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• Has the SQL Community Lost its Focus?

  - by Jonathan Kehayias

  Yesterday, Thomas LaRock’s blog post, WMI Code Creator , was brought to my attention by a member of the SQL Community.  I subscribe to Tom’s blog in my blog reader so eventually I’d like to think that his post would have come to my attention, but to be perfectly honest, I have been to busy with other obligations lately that have made reading blog posts almost impossible.  When I looked at Tom’s post, I was kind of put off when I did a copy paste of the Code from it and got the following:...(read more)

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• Happy 10th Anniversary to AskTom!

  - by jenny.gelhausen

  Happy Anniversary to Tom Kyte's AskTom.oracle.com! Ten years of nuturing and advising the Oracle Database community is certainly a milestone to celebrate. With your first question being asked and answered in early 2000 about Oracle 7.3 on a Sun 5.5.1 machine - we recognize and appreciate the value of AskTom's informaton and insight to the industry. Well done and THANK YOU Tom! the Database Insider Team

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• Can't start mysql - mysql respawning too fast, stopped

  - by Tom

  Today I did a fresh install of ubuntu 12.04 and went about setting up my local development environment. I installed mysql and edited /etc/mysql/my.cnf to optimise InnoDB but when I try to restart mysql, it fails with a error: [20:53][tom@Pochama:/var/www/website] (master) $ sudo service mysql restart start: Job failed to start The syslog reveals there is a problem with the init script: > tail -f /var/log/syslog Apr 28 21:17:46 Pochama kernel: [11840.884524] type=1400 audit(1335644266.033:184): apparmor="STATUS" operation="profile_replace" name="/usr/sbin/mysqld" pid=760 comm="apparmor_parser" Apr 28 21:17:47 Pochama kernel: [11842.603773] init: mysql main process (764) terminated with status 7 Apr 28 21:17:47 Pochama kernel: [11842.603841] init: mysql main process ended, respawning Apr 28 21:17:48 Pochama kernel: [11842.932462] init: mysql post-start process (765) terminated with status 1 Apr 28 21:17:48 Pochama kernel: [11842.950393] type=1400 audit(1335644268.101:185): apparmor="STATUS" operation="profile_replace" name="/usr/sbin/mysqld" pid=811 comm="apparmor_parser" Apr 28 21:17:49 Pochama kernel: [11844.656598] init: mysql main process (815) terminated with status 7 Apr 28 21:17:49 Pochama kernel: [11844.656665] init: mysql main process ended, respawning Apr 28 21:17:50 Pochama kernel: [11845.004435] init: mysql post-start process (816) terminated with status 1 Apr 28 21:17:50 Pochama kernel: [11845.021777] type=1400 audit(1335644270.173:186): apparmor="STATUS" operation="profile_replace" name="/usr/sbin/mysqld" pid=865 comm="apparmor_parser" Apr 28 21:17:51 Pochama kernel: [11846.721982] init: mysql main process (871) terminated with status 7 Apr 28 21:17:51 Pochama kernel: [11846.722001] init: mysql respawning too fast, stopped Any ideas? Things I tried already: I googled and found a Ubuntu bug with apparmor (https://bugs.launchpad.net/ubuntu/+source/mysql-5.5/+bug/970366), I changed apparmor from enforce mode to complain mode: sudo apt-get install apparmor-utils sudo aa-complain /usr/sbin/mysqld sudo /etc/init.d/apparmor reload but it didn't help. I still can't start mysql. I also thought the issue may be because the InnoDB logfiles were a different size than mysql was expecting. I removed the innodb log files before restarting using: sudo mv /var/lib/mysql/ib_logfile* /tmp. No luck though. Workaround: I re-installed 12.04, made sure not to touch /etc/mysql/my.cnf in any way. Mysql is working so I can get on with what I need to do. But I will need to edit it at some point - Hopefully I'll have figured out a solution, or this question will have been answered by that point...

