWideColumn varchar(1000) NOT NULL DEFAULT 'Default string value')                         ALTER TABLE myTable REBUILD PARTITION = ALL WITH (DATA_COMPRESSION = PAGE) GO INSERT INTO myTable DEFAULT VALUES GO 10 DELETE myTable WHERE myPrimaryKey % 2 = 0 DBCC TRACEON(2514) DBCC CHECKTABLE(myTable) TraceFlag 2514 will make DBCC CHECKTABLE give you an extra tidbit of information on its output. For the above script: “Ghost Record count = 5” Until next time,   -Argenis

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  • SQL Server Scripts I Use

    - by Bill Graziano
    When I get to a new client I usually find myself using the same set of scripts for maintenance and troubleshooting.  These are all drop in solutions for various maintenance issues. Reindexing.  I use Michelle Ufford’s (SQLFool) re-indexing script.  I like that it has a throttle and only re-indexes when needed.  She also has a variety of other interesting scripts on her blog too. Server Activity.  Adam Machanic is up to version 10 of sp_WhoIsActive.  It’s a great replacement for the sp_who* stored procedures and does so much more.  If a server is acting funny this is one of the first tools I use. Backups.  Tara Kizer has a great little T-SQL script for SQL Server backups.  Wait Stats.  Paul Randal has a great script to display wait stats.  The biggest benefit for me is that his script filters out at least three dozen wait stats that I just don’t care about (for example LAZYWRITER_SLEEP). Update Statistics.  I didn’t find anything I liked so I wrote a simple script to update stats myself.  The big need for me was that it had to run inside a time window and update the oldest statistics first.  Is there a better one? Diagnostic Queries.  Glenn Berry has a huge collection of DMV queries available.  He also just highlighted five of them including two I really like dealing with unused indexes and suggested indexes. Single Use Query Plans.  Kim Tripp has a script that counts the number of single-use query plans.  This should guide you in whether to enable the Optimize for Adhoc Workloads option in SQL Server 2008. Granting Permissions to Developers.  This is one of those scripts I didn’t even know I needed until I needed it.  Kendra Little wrote it to grant a login read-only permission to all the databases.  It also grants view server state and a few other handy permissions.   What else do you use?  What should I add to my list?

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  • Shrinking a large transaction log on a full drive

    - by Sam
    Someone fired off an update statement as part of some maintenance which did a cross join update on two tables with 200,000 records in each. That's 40 trillion statements, which would explain part of how the log grew to 200GB. I also did not have the log file capped, which is another problem I will be taking care of server wide - where we have almost 200 databases residing. The 'solution' I used was to backup the database, backup the log with truncate_only, and then backup the database again. I then shrunk the log file and set a cap on the log. Seeing as there were other databases using the log drive, I was in a bit of a rush to clean it out. I might have been able to back the log file up to our backup drive, hoping that no other databases needed to grow their log file. Paul Randal from http://technet.microsoft.com/en-us/magazine/2009.02.logging.aspx Under no circumstances should you delete the transaction log, try to rebuild it using undocumented commands, or simply truncate it using the NO_LOG or TRUNCATE_ONLY options of BACKUP LOG (which have been removed in SQL Server 2008). These options will either cause transactional inconsistency (and more than likely corruption) or remove the possibility of being able to properly recover the database. Were there any other options I'm not aware of?

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  • It’s the thought that counts…

