Re: WAL format changes

On Wed, Jun 20, 2012 at 8:19 PM, Magnus Hagander <magnus@hagander.net> wrote:
> On Tue, Jun 19, 2012 at 5:57 PM, Robert Haas <robertmhaas@gmail.com> wrote:
>> On Tue, Jun 19, 2012 at 4:14 AM, Heikki Linnakangas
>> <heikki.linnakangas@enterprisedb.com> wrote:
>>> Well, that was easier than I thought. Attached is a patch to make XLogRecPtr
>>> a uint64, on top of my other WAL format patches. I think we should go ahead
>>> with this.
>>
>> +1.
>>
>>> The LSNs on pages are still stored in the old format, to avoid changing the
>>> on-disk format and breaking pg_upgrade. The XLogRecPtrs stored the control
>>> file and WAL are changed, however, so an initdb (or at least pg_resetxlog)
>>> is required.
>>
>> Seems fine.
>>
>>> Should we keep the old representation in the replication protocol messages?
>>> That would make it simpler to write a client that works with different
>>> server versions (like pg_receivexlog). Or, while we're at it, perhaps we
>>> should mandate network-byte order for all the integer and XLogRecPtr fields
>>> in the replication protocol. That would make it easier to write a client
>>> that works across different architectures, in >= 9.3. The contents of the
>>> WAL would of course be architecture-dependent, but it would be nice if
>>> pg_receivexlog and similar tools could nevertheless be
>>> architecture-independent.
>>
>> I share Andres' question about how we're doing this already.  I think
>> if we're going to break this, I'd rather do it in 9.3 than 5 years
>> from now.  At this point it's just a minor annoyance, but it'll
>> probably get worse as people write more tools that understand WAL.
>
> If we are looking at breaking it, and we are especially concerned
> about something like pg_receivexlog... Is it something we could/should
> change in the protocl *now* for 9.2, to make it non-broken in any
> released version? As in, can we extract just the protocol change and
> backpatch that to 9.2beta?

pg_receivexlog in 9.2 cannot handle correctly the WAL location "FF"
(which was skipped in 9.2 or before). For example, pg_receivexlog calls
XLByteAdvance() which always skips "FF". So even if we change the protocol,
ISTM pg_receivexlog in 9.2 cannot work well with the server in 9.3 which
might send "FF". No?

Regards,

--
Fujii Masao