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• Creating block devices for openstack deployment using MAAS and juju (nova-volume deployment)

  - by Tom Van Hoof

  Hi, I'm currently trying to get a openstack deployment working by using MAAS with 9 nodes and juju. To do this I found this guide, working with ubuntu 12.04 LTS https://help.ubuntu.com/community/UbuntuCloudInfrastructure and followed it as good as I can. After a vigorous amount of trial and error I finally got to the point where I'm supposed to deploy nova-volume using the "custom" config file. However when my node is started and shows up as running in the "juju status" report the service reports the installation failed. I'm trying to install with juju jitsu by the way. I think it has something to do with the following statement in the openstack.cfg file : nova-volume: # This must be a free block device that is writable on the nova-volume host. block-device: "xvdb" overwrite: "true" I did some research and found that (at least I think) this refers to a Xen Virtual Drive/device, and because the device is not present on the node it's being deployed to, the installation fails. What I don't understand is how I am supposed to have such a block device available on a machine which was completely managed by MAAS. Does anyone here have any experience with this and knows of a way to solve this, or am I missing something big here. Some kind of missing link between the MAAS and a separate XEN host ? My MAAS server is ubuntu 12.04LTS Server. All help is welcome. Kind regards, Tom

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• Continual Professional Development - proving new skills to non-technical employers

  - by Tom

  Background I work in a non-IT based company, as a professional software developer, building a large scale internal database system. I am fortunate to have a fairly senior position within the company, and have been working here for around 4 years. Often I get asked by management "how do you learn new things?". To be honest, I don't know how to answer this. Over the last 6 months, I've really gotten my teeth into some new techniques and technologies to make my level of coding far better and hopefully improve the quality of the software. Even if it's just refreshing my skills on things I've learnt already. Like last week I dived into some complex XLinq and TPL code (.net). Nothing revolutionary, but I feel like I am a bit better than before. Question The question is, how do I prove this to my employer? It'd be nice to be able to put this on paper. Possibilities I could: Keep a journal of what I've learnt - keeping the technical bits in (nobody would understand or care, but it's better than them being omitted) ???? (I've run out of ideas already) Any ideas? Thanks, Tom

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• What is Ubuntu's Definition of a "Registered Application"?

  - by Tom

  I've run into this a few times when installing apps from source, and during the occasional hack with update-alternatives. So far, it's only been a minor annoyance (ie, not got in the way of the end-goal) but it's now a frustration as it's pointing to a hole in my knowledge-base... so when I get a message that 'foo' is "not a registered application" (or I can't use foo's default icon cuz Ubuntu has no knowledge of 'foo'): (1) what defines a "registered application"? (2) how can I define an application installed from source (and likely residing in $HOME/bin/app-name) such that it packs the same functionality as a package installed from a .deb? (if the solution is not self-evident from answer 1) Example: I download and unpack daily dev builds of sublime-text-2 to /home/tom/bin/sublime-text-2. I've created a *.desktop file with appropriate shortcuts, etc. But the icon for sublime cannot be display in any launcher even if I provide a full pathname to the option. The solution is to install a 2nd instance of sublime from a deb package. When I install sublime-text-2 from a .deb package, it installs under /usr/bin && /usr/lib, the installed .desktop file is stored under /usr/share/applications, and the relevant line reads: icon=sublime_text. Where's the linkage I'm missing? Somehow Ubuntu knows how to exact the icon from sublime_text in the latter, but not in the former (again, even with a full path provided).

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• Inconsistent accessibility error in xna.

