Re: avoiding file system caching of a table

On 02/16/2014 10:33 PM, Gabriel Sánchez Martínez wrote:
> Is there a way of asking PostgreSQL to read the files of a table
> directly off the disk, asking the OS not to use the file cache?  I am
> running PostgreSQL 9.1 on Ubuntu Server 64-bit.  The server in
> question has the maximum amount of RAM it supports, but the database
> has grown much larger.  Most of the time it doesn't matter, because
> only specific tables or parts of indexed tables are queried, and all
> of that fits in the file cache.  But we have a new requirement of
> queries to a table several times larger than the total RAM, and the
> database has slowed down considerably for the other queries.
>
> I am assuming that with every query to the large table, the OS caches
> the files containing the table's data, and since the table is larger
> than total RAM, all the old caches are cleared.  The caches that were
> useful for other smaller tables are lost, and the new caches of the
> large table are useless because on the next query caching will start
> again from the first files of the table. Please point out if there is
> a problem with this assumption.  Note that I am refering to OS file
> caching and not PostgreSQL caching.
>
> Is there a way around this?  I have read that there is a way of asking
> the OS not to cache a file when the file is opened.  Is there a way of
> telling PostgreSQL to use this option when reading files that belong a
> specific table?
>
> What about putting the table on a tablespace that is on a different
> device partition with the sync mount option?  Would that help?

I have read forum postings saying that the sync option affects writes,
and will not prevent reads from caching.  At some forum posting I came
across nocache, a utility for linux.  It is used by typing "nocache
<command>" in a shell.  But I can't do that with a postgres process when
a connection opens because postgres is the one opening the process.
Does someone know a work-around, or a different solution to the
problem?  Shouldn't PostgreSQL be smart about this and based on the
statistics collected for a table and on the query plan know the harm
that will be done if all of a very large table's pages are read and
flush the cache?

>
> All suggestions will be appreciated.
>
> Thanks,
> Gabriel