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Re: Slow pg_dump

From: "Ryan Wells" <ryan(dot)wells(at)soapware(dot)com>
To: "Tom Lane" <tgl(at)sss(dot)pgh(dot)pa(dot)us>
Cc: <pgsql-admin(at)postgresql(dot)org>
Subject: Re: Slow pg_dump
Date: 2008-04-14 22:35:32
Message-ID: EE6D03C0EF73D14E8034C37CA9B627740288D8@exchange.DOCS.COM (view raw, whole thread or download thread mbox)
Lists: pgsql-admin
The images are stored in whatever format our users load them as, so we
don't have any control over their compression or lack thereof.

I ran pg_dump with the arguments you suggested, and my 4 GB test table
finished backing up in about 25 minutes, which seems great.  The only
problem is that the resulting backup file was over 9 GB.  Using -Z2
resulting in a 55 minute 6GB backup.

Here's my interpretation of those results: the TOAST tables for our
image files are compressed by Postgres.  During the backup, pg_dump
uncompresses them, and if compression is turned on, recompresses the
backup.  Please correct me if I'm wrong there.

If we can't find a workable balance using pg_dump, then it looks like
our next best alternative may be a utility to handle filesystem backups,
which is a little scary for on-site, user-controlled servers.


-----Original Message-----
From: Tom Lane [mailto:tgl(at)sss(dot)pgh(dot)pa(dot)us] 
Sent: Saturday, April 12, 2008 9:46 PM
To: Ryan Wells
Cc: pgsql-admin(at)postgresql(dot)org
Subject: Re: [ADMIN] Slow pg_dump 

"Ryan Wells" <ryan(dot)wells(at)soapware(dot)com> writes:
> We have several tables that are used to store binary data as bytea (in

> this example image files),

Precompressed image formats, no doubt?
> pg_dump -i -h localhost -p 5432 -U postgres -F c -v -f 
> "backupTest.backup" -t "public"."images" db_name

Try it with -Z0, or even drop the -Fc completely, since it's certainly
not very helpful on a single-table dump.  Re-compressing already
compressed data is not only useless but impressively slow ...

Also, drop the -i, that's nothing but a foot-gun.

			regards, tom lane

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