  - by Tom

  Hey all, you may remember me asking a question regarding a snake game I was creating about two weeks ago. Well I'm quite far now into making the game (thanks to a brilliant tutorial I found). But I've come across the error described named above. So heres my problem; I have a SnakeFood class that has a method called "Reposition". In the game1 class I have a method called "UpdateInGame" which calls the reposition method to load an orange that spawns in a random place every second. My latest piece of code changed the reposition method to allow the snake I have on the screen to not be overlapped by the orange that randomly spawns. Now I get the error (in full): Error 1 Inconsistent accessibility: parameter type 'TheMathsSnakeGame.Snake' is less accessible than method 'TheMathsSnakeGame.SnakeFood.Reposition(TheMathsSnakeGame.Snake)' C:\Users\Tom\Documents\Visual Studio 2008\Projects\TheMathsSnakeGame\TheMathsSnakeGame\SnakeFood.cs 33 21 TheMathsSnakeGame I understand what the errors trying to tell me but having changed the accessiblity of the methods, I still can't get it to work. Sorry about the longwinded question. Thanks in advance :) Edit: Code I'm using (Game1 Class) private void UpdateInGame(GameTime gameTime) { //Calls the oranges "reposition" method every second if (gameTime.TotalGameTime.Milliseconds % 1000 == 0) orange.Reposition(sidney); sidney.Update(gameTime); } (SnakeFood Class) public void Reposition(Snake snake) { do { position = new Point(rand.Next(Grid.maxHeight), rand.Next(Grid.maxWidth)); } while (snake.IsBodyOnPoint(position)); }

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• Unable to install ubuntu on a AMD 64 bit system with a AMD Radeon HD 6670 graphics card

  - by Tom Wingrove

  I’ve been running a dual boot system (Ubuntu/Windows7) for two years or so with no problems. I recently built an AMD 64 Bit System, re-installed Windows but when I went to load Ubuntu inside Windows, hit a snag. The screen view during installation became small square blocks of colour, which obviously is a graphics drive problem. I tried various live disks both 32 & 64 bit for, Ubuntu 12.04, 11.10 & 10.10, all but Ubuntu 10.10 had the same problem. Ubuntu 10.10 loaded ok, installed the presented ATI graphics driver as usual but was left with the AMD Unsupported watermark at the bottom right of the screen. The graphics card installed in the computer is an MSI ATI Radeon HD 6670 (in effect an AMD Radeon HD 6670). I am fairly new to linux and while I can install and tweak the OS, I am rather baffled as to what to do. So my question is will an up to date ATI Driver be released in the near future for installation/live disks? Or am I going to have to downgrade my graphics card to use linux? Yours Tom Wingrove

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• Desktop goes un-usable after upgrade to 12.04

  - by Tom Nail

  I have multiple Ubuntu systems connected to a KVM, one of which I recently upgraded from Ubuntu 10 to 12.04. After the upgrade, this system desktop does fine until it is allowed to go to idle (i.e., I've switched to another system on the KVM and it locks it's desktop). When I come back to it, the screen is garbled and paging across at a rate seemingly determined by the mouse. Although no pointer is visible, I can get the screen to stop paging (and just be garbled) by moving the mouse left and right. The paging will slow down and come to a stop, if I can align things carefully enough. This condition persists even when I try to go to a CLI-based login (e.g., CTRL+Alt+F1) and will continue until I reboot the machine. Unfortunately, I'm not very familiar with the Unity desktop, so I don't know where to find things to troubleshoot. A restart of lightdm doesn't change anything, so I'm wondering if this might be more hardware based( although this machine hasn't given me any trouble previously in the same setup). The .xsession-errors file has some issues with compiz, nautilus and GConf listed, but I'm not sure those are actually germane to the issue. Thanks for any help, -=Tom

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• Authenticating Windows 7 against MIT Kerberos 5

