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Re: PG as in-memory db? How to warm up and re-populate buffers? How to read in all tuples into memory?

From: Dave Crooke <dcrooke(at)gmail(dot)com>
To: Scott Marlowe <scott(dot)marlowe(at)gmail(dot)com>
Cc: Claudio Freire <klaussfreire(at)gmail(dot)com>, Stefan Keller <sfkeller(at)gmail(dot)com>, Jeff Janes <jeff(dot)janes(at)gmail(dot)com>, postgres performance list <pgsql-performance(at)postgresql(dot)org>
Subject: Re: PG as in-memory db? How to warm up and re-populate buffers? How to read in all tuples into memory?
Date: 2012-03-01 17:38:39
Message-ID: CALi4UpgqX2Ge3v6D01CzfPwaK6zRTXpS1zs9_b=js08W4ou6Lg@mail.gmail.com (view raw or flat)
Thread:
Lists: pgsql-performance
Just curious ... has anyone tried using a ram disk as the PG primary and
DRBD as the means to make it persistent?
On Mar 1, 2012 11:35 AM, "Scott Marlowe" <scott(dot)marlowe(at)gmail(dot)com> wrote:

> On Thu, Mar 1, 2012 at 9:57 AM, Jeff Janes <jeff(dot)janes(at)gmail(dot)com> wrote:
> > On Tue, Feb 28, 2012 at 3:46 PM, Stefan Keller <sfkeller(at)gmail(dot)com>
> wrote:
> >> 2012/2/28 Claudio Freire <klaussfreire(at)gmail(dot)com>:
> >>>
> >>> In the OP, you say "There is enough main memory to hold all table
> >>> contents.". I'm assuming, there you refer to your current system, with
> >>> 4GB memory.
> >>
> >> Sorry for the confusion: I'm doing these tests on this machine with
> >> one table (osm_point) and one country. This table has a size of 2.6GB
> >> and 10 million tuples. The other machine has to deal with at least 5
> >> tables in total and will be hold more than one country plus routing
> >> etc..
> >
> > What is your shared_buffers set to?  2.6GB is uncomfortably close to
> > 4GB, considering the computer has other things it needs to use memory
> > for as well.
>
> The real danger here is that the kernel will happily swap ut
> shared_buffers memory to make room to cache more from the hard disks,
> especially if that shared_mem hasn't been touched in a while.  On a
> stock kernel with swappinness of 60 etc, it's quite likely the OP is
> seeing the DB go to get data from shared_buffers, and the OS is
> actually paging in for shared_buffers. At that point reading from
> kernel cache is MUCH faster, and reading from the HDs is still
> probably faster than swapping in shared_buffers.
>
> --
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