Re: Re: Synch Rep: direct transfer of WAL file from the primary to the standby

Hi,

On Wed, Jul 8, 2009 at 4:00 AM, Heikki
Linnakangas<heikki(dot)linnakangas(at)enterprisedb(dot)com> wrote:
>>  I would envision the slaves
>> connecting to the master's replication port and asking "feed me WAL
>> beginning at LSN position thus-and-so", with no notion of WAL file
>> boundaries exposed anyplace.
>
> Yep, that's the way I envisioned it to work in my protocol suggestion
> that Fujii adopted
> (http://archives.postgresql.org/message-id/4951108A.5040608@enterprisedb.com)
> The <begin> and <end> values are XLogRecPtrs, not WAL filenames.

If <begin> indicates the middle of the XLOG file, the file written to the
standby is partial. Is this OK? After two server failed, the XLOG file
including <begin> might still be required for crash recovery of the
standby server. But, since it's partial, the crash recovery would fail.
I think that any XLOG file should be written to the standby as it can
be replayed by a normal recovery.

>>The point about not wanting to archive
>> lots of WAL on the master would imply that the master reserves the right
>> to fail if the requested starting position is too old, whereupon the
>> slave needs some way to resync --- but that probably involves something
>> close to taking a fresh base backup to copy to the slave.

What if the XLOG file required for recovery has gone while doing
resync a large data? In this case, the standby might never start
because the requested starting position is always too old.

Regards,

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