Re: How to keep a table in memory?

Hi,

On Mon, 2007-11-12 at 09:12 +0000, Heikki Linnakangas wrote:

> Just leave it to the cache management algorithms in Postgres and
> Linux. If it really is frequently accessed, it should stay in
> Postgres shared buffers.

How is "frequently accessed" determined by PostgreSQL?

I mean... You know, OS caches either inodes, or pages. Page caches are
pretty ignorable, since it means the data is already in virtual memory.
So, we have inode caching, and IIRC it results in i/o requests from the
disk -- and sure, it uses i/o scheduler of the kernel (like the all of
the applications running on that machine -- including a basic login
session). *If* the data hadn't been deleted, it returns from i/o
scheduler.

So there is no 100% guarantee that the table is in the memory. If we
could use the ram (some (or a :) ) database(s) can do that IIRC), we
will avoid i/o scheduler, which will really speed up the process. (Ok,
AFAIK, you can "pin" your objects to memory with Oracle).

... and one more thing with ramfs: Since there is a fs on ramfs, it
passes through VFS -- and goes through kernel schedulers again.

So, IMHO, saying "trust your OS + PostgreSQL" is not a 100% perfect
approach for the people who are asking to keep their objects on RAM,
even though I know that there is nothing we can say right now.

Regards,
--
Devrim GÜNDÜZ , RHCE
PostgreSQL Replication, Consulting, Custom Development, 24x7 support
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