  - by tommed

  Hi There, I've been wracking my brains trying to get Windows 7 authenticating against a MIT Kerberos 5 Realm (which is running on an Arch Linux server). I've done the following on the server (aka dc1): Installed and configured a NTP time server Installed and configured DHCP and DNS (setup for the domain tnet.loc) Installed Kerberos from source Setup the database Configured the keytab Setup the ACL file with: *@TNET.LOC * Added a policy for my user and my machine: addpol users addpol admin addpol hosts ank -policy users [email protected] ank -policy admin tom/[email protected] ank -policy hosts host/wdesk3.tnet.loc -pw MYPASSWORDHERE I then did the following to the windows 7 client (aka wdesk3): Made sure the ip address was supplied by my DHCP server and dc1.tnet.loc pings ok Set the internet time server to my linux server (aka dc1.tnet.loc) Used ksetup to configure the realm: ksetup /SetRealm TNET.LOC ksetup /AddKdc dc1.tnet.loc ksetip /SetComputerPassword MYPASSWORDHERE ksetip /MapUser * * After some googl-ing I found that DES encryption was disabled by Windows 7 by default and I turned the policy on to support DES encryption over Kerberos Then I rebooted the windows client However after doing all that I still cannot login from my Windows client. :( Looking at the logs on the server; the request looks fine and everything works great, I think the issue is that the response from the KDC is not recognized by the Windows Client and a generic login error appears: "Login Failure: User name or password is invalid". The log file for the server looks like this (I tail'ed this so I know it's happening when the Windows machine attempts the login): Screen-shot: http://dl.dropbox.com/u/577250/email/login_attempt.png If I supply an invalid realm in the login window I get a completely different error message, so I don't think it's a connection problem from the client to the server? But I can't find any error logs on the Windows machine? (anyone know where these are?) If I try: runas /netonly /user:[email protected] cmd.exe everything works (although I don't get anything appear in the server logs, so I'm wondering if it's not touching the server for this??), but if I run: runas /user:[email protected] cmd.exe I get the same authentication error. Any Kerberos Gurus out there who can give me some ideas as to what to try next? pretty please?

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• Is Subversion(SVN) supported on Ubuntu 10.04 LTS 32bit?

  - by Chad

  I've setup subversion on Ubuntu 10.04, but can't get authentication to work. I believe all my config files are setup correctly, However I keep getting prompted for credentials on a SVN CHECKOUT. Like there is an issue with apache2 talking to svnserve. If I allow anonymous access checkout works fine. Does anybody know if there is a known issue with subversion and 10.04 or see a error in my configuration? below is my configuration: # fresh install of Ubuntu 10.04 LTS 32bit sudo apt-get install apache2 apache2-utils -y sudo apt-get install subversion libapache2-svn subversion-tools -y sudo mkdir /svn sudo svnadmin create /svn/DataTeam sudo svnadmin create /svn/ReportingTeam #Setup the svn config file sudo vi /etc/apache2/mods-available/dav_svn.conf #replace file with the following. <Location /svn> DAV svn SVNParentPath /svn/ AuthType Basic AuthName "Subversion Server" AuthUserFile /etc/apache2/dav_svn.passwd Require valid-user AuthzSVNAccessFile /etc/apache2/svn_acl </Location> sudo touch /etc/apache2/svn_acl #replace file with the following. [groups] dba_group = tom, jerry report_group = tom [DataTeam:/] @dba_group = rw [ReportingTeam:/] @report_group = rw #Start/Stop subversion automatically sudo /etc/init.d/apache2 restart cd /etc/init.d/ sudo touch subversion sudo cat 'svnserve -d -r /svn' > svnserve sudo cat '/etc/init.d/apache2 restart' >> svnserve sudo chmod +x svnserve sudo update-rc.d svnserve defaults #Add svn users sudo htpasswd -cpb /etc/apache2/dav_svn.passwd tom tom sudo htpasswd -pb /etc/apache2/dav_svn.passwd jerry jerry #Test by performing a checkout sudo svnserve -d -r /svn sudo /etc/init.d/apache2 restart svn checkout http://127.0.0.1/svn/DataTeam /tmp/DataTeam

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• django + ExtJs - list of example implementations with source code

