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• Slow Memcached: Average 10ms memcached `get`

  - by Chris W.

  We're using Newrelic to measure our Python/Django application performance. Newrelic is reporting that across our system "Memcached" is taking an average of 12ms to respond to commands. Drilling down into the top dozen or so web views (by # of requests) I can see that some Memcache get take up to 30ms; I can't find a single use of Memcache get that returns in less than 10ms. More details on the system architecture: Currently we have four application servers each of which has a memcached member. All four memcached members participate in a memcache cluster. We're running on a cloud hosting provider and all traffic is running across the "internal" network (via "internal" IPs) When I ping from one application server to another the responses are in ~0.5ms Isn't 10ms a slow response time for Memcached? As far as I understand if you think "Memcache is too slow" then "you're doing it wrong". So am I doing it wrong? Here's the output of the memcache-top command: memcache-top v0.7 (default port: 11211, color: on, refresh: 3 seconds) INSTANCE USAGE HIT % CONN TIME EVICT/s GETS/s SETS/s READ/s WRITE/s cache1:11211 37.1% 62.7% 10 5.3ms 0.0 73 9 3958 84.6K cache2:11211 42.4% 60.8% 11 4.4ms 0.0 46 12 3848 62.2K cache3:11211 37.5% 66.5% 12 4.2ms 0.0 75 17 6056 170.4K AVERAGE: 39.0% 63.3% 11 4.6ms 0.0 64 13 4620 105.7K TOTAL: 0.1GB/ 0.4GB 33 13.9ms 0.0 193 38 13.5K 317.2K (ctrl-c to quit.) ** Here is the output of the top command on one machine: ** (Roughly the same on all cluster machines. As you can see there is very low CPU utilization, because these machines only run memcache.) top - 21:48:56 up 1 day, 4:56, 1 user, load average: 0.01, 0.06, 0.05 Tasks: 70 total, 1 running, 69 sleeping, 0 stopped, 0 zombie Cpu(s): 0.0%us, 0.0%sy, 0.0%ni, 99.7%id, 0.0%wa, 0.0%hi, 0.0%si, 0.3%st Mem: 501392k total, 424940k used, 76452k free, 66416k buffers Swap: 499996k total, 13064k used, 486932k free, 181168k cached PID USER PR NI VIRT RES SHR S %CPU %MEM TIME+ COMMAND 6519 nobody 20 0 384m 74m 880 S 1.0 15.3 18:22.97 memcached 3 root 20 0 0 0 0 S 0.3 0.0 0:38.03 ksoftirqd/0 1 root 20 0 24332 1552 776 S 0.0 0.3 0:00.56 init 2 root 20 0 0 0 0 S 0.0 0.0 0:00.00 kthreadd 4 root 20 0 0 0 0 S 0.0 0.0 0:00.00 kworker/0:0 5 root 20 0 0 0 0 S 0.0 0.0 0:00.02 kworker/u:0 6 root RT 0 0 0 0 S 0.0 0.0 0:00.00 migration/0 7 root RT 0 0 0 0 S 0.0 0.0 0:00.62 watchdog/0 8 root 0 -20 0 0 0 S 0.0 0.0 0:00.00 cpuset 9 root 0 -20 0 0 0 S 0.0 0.0 0:00.00 khelper ...output truncated...

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• Ignoring GET parameters in Varnish VCL

  - by JamesHarrison

  Okay: I've got a site set up which has some APIs we expose to developers, which are in the format /api/item.xml?type_ids=34,35,37&region_ids=1000002,1000003&key=SOMERANDOMALPHANUM In this URI, type_ids is always set, region_ids and key are optional. The important thing to note is that the key variable does not affect the content of the response. It is used for internal tracking of requests so we can identify people who make slow or otherwise unwanted requests. In Varnish, we have a VCL like this: if (req.http.host ~ "the-site-in-question.com") { if (req.url ~ "^/api/.+\.xml") { unset req.http.cookie; } } We just strip cookies out and let the backend do the rest as far as times are concerned (this is a hackaround since Rails/authlogic sends session cookies with API responses). At present though, any distinct developers are basically hitting different caches since &key=SOMEALPHANUM is considered as part of the Varnish hash for storage. This is obviously not a great solution and I'm trying to work out how to tell Varnish to ignore that part of the URI.

