Re: Online base backup from the hot-standby

On Wed, Aug 17, 2011 at 6:19 AM, Fujii Masao <masao(dot)fujii(at)gmail(dot)com> wrote:
> 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.

Seems like something we could add to XLOG_PARAMETER_CHANGE fairly easily.

--
Robert Haas
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