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

On Thu, May 23, 2013 at 11:34 AM, Fujii Masao <masao(dot)fujii(at)gmail(dot)com> wrote:
> On Thu, May 23, 2013 at 8:55 PM, Robert Haas <robertmhaas(at)gmail(dot)com> 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.
>
> Yes in asynchronous replication case. But in synchronous replication case,
> after WAL forks, hint bits would not be set if their corresponding commit record
> is not replicated to the standby. The transaction commit keeps waiting
> for the reply
> from the standby before updating clog. So, this data corruption would not happen
> in sync case.

Not necessarily. SyncRepWaitForLSN() can be interrupted via a cancel signal.

--
Robert Haas
EnterpriseDB: http://www.enterprisedb.com
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