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• How to analyze webalizer's dns_cache.db file under windows?

  - by flickaswitch

  Hi guys, I am trying to take a look at the dns_cache.db file that is generated by webalizer when doing DNS lookups to find out why some host files seem not to be resolved resolved properly. Is there any way I can do that under windows (wcmgr does not work there)? Thank you & regards, flickaswitch

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• SQL Service Broker enabled causes 100% CPU

  - by user40373

  I have new set of code for a website that is using SqlCacheDependencies based on sql commands. I have enabled SQL Service Broker and some triggers on update/insert/delete and it is causing 100% CPU. Any ideas if I am doing something wrong or suggestions to improve? Here are the SQLchanges I ran: alter database DATABASE_NAME set enable_broker WITH ROLLBACK IMMEDIATE grant subscribe query notifications to CONNECTION_USER_NAME grant send on service::sqlquerynotificationservice to CONNECTION_USER_NAME ALTER AUTHORIZATION ON DATABASE::DATABASE_NAME TO CONNECTION_USER_NAME;

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• Linux not buffering block I/O when the device is not "in use" (i.e. mounted)

  - by Radek Hladík

  I am installing new server and I've found an interesting issue. The server is running Fedora 19 (3.11.7-200.fc19.x86_64 kernel) and is supposed to host a few KVM/Qemu virtual servers (mail server, file server, etc..). The HW is Intel(R) Xeon(R) CPU 5160 @ 3.00GHz with 16GB RAM. One of the most important features will be Samba server and we have decided to make it as virtual machine with almost direct access to the disks. So the real HDD is cached on SSD (via bcache) then raided with md and the final device is exported into the virtual machine via virtio. The virtual machine is again Fedora 19 with the same kernel. One important topic to find out is whether the virtualization layer will not introduce high overload into disk I/Os. So far I've been able to get up to 180MB/s in VM and up to 220MB/s on real HW (on the SSD disk). I am still not sure why the overhead is so big but it is more than the network can handle so I do not care so much. The interesting thing is that I've found that the disk reads are not buffered in the VM unless I create and mount FS on the disk or I use the disks somehow. Simply put: Lets do dd to read disk for the first time (the /dev/vdd is an old Raptor disk 70MB/s is its real speed): [root@localhost ~]# dd if=/dev/vdd of=/dev/null bs=256k count=10000 ; cat /proc/meminfo | grep Buffers 2621440000 bytes (2.6 GB) copied, 36.8038 s, 71.2 MB/s Buffers: 14444 kB Rereading the data shows that they are cached somewhere but not in buffers of the VM. Also the speed increased to "only" 500MB/s. The VM has 4GB of RAM (more that the test file) [root@localhost ~]# dd if=/dev/vdd of=/dev/null bs=256k count=10000 ; cat /proc/meminfo | grep Buffers 2621440000 bytes (2.6 GB) copied, 5.16016 s, 508 MB/s Buffers: 14444 kB [root@localhost ~]# dd if=/dev/vdd of=/dev/null bs=256k count=10000 ; cat /proc/meminfo | grep Buffers 2621440000 bytes (2.6 GB) copied, 5.05727 s, 518 MB/s Buffers: 14444 kB Now lets mount the FS on /dev/vdd and try the dd again: [root@localhost ~]# mount /dev/vdd /mnt/tmp [root@localhost ~]# dd if=/dev/vdd of=/dev/null bs=256k count=10000 ; cat /proc/meminfo | grep Buffers 2621440000 bytes (2.6 GB) copied, 4.68578 s, 559 MB/s Buffers: 2574592 kB [root@localhost ~]# dd if=/dev/vdd of=/dev/null bs=256k count=10000 ; cat /proc/meminfo | grep Buffers 2621440000 bytes (2.6 GB) copied, 1.50504 s, 1.7 GB/s Buffers: 2574592 kB While the first read was the same, all 2.6GB got buffered and the next read was at 1.7GB/s. And when I unmount the device: [root@localhost ~]# umount /mnt/tmp [root@localhost ~]# cat /proc/meminfo | grep Buffers Buffers: 14452 kB [root@localhost ~]# dd if=/dev/vdd of=/dev/null bs=256k count=10000 ; cat /proc/meminfo | grep Buffers 2621440000 bytes (2.6 GB) copied, 5.10499 s, 514 MB/s Buffers: 14468 kB The bcache was disabled while testing and the results are same on faster (newer) HDDs and on SSD (except for the initial read speed of course). To sum it up. When I read from the device via dd first time, it gets read from the disk. Next time I reread it gets cached in the host but not in the guest (thats actually the same issue, more on that later). When I mount the filesystem but try to read the device directly it gets cached in VM (via buffers). As soon as I stop "using" it, buffers are discarded and the device is not cached anymore in the VM. When I looked into buffers value on the host I realized that the situation is the same. The block I/O gets buffered only when the disk is in use, in this case it means "exported to a VM". On host, after all the measurement done: 3165552 buffers On the host, after the VM shutdown: 119176 buffers I know it is not important as the disks will be mounted all the time but I am curious and I would like to know why it is working like this.

