Re: SR fails to send existing WAL file after off-line copy

On 01.11.2010 09:37, Heikki Linnakangas wrote:
> On 31.10.2010 23:31, Greg Smith wrote:
>> LOG: replication connection authorized: user=rep host=127.0.0.1
>> port=52571
>> FATAL: requested WAL segment 000000010000000000000000 has already been
>> removed
>>
>> Which is confusing because that file is certainly on the master still,
>> and hasn't even been considered archived yet much less removed:
>>
>> [master@pyramid pg_log]$ ls -l $PGDATA/pg_xlog
>> -rw------- 1 master master 16777216 Oct 31 16:29 000000010000000000000000
>> drwx------ 2 master master 4096 Oct 4 12:28 archive_status
>> [master@pyramid pg_log]$ ls -l $PGDATA/pg_xlog/archive_status/
>> total 0
>>
>> So why isn't SR handing that data over? Is there some weird unhandled
>> corner case this exposes, but that wasn't encountered by the systems the
>> tutorial was tried out on?
>
> Yes, indeed there is a corner-case bug when you try to stream the very
> first WAL segment, with log==seg==0. We keep track of the last removed
> WAL segment, and before a piece of WAL is sent to the standby, walsender
> checks that the requested WAL segment is > the last removed. Before any
> WAL segments have been removed since postmaster startup, the latest
> removed segment is initialized to 0/0, with the idea that 0/0 precedes
> any valid WAL segment. That's clearly not true though, it does not
> precede the very first WAL segment after initdb, 0/0.
>
> Seems that we need to change the meaning of the last removed WAL segment
> to avoid the ambiguity of 0/0. Let's store the (last removed)+1 in the
> global variable instead.

Committed that. Thanks for the report, both of you. I'm not subscribed 
to pgsql-admin which is why I didn't see Matt's original report.

--   Heikki Linnakangas  EnterpriseDB   http://www.enterprisedb.com