Re: Online base backup from the hot-standby

2011/8/17 Jun Ishiduka <ishizuka(dot)jun(at)po(dot)ntts(dot)co(dot)jp>:
>> I see in xlog.h XLR_BKP_REMOVABLE, the comment above it says that this
>> flag is used to indicate that the archiver can compress the full page
>> blocks to non-full page blocks. I am not familiar with where in the code
>> this actually happens but will this cause issues if the first standby is
>> processing WAL files from the archive?
>
> I confirmed the flag in xlog.c, so I seemed to only insert it in
> XLogInsert(). I consider whether it is available.

That flag is not available to check whether full-page writing was
skipped or not.
Because it's in full-page data, not non-full-page one.

The straightforward approach to address the problem you raised is to log
the change of full_page_writes on the master. Since such a WAL record is also
replicated to the standby, the standby can know whether full_page_writes is
enabled or not in the master, from the WAL record. If it's disabled,
pg_start_backup() in the standby should emit an error and refuse standby-only
backup. If the WAL record indicating that full_page_writes was disabled
on the master arrives during standby-only backup, the standby should cancel
the backup.

Regards,

--
Fujii Masao
NIPPON TELEGRAPH AND TELEPHONE CORPORATION
NTT Open Source Software Center