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• Per-machine decentralised DNS caching - nscd/lwresd/etc

  - by Dan Carley

  Preface: We have caching resolvers at each of our geographic network locations. These are clustered for resiliency and their locality reduces the latency of internal requests generated by our servers. This works well. Except that a vast quantity of the requests seen over the wire are lookups for the same records, generated by applications which don't perform any DNS caching of their own. Questions: Is there a significant benefit to running lightweight caching daemons on the individual servers in order to reduce repeated requests from hitting the network? Does anyone have experience of using [u]nscd, lwresd or dnscache to do such a thing? Are there any other packages worth looking at? Any caveats to beware of? Besides the obvious, caching and negative caching stale results.

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• postfix concurrency limit with round robin dns

  - by goose

  Take the following internal round robin dns setup mymta.com. IN A 172.31.1.1 mymta.com. IN A 172.31.1.2 mymta.com. IN A 172.31.1.3 mymta.com. IN A 172.31.1.4 mymta.com. IN A 172.31.1.5 mymta.com. IN A 172.31.1.6 mymta.com. IN A 172.31.1.7 mymta.com. IN A 172.31.1.8 mymta.com. IN A 172.31.1.9 mymta.com. IN A 172.31.1.10 Now assume the following postfix setup (assume these are the only tweaks from defaults in debian package) main.cf: smtp_connection_cache_destinations = mymta.com smtp_connection_cache_reuse_limit = 750 smtp_destination_concurrency_limit = 75 transport * :[mymta.com] I would expect 75 concurrent connections spread across the 10 A records I've set in DNS. However I'm seeing more than a few hundred connections to mymta.com and I'm wondering if Postfix is "smart" enough to set up 75 concurrent connections for each IP address. Thoughts?

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• IIS seems to be caching files on a system share?

  - by scott novell

  Switching over to windows 2008 and IIS 7.5 and it seems whenever I make a change to a css file on a system share it does not show through the browser for a few mins. It is shown through the browser using an ISAPI filter. I have turned off output caching in IIS and also turned off caching on the share itself. The browser is not caching either forcing a 200 and it is cached. Any ideas

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• Squid closing the connection on long HTTP GET requests

  - by Rhys

  Hello, When running a database query on a specific external site we use, Squid seems to cut off the connection after a consistent period of time (just over a minute). The query is submitted through a standard web form is that uses GET to query their database. Firefox 3 just displays a blank page. Internet Explorer throws a 'Page Cannot Be Displayed' error (tested in v6 and v8). When we perform the same query on the same machine, but bypass the Squid proxy, it works fine. The query takes about two and a half minutes to complete. There are a few timeout settings in Squid, but I honestly don't know what one to be looking at. Any possible solutions would be much appreciated. Cheers

