Message-ID: <524B22C7.2010300@pasteleros.org.ar>
Date: Tue, 01 Oct 2013 16:30:15 -0300
From: Alejandro Brust <alejandrob@pasteleros.org.ar>
Reply-To: alejandrob@pasteleros.org.ar
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To: pgsql-admin@postgresql.org
Subject: Re: PostgreSQL 9.2 - pg_dump out of memory when backuping
 a database with 300000000 large objects
References: <524A90D1.5000109@iqbuzz.ru>
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Did U perform  any vacuumdb / reindexdb before the Pg_dump?


El 01/10/2013 09:49, Magnus Hagander escribió:
> On Tue, Oct 1, 2013 at 11:07 AM, Sergey Klochkov <klochkov@iqbuzz.ru> wrote:
>> Hello All,
>>
>> While trying to backup a database of relatively modest size (160 Gb) I ran
>> into the following issue:
>>
>> When I run
>> $ pg_dump -f /path/to/mydb.dmp -C -Z 9 mydb
>>
>> File /path/to/mydb.dmp does not appear (yes, I've checked permissions and so
>> on). pg_dump just begins to consume memory until it eats up all avaliable
>> RAM (96 Gb total on server, >64 Gb available) and is killed by the oom
>> killer.
>>
>> According to pg_stat_activity, pg_dump runs the following query
>>
>> SELECT oid, (SELECT rolname FROM pg_catalog.pg_roles WHERE oid = lomowner)
>> AS rolname, lomacl FROM pg_largeobject_metadata
>>
>> until it is killed.
>>
>> strace shows that pg_dump is constantly reading a large amount of data from
>> a UNIX socket. I suspect that it is the result of the above query.
>>
>> There are >300000000 large objects in the database. Please don't ask me why.
>>
>> I tried googling on this, and found mentions of pg_dump being killed by oom
>> killer, but I failed to find anything related to the huge large objects
>> number.
>>
>> Is there any method of working around this issue?
> I think this problem comes from the fact that pg_dump treats each
> large object as it's own item. See getBlobs() which allocates a
> BlobInfo struct for each LO (and a DumpableObject if there are any,
> but that's just one).
>
> I assume the query (from that file):
> SELECT oid, lomacl FROM pg_largeobject_metadata
>
> returns 300000000 rows, which are then looped over?
>
> I ran into a similar issue a few years ago with a client using a
> 32-bit version of pg_dump, and got it worked around by moving to
> 64-bit. Did unfortunately not have time to look at the underlying
> issue.
>
>


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