Re: Online base backup from the hot-standby

On Wed, Sep 21, 2011 at 5:34 PM, Magnus Hagander <magnus(at)hagander(dot)net> wrote:
> On Wed, Sep 21, 2011 at 08:23, Fujii Masao <masao(dot)fujii(at)gmail(dot)com> wrote:
>> On Wed, Sep 21, 2011 at 2:13 PM, Magnus Hagander <magnus(at)hagander(dot)net> wrote:
>>> Presumably pg_start_backup() will check this. And we'll somehow track
>>> this before pg_stop_backup() as well? (for such evil things such as
>>> the user changing FPW from on to off and then back to on again during
>>> a backup, will will make it look correct both during start and stop,
>>> but incorrect in the middle - pg_stop_backup needs to fail in that
>>> case as well)
>>
>> Right. As I suggested upthread, to address that problem, we need to log
>> the change of FPW on the master, and then we need to check whether
>> such a WAL is replayed on the standby during the backup. If it's done,
>> pg_stop_backup() should emit an error.
>
> I somehow missed this thread completely, so I didn't catch your
> previous comments - oops, sorry. The important point being that we
> need to track if when this happens even if it has been reset to a
> valid value. So we can't just check the state of the variable at the
> beginning and at the end.

Right. Let me explain again what I'm thinking.

When FPW is changed, the master always writes the WAL record
which contains the current value of FPW. This means that the standby
can track all changes of FPW by reading WAL records.

The standby has two flags: One indicates whether FPW has always
been TRUE since last restartpoint. Another indicates whether FPW
has always been TRUE since last pg_start_backup(). The standby
can maintain those flags by reading WAL records streamed from
the master.

If the former flag indicates FALSE (i.e., the WAL records which
the standby has replayed since last restartpoint might not contain
required FPW), pg_start_backup() fails. If the latter flag indicates
FALSE (i.e., the WAL records which the standby has replayed
during the backup might not contain required FPW),
pg_stop_backup() fails.

If I'm not missing something, this approach can address the problem
which you're concerned about.

Regards,

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