Horton

Re: Hot Backup with rsync fails at pg_clog if under load

Simon Riggs <simon@2ndquadrant.com>

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From: Simon Riggs <simon@2ndQuadrant.com>
To: Aidan Van Dyk <aidan@highrise.ca>
Cc: Florian Pflug <fgp@phlo.org>, Daniel Farina <daniel@heroku.com>, Chris Redekop <chris@replicon.com>, PostgreSQL-development <pgsql-hackers@postgresql.org>
Date: 2011-10-26T13:12:27Z
Lists: pgsql-hackers

Commits

Same data as JSON: GET /api/v1/messages/:b64id/commits the thread's linked commits as JSON, with link sources. API reference →

  1. Derive oldestActiveXid at correct time for Hot Standby.

  2. Start Hot Standby faster when initial snapshot is incomplete.

  3. Fix timing of Startup CLOG and MultiXact during Hot Standby

On Wed, Oct 26, 2011 at 12:54 PM, Aidan Van Dyk <aidan@highrise.ca> wrote:

> The read fails because their is no data at the location it's trying to
> read from, because clog hasn't been extended yet by recovery.

You don't actually know that, though I agree it seems a reasonable
guess and was my first thought also.

The error is very specifically referring to 22811359, which is the
nextxid from pg_control and updated by checkpoint.

22811359 is mid-way through a clog page, so prior xids will already
have been allocated, pages extended and then those pages fsyncd before
the end of pg_start_backup().  So it shouldn't be possible for that
page to be absent from the base backup, unless the base backup was
taken without a preceding checkpoint, which seems is not the case from
the script output.

Note that if you are correct, then the solution is to extend clog,
which Florian disagrees with as a solution.

-- 
 Simon Riggs                   http://www.2ndQuadrant.com/
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