A while ago I wrote a script based on Dave Plonka work
http://net.doit.wisc.edu/~plonka/fincore/
My script monitors system buffers and shared buffers
(if pg_buffercache installed) and I found it's almost useless to
check system buffers, since I got rather ridiculous numbers.
I use it to investigate OS cacheing of PostgreSQL files and was
surprized on 24 Gb server, total cache was about 30 Gb. How this is
possible ?
I can send script and perl module if you want to play with.
Oleg
On Mon, 29 Sep 2008, Greg Smith wrote:
On Mon, 29 Sep 2008, Matthew Pulis wrote:
I need to perform some timed testing, thus need to make sure that disk
cache
does not affect me. Is clearing the OS (Ubuntu) disk cache, ( by running:
sudo echo 3 | sudo tee /proc/sys/vm/drop_caches ) enough to do this?
What you should do is:
1) Shutdown the database server (pg_ctl, sudo service postgresql stop, etc.)
2) sync
3) sudo echo 3 > /proc/sys/vm/drop_caches
4) Start the database server
That will clear both the database and OS cache with a minimum of junk left
behind in the process; clearing the cache without a sync is a bad idea.
Note that all of this will still leave behind whatever cache is in your disk
controller card or on the disk themselves available. There are some other
techniques you could consider. Add a setp 2.5 that generates a bunch of data
unused by the test, then sync again, and you've turned most of that into
useless caching.
Ideally, your test should be running against a large enough data set that the
dozens or couple of hundred megabytes that might be in those will only add a
bit of noise to whatever you're testing. If you're not running a larger test
or going through tasts to make the caches clear, the only easy way to make
things more clear is to reboot the whole server.
--
* Greg Smith [EMAIL PROTECTED] http://www.gregsmith.com Baltimore, MD
Regards,
Oleg
_____________________________________________________________
Oleg Bartunov, Research Scientist, Head of AstroNet (www.astronet.ru),
Sternberg Astronomical Institute, Moscow University, Russia
Internet: [EMAIL PROTECTED], http://www.sai.msu.su/~megera/
phone: +007(495)939-16-83, +007(495)939-23-83
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