Re: pg_rewind, a tool for resynchronizing an old master after failover

On 23.05.2013 07:55, Robert Haas wrote:
> On Thu, May 23, 2013 at 7:10 AM, Heikki Linnakangas
> <hlinnakangas(at)vmware(dot)com> wrote:
>> 1. Scan the WAL log of the old cluster, starting from the point where
>> the new cluster's timeline history forked off from the old cluster. For each
>> WAL record, make a note of the data blocks that are touched. This yields a
>> list of all the data blocks that were changed in the old cluster, after the
>> new cluster forked off.
>
> Suppose that a transaction is open and has written tuples at the point
> where WAL forks. After WAL forks, the transaction commits. Then, it
> hints some of the tuples that it wrote. There is no record in WAL
> that those blocks are changed, but failing to revert them leads to
> data corruption.

Bummer, you're right. Hmm, if you have checksums enabled, however, we'll
WAL log a full-page every time a page is dirtied for setting a hint bit,
which fixes the problem. So, there's a caveat with pg_rewind; you must
have checksums enabled.

- Heikki