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• If-Modified-Since vs If-None-Match

  - by Roger

  This question is based on this article response header HTTP/1.1 200 OK Last-Modified: Tue, 12 Dec 2006 03:03:59 GMT ETag: "10c24bc-4ab-457e1c1f" Content-Length: 12195 request header GET /i/yahoo.gif HTTP/1.1 Host: us.yimg.com If-Modified-Since: Tue, 12 Dec 2006 03:03:59 GMT If-None-Match: "10c24bc-4ab-457e1c1f" HTTP/1.1 304 Not Modified In this case browser is sending both If-None-Match and If-Modified-Since. My question is on the server side do I need to match BOTH etag and If-Modified-Since before I send 304. Or Should I just look at etag and send 304 if etag is a match. In this case I am ignoring If-Modified-Since .

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• Dual boot new laptop win 7 / ubuntu 12.04 - 750gb + 32gb SSD

  - by Alex Waters

  I have just purchased a new HP dv7t-7000 and I would like to run Windows 7 / Ubuntu. How do I setup the dual boot? Can I install both operating systems with an 8gb USB drive? Can I still make use of the 32gb SSD? I'm unfamiliar with the efficacy of using an SSD for caching with a 750gb 7200rpm sata 3 drive. I can only see using it for windows 7 - which I have installed in order to play games. Thank you!

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• How to configure squid for retrieving (and caching) directly my static resources?

  - by fabien7474

  I have an Apache/Tomcat/Spring tc Server running on CentOS EC2 VM. I would like to install squid on the same machine as a proxy for retrieving (directly i.e. without forwarding the request to Apache/Tomcat) and caching static content ONLY identified by URIs : /images, /css or /js. Other URIs should be forwarded to the normal Web Server and not cached. Since I am a newbie, I didn't find from squid documentation how to configure squid for this desired behavior (and if it is even possible). Could you please help me and tell me how should I configure squid for this purpose? Thank you.

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• Varnish + Tomcat vs Apache + mod_jk + Tomcat

  - by Adrian Ber

  Does anyone have some comparison data in terms of performance for using in front of Tomcat either Varnish or Apache with mod_jk. I know that AJP connector suppose to be faster than HTTP, but I was thinking that in combination Varnish which is lighter and highly optimized could perform better. There is also the discussion between static resources (which I think will perform faster with Varnish than Apache, even with mod_cache) and dynamic pages.

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• Should I disable write caching on my Windows 2008 VM?

  - by javano

  I have a Windows Server 2008 x64 Standard virtual machine that runs on a machine with a hardware RAID controller, a Perc 6/i, which has a battery on-board. Doing everything I can for additional performance, I think I should disable this. Is this very dangerous though? My understand is that Battery Backed Write Caching gives a performance boost to the host OS, telling it the write was complete when they are still sitting in flash waiting to be written. However, I can't see how it would be detrimental to performance, but is there a gain (even if marginal) to enabling it / disabling it? P.s. There machine has a backup power. Here is a screen shot for clarification:

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• Weird caching bug where old version of the same web page (same filename) is still called (Windows 2008 R2, Tomcat 5.5)

  - by user717236

  This is definitely one of the strangest errors I've seen and it occurs intermittently. I am running Windows 2008 R2, IIS 7.5, and Apache Tomcat 5.5, by the way. Let's say I have two machines, A and B. Both A and B are running Windows 2008 R2. I have a web page called login.jsp on machine A, and I have a newer, modified version of login.jsp on machine B . Now, I copy the new login.jsp from machine B and paste it to machine A, replacing the older version with the same filename. For whatever reason, when I hit up the web page in my browser from a local machine (i.e. my laptop), it still recalls the old version of the web page, even though it's been replaced! I tried restarting IIS and Apache Tomcat. That didn't work. I tried restarting machine A and that didn't work. I tried a cold reboot of my local machine and that didn't work, either. So, I spoke to someone I can confide in for help. He said to open the login.jsp page in notepad, put a space in, save the file, and try again. Sure enough, it worked. He said he hasn't seen it in Windows 2003, but this is occurring with Windows 2008. What I don't understand is why did it work and what the heck is this error and I do I really diagnose it and resolve it for good, instead of the hack my colleague proposed? Is this bug related to Windows 2008, Windows 2008 R2, Tomcat, or something else entirely? Anyone else have the same problem? Thank you for any help.

