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Re: Keeping separate WAL segments for each database

From: Alvaro Herrera <alvherre(at)commandprompt(dot)com>
To: Devrim GÜNDÜZ <devrim(at)gunduz(dot)org>
Cc: PostgreSQL Hackers ML <pgsql-hackers(at)postgresql(dot)org>
Subject: Re: Keeping separate WAL segments for each database
Date: 2010-06-30 21:05:51
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Lists: pgsql-hackers
Excerpts from Devrim GÜNDÜZ's message of mié jun 30 14:54:06 -0400 2010:

> One of the things that interested me was parallel recovery feature. They
> said that they are keeping separate xlogs for each database, which
> speeds ups recovery in case of a crash. It also would increase
> performance, since we could write xlogs to separate disks.

I'm not sure about this. You'd need to have one extra WAL stream, for
shared catalogs; and what would you do to a transaction that touches
both shared catalogs and also local objects?  You'd have to split the
WAL entries in those two WAL streams.

I think you could try to solve this by having yet another WAL stream for
transaction commit, and have the database-specific streams reference
that one.  Operations touching shared catalogs would act as barriers:
all other databases' WAL streams would have to be synchronized to that
one.  This would still allow you to have some concurrency because,
presumably, operations on shared catalogs are rare.

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