Re: A few questions to real pgsql gurus

From: Larry Rosenman <ler(at)lerctr(dot)org>
To: Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>, gearond(at)cvc(dot)net
Cc: Dmitry Tkach <dmitry(at)openratings(dot)com>, mike McGire <mmcgire(at)hotmail(dot)com>, pgsql-general(at)postgresql(dot)org
Subject: Re: A few questions to real pgsql gurus
Date: 2003-02-24 18:10:52
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--On Monday, February 24, 2003 12:28:03 -0500 Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>

> Dennis Gearon <gearond(at)cvc(dot)net> writes:
>> And what about all the stuff in memory, pending transactions, etc?
> If he's got an accurate snapshot of the WAL files, everything the
> database has claimed to be committed will be committed. It's the same
> scenario as a crash.
> The trouble with most "I'll just take a tar dump/rsync copy/whatever
> of the $PGDATA directory" backup solutions is that you don't get a copy
> of the WAL files that's consistent with what you copy out of the other
> subdirectories --- and any time skew there will burn you. See recent
> reports of duplicate rows for an example. But if NetApp can implement
> an instantaneous copy (and offhand it seems that they should be able to,
> since they sit between PG and the disk) then it should work.
> I'd still recommend some tests before trusting it ;-)
the netapp facility is called SnapShot.

What happens is that the original blocks of the file are write protected,
and any changes are put elsewhere. (I don't recall whether it's the new or
the old blocks that get saved, but it doesn't matter, the effect is the

The snapshot stuff in Veritas' VxFS is very similar.

It makes a consistent view of the filesystem available for backup.


> regards, tom lane
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Larry Rosenman
Phone: +1 972-414-9812 E-Mail: ler(at)lerctr(dot)org
US Mail: 1905 Steamboat Springs Drive, Garland, TX 75044-6749

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