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• Record browsing history

  - by nc3b

  How can I record everything I browse so that, ideally, it might later enable me to re-surf the same pages without internet access ? For instance, if I go to http://www.example.com/example.html I would like to be able to view the same page later exactly as initially (but without reconnecting to www.example.com). Thank you in advance.

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• Windows, why 8 GB of RAM feel like a few MB?

  - by Desmond Hume

  I'm on Windows 7 x64 with 4-core Intel i7 and 8 GB of RAM, but lately it feels like my computer's "RAM" is located solely on the hard drive. Here is what the task manager shows: The total amount of memory used by the processes in the list is just about 1 GB. And what is happening on my computer for a few days now is that one program (Cataloger.exe) is continually processing large quantities of (rather big) files, repeatedly opening and reading them for the purposes of cataloging. But it doesn't grow too much in memory and stays about that size, about 90 MB. However, the amount of data it processes in, say, 30 minutes can be measured in gigabytes. So my guess was that Windows file caching has something to do with it. And after some research on the topic, I came across this program, called RamMap, that displays detailed info on a computer's RAM. Here is the screenshot: So to me it looks like Windows keeps in RAM huge amounts of data that is no longer needed, redirecting any RAM allocation requests to the pagefile on the hard drive. Even when I close Cataloger.exe, the RamMap reports the size of the mapped file as about the same for a long time on. And it's not just this particular program. Earlier I noticed that similar slowdown occurred after some massive file operations with other programs. So it's really not an exception. Whatever it is, it slows down the computer by like 50 times. Opening a new tab in Chrome takes 20-30 seconds, opening a new program can take up to a minute. Due to the slowdown, some programs even crash. So what do you think, is the problem hiding in file caching or somewhere else? How do I solve it?

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• Torrent caching

  - by TiFFolk

  Hi friends! I am looking for torrent caching software (like SQUID for http). Do you know any?

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• nginx status code 200 and 304

  - by Chamnap

  I'm using nginx + passenger. I'm trying to understand the nginx response 200 and 304. What does this both means? Sometimes, it responses back in 304 and others only 200. Reading the YUI blog, it seems browser needs the header "Last-Modified" to verify with the server. I'm wondering why the browser need to verify the last modified date. Here is my nginx configuration: location / { root /var/www/placexpert/public; # <--- be sure to point to 'public'! passenger_enabled on; rack_env development; passenger_use_global_queue on; if ($request_filename ~* ^.+\.(jpg|jpeg|gif|png|ico|css|js|swf)$) { expires max; break; } } How would I add the header "Last-Modified" to the static files? Which value should I set?

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• How do i restart my linux server in every 2 days via crontab?

  - by Barkat Ullah

  I have a Linux server containing the os version below: Linux 2.6.32-220.7.1.el6.x86_64 I want to restart it in every 2 days, please help me, I want to do it via crontab. Another help, I used a code below to drop my memory caches in every hour. 0 * * * * /root/clearcache.sh #!/bin/sh sync; echo 3 > /proc/sys/vm/drop_caches But 1st 15 in every hour my server remain so slow after cleaning the caches. My sites do not load during every hour in 1st 15 minutes. In another way if I restart my server then also caches are removed. So I decided to restart my server in every 2 days to drop my caches. Will it be helpful to restart? Or is there any other way to drop my memory caches that will not down my server?

