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

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/
 PostgreSQL Development, 24x7 Support, Training & Services