    - by Tony Davis
    I recently finished editing a book called Tribal SQL, and it was a fantastic experience. It’s a community-sourced book written by first-timers. Fifteen previously unpublished authors contributed one chapter each, with the seemingly simple remit to write about “what makes them passionate about working with SQL Server, something that all SQL Server DBAs and developers really need to know”. Sure, some of the writing skills were a bit rusty as one would expect from busy people, but the ideas and energy were sheer nectar. Any seasoned editor can deal easily with the problem of fixing the output of untrained writers. We can handle with the occasional technical error too, which is why we have technical reviewers. The editor’s real job is to hone the clarity and flow of ideas, making the author’s knowledge and experience accessible to as many others as possible. What the writer needs to bring, on the other hand, is enthusiasm, attention to detail, common sense, and a sense of the person behind the writing. If any of these are missing, no editor can fix it. We can see these essential characteristics in many of the more seasoned and widely-published writers about SQL. To illustrate what I mean by enthusiasm, or passion, take a look at the work of Laerte Junior or Fabiano Amorim. Both authors have English as a second language, but their energy, enthusiasm, sheer immersion in a technology and thirst to know more, drives them, with a little editorial help, to produce articles of far more practical value than one can find in the “manuals”. There’s the attention to detail of the likes of Jonathan Kehayias, or Paul Randal. Read their work and one begins to understand the knowledge coupled with incredible rigor, the willingness to bend and test every piece of advice offered to make sure it’s correct, that marks out the very best technical writing. There’s the common sense of someone like Louis Davidson. All writers, including Louis, like to stretch the grey matter of their readers, but some of the most valuable writing is that which takes a complicated idea, or distils years of experience, and expresses it in a way that sounds like simple common sense. There’s personality and humor. Contrary to what you may have been told, they can and do mix well with technical writing, as long as they don’t become a distraction. Read someone like Rodney Landrum, or Phil Factor, for numerous examples of articles that teach hard technical lessons but also make you smile at least twice along the way. Writing well is not easy and it takes a certain bravery to expose your ideas and knowledge for dissection by others, but it doesn’t mean that writing should be the preserve only of those trained in the art, or best left to the MVPs. I believe that Tribal SQL is testament to the fact that if you have passion for what you do, and really know your topic then, with a little editorial help, you can write, and people will learn from what you have to say. You can read a sample chapter, by Mark Rasmussen, in this issue of Simple-Talk and I hope you’ll consider checking out the book (if you needed any further encouragement, it’s also for a good cause, Computers4Africa). Cheers, Tony  

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  • Top 10 Linked Blogs of 2010

    - by Bill Graziano
    Each week I send out a SQL Server newsletter and include links to interesting blog posts.  I’ve linked to over 500 blog posts so far in 2010.  Late last year I started storing those links in a database so I could do a little reporting.  I tend to link to posts related to the OLTP engine.  I also try to link to the individual blogger in the group blogs.  Unfortunately that wasn’t possible for the SQLCAT and CSS blogs.  I also have a real weakness for posts related to PASS. These are the top 10 blogs that I linked to during the year ordered by the number of posts I linked to. Paul Randal – Paul writes extensively on the internals of the relational engine.  Lots of great posts around transactions, transaction log, disaster recovery, corruption, indexes and DBCC.  I also linked to many of his SQL Server myths posts. Glenn Berry – Glenn writes very interesting posts on how hardware affects SQL Server.  I especially like his posts on the various CPU platforms.  These aren’t necessarily topics that I’m searching for but I really enjoy reading them. The SQLCAT Team – This Microsoft team focuses on the largest and most interesting SQL Server installations.  The regularly publish white papers and best practices. SQL Server CSS Team – These are the top engineers from the Microsoft Customer Service and Support group.  These are the folks you finally talk to after your case has been escalated about 20 times.  They write about the interesting problems they find. Brent Ozar – The posts I linked to mostly focused on the relational engine: CPU, NUMA, SSD drives, performance monitoring, etc.  But Brent writes about a real variety of topics including blogging, social networking, speaking, the MCM, SQL Azure and anything else that seems to strike his fancy.  His posts are always well written and though provoking. Jeremiah Peschka – A number of Jeremiah’s posts weren’t about SQL Server.  He’s very active in the “NoSQL” area and I linked to a number of those posts.  I think it’s important for people to know what other technologies are out there. Brad McGehee – Brad writes about being a DBA including maintenance plans, DBA checklists, compression and audit. Thomas LaRock – I linked to a variety of posts from PBM to networking to 24 Hours of PASS to TDE.  Just a real variety of topics.  Tom always writes with an interesting style usually mixing in a movie theme and/or bacon. Aaron Bertrand – Many of my links this year were Denali features.  He also had a great series on bad habits to kick. Michael J. Swart – This last one surprised me.  There are some well known SQL Server bloggers below Michael on this list.  I linked to posts on indexes, hierarchies, transactions and I/O performance and a variety of other engine related posts.  All are interesting and well thought out.  Many of his non-SQL posts are also very good.  He seems to have an interest in puzzles and other brain teasers.  Michael, I won’t be surprised again!

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