  - by Tom Tom

  Hi, I want to learn ExtJs http://www.extjs.com/ in combination with Django (I know Django already .. a bit ;-) and I'm on the search of examples I can learn from. I found already this post. Would be great if you post your links to examples here. Maybe we get a nice list of examplary implementations that help users to start with django + ExtJs. Ext js Poll Tutorial Ext js vs. Jquery "Show how you can use extjs to display a table and how to use jqgrid 3.2 to get the same as in extjs. Incluses extjs and jqgird libraries. I have had to patch jqgrid adding" RESTful Web apps with Django, Piston and Ext JS Convert your forms.Form and forms.ModelForm to extjs and handles the form submission like any django form.

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• Django custom managers - how do I return only objects created by the logged-in user?

  - by Tom Tom

  I want to overwrite the custom objects model manager to only return objects a specific user created. Admin users should still return all objects using the objects model manager. Now I have found an approach that could work. They propose to create your own middleware looking like this: #### myproject/middleware/threadlocals.py try: from threading import local except ImportError: # Python 2.3 compatibility from django.utils._threading_local import local _thread_locals = local() def get_current_user(): return getattr(_thread_locals, 'user', None) class ThreadLocals(object): """Middleware that gets various objects from the request object and saves them in thread local storage.""" def process_request(self, request): _thread_locals.user = getattr(request, 'user', None) #### end And in the Custom manager you could call the get_current_user() method to return only objects a specific user created. class UserContactManager(models.Manager): def get_query_set(self): return super(UserContactManager, self).get_query_set().filter(creator=get_current_user()) Is this a good approach to this use-case? Will this work? Or is this like "using a sledgehammer to crack a nut" ? ;-) Just using: Contact.objects.filter(created_by= user) in each view doesn`t look very neat to me. EDIT Do not use this middleware approach !!! use the approach stated by Jack M. below After a while of testing this approach behaved pretty strange and with this approach you mix up a global-state with a current request. Use the approach presented below. It is really easy and no need to hack around with the middleware. create a custom manager in your model with a function that expects the current user or any other user as an input. #in your models.py class HourRecordManager(models.Manager): def for_user(self, user): return self.get_query_set().filter(created_by=user) class HourRecord(models.Model): #Managers objects = HourRecordManager() #in vour view you can call the manager like this and get returned only the objects from the currently logged-in user. hr_set = HourRecord.objects.for_user(request.user)

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• additional security measures besides a login with user-password - what can you think of?

  - by Tom Tom

  I'm wondering which additional security measures one could take besides a traditional login with user and password. What do you think of this one: _manually adding a cookie to each client which includes a secret key _this cookie is not served by the webserver, it is actually copied "by hand" to each client computer _if a client connects to the web-app the server graps that cookie and if the containing secret key is ok, the traditional login box is presented where the user has to enter the user-password combination _communication between client and server is encrypted with https Thus a potential intruder would first need to get the cookie from the clients computer, which is only possible with having access to the clients computer. This would work only for a very small user-base and an admin willing to do this manual work.

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• django return file over HttpResonse - file is not served correctly

  - by Tom Tom

  I want to return some files in a HttpResponse and I'm using the following function. The file that is returned always has a filesize of 1kb and I do not know why. I can open the file, but it seems that it is not served correctly. Thus I wanted to know how one can return files with django/python over a HttpResponse. @login_required def serve_upload_files(request, file_url): import os.path import mimetypes mimetypes.init() try: file_path = settings.UPLOAD_LOCATION + '/' + file_url fsock = open(file_path,"r") #fsock = open(file_path,"r").read() file_name = os.path.basename(file_path) mime_type_guess = mimetypes.guess_type(file_name) try: if mime_type_guess is not None: response = HttpResponse(mimetype=mime_type_guess[0]) response['Content-Disposition'] = 'attachment; filename=' + file_name response.write(fsock) finally: fsock.close() except IOError: response = HttpResponseNotFound() return response

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• django media url is not resolved in 500 internal server error template

  - by Tom Tom

  Hi, I'm using a 500.html template for my app, which is an identical copy of the 404.html with some minor text changes. Interestingly the {{ media_url }} context variable will not be resolved by the server if the 500.html is presented (e.g. when I force an internal server error), resulting in a page without any css loaded. An easy way to circumvent this would be to hardcode the links to the css, but I m just curious why the media_url is not resolved. Probably it is because the server encounters a internal server error and that leads to context variables not any more available!?

