Re: Is full_page_writes=off safe in conjunction with PITR?

Bruce Momjian <pgman@candle.pha.pa.us> writes:
> Tom Lane wrote:
>> If we were to do this, I'd want some more-bulletproof mechanism for
>> forcing full_page_writes on during the backup.  We could probably
>> keep a "backup in progress" flag in shared memory, and examine that
>> along with the GUC variable before deciding to omit a full-page write.

> Yes, good point.  The setting has to be seen by all backends at the same
> time, so yea, a shared memory variable seems required.

I've applied a patch for this.  On reflection, the CHECKPOINT during
pg_start_backup was actually necessary for torn-page safety even without
full_page_writes off.  The reason is that the torn-page risk occurs when
we write a page from shared memory, not when we modify it in memory.
Without a CHECKPOINT, a page modified just before pg_start_backup could
be dumped during the backup and then be saved in a torn state, even
though no WAL record for it is emitted anytime during the backup
procedure.  So that comment's been wrong all along.
        regards, tom lane