Re: [ADMIN] Can we Flush the Postgres Shared Memory ? - Mailing list pgsql-general

Is there a particular one of Oracle's memory clearning features you want to use in PostgreSQL?   In Oracle you cannot flush the entire SGA without a restart, but you can flush three parts of the SGA using three separate commands.

1.  In Oracle you can flush the redo buffer by issuing a COMMIT or by rotating the logs.  You can force a log switch in PostgreSQL using "select pg_switch_xlog();".

2.  In Oracle when you flush the shared pool this does three things: (a) removes sql and pl/sql statements from the shared library cache, (b) flushes the dictionary cache of object info and security data, and (c) flushes the query result cache (11g only).  I am relatively new to PostgreSQL and have not seen an equivalent in PostgreSQL to these things.  Based on other replies it does not seem possible to flush the catalog cache in PostgreSQL.

3.  In Oracle when you request a flush of the buffer cache it signals a checkpoint to ensure all dirty buffers are written out AND later it will remove the dirty buffers from memory.  This can take anywhere from a few seconds on very small systems to several minutes on VLDB systems, per my observations.  The Oracle checkpoint is fast, and the SQL prompt comes back very fast, but the removal of dirty buffers from memory runs in the background with a low priority over a long period of time.  If you are planning to use "alter system flush buffer_cache" to clear memory in between tests you actually have no way to know when memory is clear except to wait a long time and then assume all is well (yes, this is also true with ASM and direct i/o to raw devices).  In PostgreSQL, you can checkpoint manually to signal bgwriter to flush dirty pages to the operating system's cache and from there you will see a lazy write to disk (e.g., watch pdflush on linux), so immediately re-running a query will still get some caching benefits eventhough the checkpoint is complete.  There are operating system commands that you could use for that ("cat /proc/meminfo" to see what's there, "sync" to write dirty pages to disk, then "echo 3 > /proc/sys/vm/drop_caches" to remove the now clean pages, and then "cat /proc/meminfo" one more time).  And, if you are using SAN consider array based caching as well.

Sincerely,

Mark R. Johnson
Owner, Remington Database Solutions, LLC
Author, Oracle Database 10g: From Nuts to Soup 



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-----Original Message-----
From: Raghavendra [mailto:raghavendra.rao@enterprisedb.com]
Sent: Tuesday, May 3, 2011 08:31 AM
To: 'Simon Riggs'
Cc: 'raghu ram', 'pgsql-admin', 'pgsql-general', 'pgsql novice'
Subject: Re: [ADMIN] Can we Flush the Postgres Shared Memory ?

On Tue, May 3, 2011 at 5:37 PM, Simon Riggs <simon@2ndquadrant.com> wrote:

On Tue, May 3, 2011 at 11:54 AM, raghu ram <raghuchennuru@gmail.com> wrote:

> It may be a silly question, still out of curiosity I want to know, is there
> any possible way to flush the Postgres Shared Memory without restarting the
> cluster.
> In Oracle, we can flush the SGA, can we get the same feature here..
> Thanks in Advance.

The CHECKPOINT command will do this for you.

This command will empty the PSM...

Best Regards,

Raghavendra

EnterpriseDB Corporation

Blog: http://raghavt.blogspot.com/