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• django custom management command does not show up in production

  - by Tom Tom

  I wrote a custom management command for django. Locally with my dev settings everything works fine. Now I deployed my project onto the production server and the management command does not show up, respectively is not available. But I did not get an error message deploying the project (syncdb). Any ideas where I could try to begin to search? Is there a special command that all custom management commands are "autodiscovered"?

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• serving files using django - is this a security vulnerability

  - by Tom Tom

  I'm using the following code to serve uploaded files from a login secured view in a django app. Do you think that there is a security vulnerability in this code? I'm a bit concerned about that the user could place arbitrary strings in the url after the upload/ and this is directly mapped to the local filesystem. Actually I don't think that it is a vulnerability issue, since the access to the filesystem is restricted to the files in the folder defined with the UPLOAD_LOCATION setting. UPLOAD_LOCATION = is set to a not publicly available folder on the webserver url(r'^upload/(?P<file_url>[/,.,\s,_,\-,\w]+)', 'aeon_infrastructure.views.serve_upload_files', name='project_detail'), @login_required def serve_upload_files(request, file_url): import os.path import mimetypes mimetypes.init() try: file_path = settings.UPLOAD_LOCATION + '/' + file_url fsock = open(file_path,"r") file_name = os.path.basename(file_path) file_size = os.path.getsize(file_path) print "file size is: " + str(file_size) mime_type_guess = mimetypes.guess_type(file_name) if mime_type_guess is not None: response = HttpResponse(fsock, mimetype=mime_type_guess[0]) response['Content-Disposition'] = 'attachment; filename=' + file_name #response.write(file) except IOError: response = HttpResponseNotFound() return response

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• django handling file uploads - target different than media folder

  - by Tom Tom

  Hi, I want to enable the user to upload media which will not be saved in the media folder. When I use the following line of code data will be uploaded to media/upload/logo . logo_img = models.FileField(upload_to='upload/logo', blank=True) I'm wondering how I can change this behaviour. I would try to write a custom FileField and a view that serves the data based on the database entries. I do not want to place the data the user uploads to the media folder, since it is no public data. Is this approach correct? Are there solutions out there which do exactly what I want and I would reinvent the wheel with implementing this by myself? Would appreciate any help!

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• SQL SERVER – Signal Wait Time Introduction with Simple Example – Wait Type – Day 2 of 28

