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Re: Enabling Checksums

From: Simon Riggs <simon(at)2ndQuadrant(dot)com>
To: Jeff Davis <pgsql(at)j-davis(dot)com>
Cc: Robert Haas <robertmhaas(at)gmail(dot)com>, Pavel Stehule <pavel(dot)stehule(at)gmail(dot)com>, Josh Berkus <josh(at)agliodbs(dot)com>, pgsql-hackers(at)postgresql(dot)org
Subject: Re: Enabling Checksums
Date: 2013-01-10 09:10:33
Message-ID: CA+U5nM+HeZ5u4Rs2w1RKjFJdif4uprY3WmLqhBNCyusoSdAu3w@mail.gmail.com (view raw or flat)
Thread:
Lists: pgsql-hackers
On 10 January 2013 06:06, Jeff Davis <pgsql(at)j-davis(dot)com> wrote:

> The checksums patch also introduces another behavior into
> SetBufferCommitInfoNeedsSave, which is to write an XLOG_HINT WAL record
> if checksums are enabled (to avoid torn page hazards). That's only
> necessary for changes where the caller does not write WAL itself and
> doesn't bump the LSN of the data page. (There's a reason the caller
> can't easily write the XLOG_HINT WAL itself.) So, we could introduce
> another flag "needsWAL" that would control whether we write the
> XLOG_HINT WAL or not (only applies with checksums on, of course).

That wouldn't work because it can't know the exact answer to that, but
the way the patch does this is already correct.

XLOG_HINT_WAL doesn't always write a WAL record, it only does it when
necessary. See XLogInsert()

Didn't fully understand other comments. Do we we need an answer now?
My head is somewhere else.

-- 
 Simon Riggs                   http://www.2ndQuadrant.com/
 PostgreSQL Development, 24x7 Support, Training & Services


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