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
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