  - by pinaldave

  In this post, let’s delve a bit more in depth regarding wait stats. The very first question: when do the wait stats occur? Here is the simple answer. When SQL Server is executing any task, and if for any reason it has to wait for resources to execute the task, this wait is recorded by SQL Server with the reason for the delay. Later on we can analyze these wait stats to understand the reason the task was delayed and maybe we can eliminate the wait for SQL Server. It is not always possible to remove the wait type 100%, but there are few suggestions that can help. Before we continue learning about wait types and wait stats, we need to understand three important milestones of the query life-cycle. Running - a query which is being executed on a CPU is called a running query. This query is responsible for CPU time. Runnable – a query which is ready to execute and waiting for its turn to run is called a runnable query. This query is responsible for Signal Wait time. (In other words, the query is ready to run but CPU is servicing another query). Suspended – a query which is waiting due to any reason (to know the reason, we are learning wait stats) to be converted to runnable is suspended query. This query is responsible for wait time. (In other words, this is the time we are trying to reduce). In simple words, query execution time is a summation of the query Executing CPU Time (Running) + Query Wait Time (Suspended) + Query Signal Wait Time (Runnable). Again, it may be possible a query goes to all these stats multiple times. Let us try to understand the whole thing with a simple analogy of a taxi and a passenger. Two friends, Tom and Danny, go to the mall together. When they leave the mall, they decide to take a taxi. Tom and Danny both stand in the line waiting for their turn to get into the taxi. This is the Signal Wait Time as they are ready to get into the taxi but the taxis are currently serving other customer and they have to wait for their turn. In other word they are in a runnable state. Now when it is their turn to get into the taxi, the taxi driver informs them he does not take credit cards and only cash is accepted. Neither Tom nor Danny have enough cash, they both cannot get into the vehicle. Tom waits outside in the queue and Danny goes to ATM to fetch the cash. During this time the taxi cannot wait, they have to let other passengers get into the taxi. As Tom and Danny both are outside in the queue, this is the Query Wait Time and they are in the suspended state. They cannot do anything till they get the cash. Once Danny gets the cash, they are both standing in the line again, creating one more Signal Wait Time. This time when their turn comes they can pay the taxi driver in cash and reach their destination. The time taken for the taxi to get from the mall to the destination is running time (CPU time) and the taxi is running. I hope this analogy is bit clear with the wait stats. You can check the Signalwait stats using following query of Glenn Berry. -- Signal Waits for instance SELECT CAST(100.0 * SUM(signal_wait_time_ms) / SUM (wait_time_ms) AS NUMERIC(20,2)) AS [%signal (cpu) waits], CAST(100.0 * SUM(wait_time_ms - signal_wait_time_ms) / SUM (wait_time_ms) AS NUMERIC(20,2)) AS [%resource waits] FROM sys.dm_os_wait_stats OPTION (RECOMPILE); Higher the Signal wait stats are not good for the system. Very high value indicates CPU pressure. In my experience, when systems are running smooth and without any glitch the Signal wait stat is lower than 20%. Again, this number can be debated (and it is from my experience and is not documented anywhere). In other words, lower is better and higher is not good for the system. In future articles we will discuss in detail the various wait types and wait stats and their resolution. Read all the post in the Wait Types and Queue series. Reference: Pinal Dave (http://blog.SQLAuthority.com) Filed under: Pinal Dave, PostADay, SQL, SQL Authority, SQL DMV, SQL Performance, SQL Query, SQL Scripts, SQL Server, SQL Tips and Tricks, SQL Wait Stats, SQL Wait Types, T SQL, Technology

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• SQL SERVER – Single Wait Time Introduction with Simple Example – Wait Type – Day 2 of 28

