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

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.

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