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• Increase memory to memcached

  - by Petrus

  I need to increase the memory size for memcached. I have done this before, but I cannot remember all the steps that I took. If I remember correctly, I downloaded /etc/sysconfig/memcached and changed CACHESIZE=64 to CACHESIZE=1024. However I am not sure if that is how it is supposed to be done. Anyone that could guide me into how I do this? Also a command that confirms the change would be useful. I am running RedHat x86_64 es5.

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• Use Cherokee Instead of nginx in Front of Varnish to Get HTTP 1.1 Optimizations?

  - by espeed

  We have been running nginx - uWSGI, and now we are evaluating putting Varnish as a caching layer between nginx and uWSGI (similar to http://www.heroku.com/how/architecture). But, nginx only supports HTTP 1.0 on the back so it will have to create new connections with Varnish for each request. Many recommend running nginx in front of Varnish, but wouldn't it make much more sense to use something like Cherokee so that you eliminate the HTTP connection overhead since it supports HTTP 1.1 on the back?

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• Nginx location to match query parameters

  - by Dave

  Is it possible in nginx to have a location {} block that matches query parameters. For example I want to pick up that "preview=true" in this url and then instruct it to do several different things, all possible in a location block. http://192.158.0.1/web/test.php?hello=test&preview=true&another=var The problem I'm having is that my test stuff doesn't seem to match, it seems like I can only match the URL itself? E.g. location ~ ^(.*)(preview)(.*)$ Or something aloong those lines?

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• VCL - configuration for Magento and Varnish 3.0.2

  - by Tomas

  I would like to kindly ask if there's someone who can help me configure Varnish for Magento to reach far more hits. My current ratio from varnishstat is: cache_hit=271 cache_miss=926 I'm kindly asking this because I've googled almost every site related to this theme, but 99.9% of configurations don't work because of outdated code. Details of my set-up: I use Varnish on port 80, Apache on port 81, PageCache as Magento varnish module, APC for PHP speed and Memcached for dynamic caching. Load speed is about 1.5s on home-page (Pingdom.com average results) USA ping & 2.5s Europe. Servers are located in Toronto, Canada. EDIT: This is my full VCL configuration http://pastebin.com/885BzHCs (I just use xxx.xxx.xxx.xxx for my IPs) This is the info from the command (varnishtop -i TxHeader -I Cookie): TxHeader Cookie: frontend=965b5...(*lots of numbers); adminhtml=3ae65...(*lots of numbers); EXTERNAL_NO_CACHE=1 "(*lots of numbers)" is just my adding to the info Any idea how to avoid Varnish hitting this cookies? (If I got correctly the idea about avoiding Vanrish hitting the cookie and not caching the home page). Thank you for any help!

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• To what extent is size a factor in SSD performance?

  - by artif

  To what extent is the size of an SSD a factor in its performance? In my mind, correct me if I'm wrong, a bigger SSD should be, everything else being equal, faster than a smaller one. A bigger SSD would have more erase blocks and thus more leeway for the FTL (flash translation layer) to do garbage collection optimization. Also there would be more time before TRIM became necessary. I see on Wikipedia that it remarks that "The performance of the SSD can scale with the number of parallel NAND flash chips used in the device" so it seems throughput also increases significantly. Also many SSDs contain internal caches of some sort and presumably those caches are larger for correspondingly large SSDs. But supposing this effect exists, I would like a quantitative analysis. Does throughput increase linearly? How much is garbage collection impacted, if at all? Does latency stay the same? And so on. Would the performance of a 8 GB SSD be significantly different from, for example, an 80 GB SSD assuming both used high quality chips, controllers, etc? Are there any resources (webpages, research papers, presentations, books, etc) that discuss correlations between SSD performance (4 KB random write speed, latency, maximum sequential throughput, etc) and size? I realize this does not really sound like a programming question but it is relevant for what I'm working on (using flash for caching hard drive data) which does involve programming. If there is a better place to ask this question, eg a more hardware oriented site, what would that be? Something like the equivalent of stack overflow (or perhaps a forum) for in-depth questions on hardware interfaces, internals, etc would be appreciated.

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