Re: [GENERAL] Can we Flush the Postgres Shared Memory ?

2011/5/3 Merlin Moncure <mmoncure(at)gmail(dot)com>:
> On Tue, May 3, 2011 at 8:55 AM, Cédric Villemain
> <cedric(dot)villemain(dot)debian(at)gmail(dot)com> wrote:
>> 2011/5/3 Merlin Moncure <mmoncure(at)gmail(dot)com>:
>>>
>>> no it will not, or at least there is no guarantee it will be.  the
>>> only way to reset the buffers in that sense is to restart the database
>>> (and even then they might not be read from disk, because they could
>>> sit in the o/s cache).  to force a read from the drive you'd have to
>>> reboot the server, or at least shut it down and use a lot of memory
>>> for some other purpose.
>>
>> with linux, you can : "echo 3 > /proc/sys/vm/drop_caches" for the OS cache
>>
>
> yeah -- good point.  aside: does that also drop cache on the drive/raid card?

no -- good point too ! (damn! how SAN users will do...maybe EMC or
other are good enough to provide some control panel for that ? )

and as I read on the link provided by Tomas, it is better to issue a
'sync' before trying to drop cache (I do that sometime, but postgresql
flush its write before shutdown, so I expected the dirty pages in OS
cache not to be relative to postgresql files.)

--
Cédric Villemain               2ndQuadrant
http://2ndQuadrant.fr/     PostgreSQL : Expertise, Formation et Support