  - by pinaldave

  In this post, let’s delve a bit more in depth regarding wait stats. The very first question: when do the wait stats occur? Here is the simple answer. When SQL Server is executing any task, and if for any reason it has to wait for resources to execute the task, this wait is recorded by SQL Server with the reason for the delay. Later on we can analyze these wait stats to understand the reason the task was delayed and maybe we can eliminate the wait for SQL Server. It is not always possible to remove the wait type 100%, but there are few suggestions that can help. Before we continue learning about wait types and wait stats, we need to understand three important milestones of the query life-cycle. Running - a query which is being executed on a CPU is called a running query. This query is responsible for CPU time. Runnable – a query which is ready to execute and waiting for its turn to run is called a runnable query. This query is responsible for Single Wait time. (In other words, the query is ready to run but CPU is servicing another query). Suspended – a query which is waiting due to any reason (to know the reason, we are learning wait stats) to be converted to runnable is suspended query. This query is responsible for wait time. (In other words, this is the time we are trying to reduce). In simple words, query execution time is a summation of the query Executing CPU Time (Running) + Query Wait Time (Suspended) + Query Single Wait Time (Runnable). Again, it may be possible a query goes to all these stats multiple times. Let us try to understand the whole thing with a simple analogy of a taxi and a passenger. Two friends, Tom and Danny, go to the mall together. When they leave the mall, they decide to take a taxi. Tom and Danny both stand in the line waiting for their turn to get into the taxi. This is the Signal Wait Time as they are ready to get into the taxi but the taxis are currently serving other customer and they have to wait for their turn. In other word they are in a runnable state. Now when it is their turn to get into the taxi, the taxi driver informs them he does not take credit cards and only cash is accepted. Neither Tom nor Danny have enough cash, they both cannot get into the vehicle. Tom waits outside in the queue and Danny goes to ATM to fetch the cash. During this time the taxi cannot wait, they have to let other passengers get into the taxi. As Tom and Danny both are outside in the queue, this is the Query Wait Time and they are in the suspended state. They cannot do anything till they get the cash. Once Danny gets the cash, they are both standing in the line again, creating one more Single Wait Time. This time when their turn comes they can pay the taxi driver in cash and reach their destination. The time taken for the taxi to get from the mall to the destination is running time (CPU time) and the taxi is running. I hope this analogy is bit clear with the wait stats. You can check the single wait stats using following query of Glenn Berry. -- Signal Waits for instance SELECT CAST(100.0 * SUM(signal_wait_time_ms) / SUM (wait_time_ms) AS NUMERIC(20,2)) AS [%signal (cpu) waits], CAST(100.0 * SUM(wait_time_ms - signal_wait_time_ms) / SUM (wait_time_ms) AS NUMERIC(20,2)) AS [%resource waits] FROM sys.dm_os_wait_stats OPTION (RECOMPILE); Higher the single wait stats are not good for the system. Very high value indicates CPU pressure. In my experience, when systems are running smooth and without any glitch the single wait stat is lower than 20%. Again, this number can be debated (and it is from my experience and is not documented anywhere). In other words, lower is better and higher is not good for the system. In future articles we will discuss in detail the various wait types and wait stats and their resolution. Read all the post in the Wait Types and Queue series. Reference: Pinal Dave (http://blog.SQLAuthority.com) Filed under: Pinal Dave, PostADay, SQL, SQL Authority, SQL DMV, SQL Performance, SQL Query, SQL Scripts, SQL Server, SQL Tips and Tricks, SQL Wait Stats, SQL Wait Types, T SQL, Technology

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• pci-express ssd running an OS

  - by tom

  Hi, Simple question really, is it possible to install an OS such as windows or ubuntu on a PCI-Express SSD (solid state drive)? And if so is it as straight forward as selecting that drive on install? Thanks Tom

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• Replace sound in another YouTube video

  - by Tom

  I have received permission from someone to translate the audio in their movies. The problem I am facing is that the video quality is quite poor and the author does not have the original videos any more. How can I replace the audio in the YouTube videos without further degrading the quality of the videos? Thanks, Tom

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• Setting Manager path in Tomcat6

  - by Tom

  Hi gurus I want to switch context path to Manager app in Tomcat6 http://tomcat.apache.org/tomcat-6.0-doc/manager-howto.html I change $CATALINA_BASE/conf/[enginename]/[hostname]/manager.xml to: <Context path="/adm" docBase="${catalina.home}/webapps/manager" privileged="true" antiResourceLocking="false" antiJARLocking="false"> notice to: path="/adm", but manager app is always in /manager. Please, how can I change manager path in Tomcat6? Thanks a lot. Tom

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• How can I disable the css {position:fixed} side-bar on Slashdot?

  - by Tom

  When I look at a story on Slashdot, I see a side-bar that sits constantly in the upper-left corner. Every time I open a story, I have to click on the "slash" sign in its upper-right corner. How can I disable it, so it will always have css position absolute, relative or static? Is there an option for it? Because I find it is slowing my browser down when I am scrolling the page. Thank you, Tom

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