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

Tom Lane wrote:
> Hannu Krosing <hannu@skype.net> writes:
> > Ühel kenal päeval, R, 2006-04-14 kell 16:40, kirjutas Tom Lane:
> >> If the backup-taker reads, say, 4K at a time then it's
> >> certainly possible that it gets a later version of the second half of a
> >> page than it got of the first half.  I don't know about you, but I sure
> >> don't feel comfortable making assumptions at that level about the
> >> behavior of tar or cpio.
> >> 
> >> I fear we still have to disable full_page_writes (force it ON) if
> >> XLogArchivingActive is on.  Comments?
> 
> > Why not just tell the backup-taker to take backups using 8K pages ? 
> 
> How?  (No, I don't think tar's blocksize options control this
> necessarily --- those indicate the blocking factor on the *tape*.
> And not everyone uses tar anyway.)
> 
> Even if this would work for all popular backup programs, it seems
> far too fragile: the consequence of forgetting the switch would be
> silent data corruption, which you might not notice until the slave
> had been in live operation for some time.

Yea, it is a problem.  Even a 10k read is going to read 2k into the next
page.

I am thinking we should throw an error on pg_start_backup() and
pg_stop_backup if full_page_writes is off.  Seems archive_command and
full_page_writes can still be used if we are not in the process of doing
a file system backup.

In fact, could we have pg_start_backup() turn on full_page_writes and
have pg_stop_backup turn it off, if postgresql.conf has it off.

--  Bruce Momjian   http://candle.pha.pa.us EnterpriseDB    http://www.enterprisedb.com
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