Last week we got this report from Matt Chesler:
http://archives.postgresql.org/pgsql-admin/2010-10/msg00221.php that he
was getting errors when trying to do a simple binary replication test.
The problem is that what appears to be a perfectly good WAL segment
doesn't get streamed to the standby. No one responded.
One of our testers just ran into the same thing. I just investigated,
and I'm baffled as to what's going on myself. Can't tell if this is a
bug or an under-documented restriction, but this makes two reports of
the problem now. (Mine is happening on a standard 9.0.0 RPM set, didn't
notice any changes in 9.0.1 that would impact this; afraid to upgrade
while I have a repeatable test case for this sitting here)
The setup is intended to get a simple test replication setup going
without even having to do the whole pg_start_backup shuffle, by copying
the whole server directory when it's down. Basic steps are:
-Follow the first set of instructions at
http://wiki.postgresql.org/wiki/Binary_Replication_Tutorial to setup a
master compatible with replication, then duplicate it after stopping it
using rsync. Note that you may have to manually create an empty pg_xlog
directory on the standby, depending on what was there before you started
replication.
To rule out one possible source of problems here, I made an additional
change on the master not listed there:
[mas...@pyramid pg_log]$ psql -d postgres -c "show wal_keep_segments"
wal_keep_segments
-------------------
10
I wondered if having this set to 0 (the default) was causing the issue,
thinking perhaps it doesn't do any looking for existing segments at all
in that situation. Problem still happens for me.
-Create a recovery.conf pointing to the master as described in the tutorial.
-Start the standby. Make sure that it has reached the point where it's
requesting WAL segments from the master; you want to see it looping
doing periodic "FATAL: could not connect to the primary server: could
not connect to server: Connection refused" before touching the master.
-Start the master
What I expect to happen now is that the current WAL file that was in
progress at the point the data was copied over gets streamed over. That
doesn't seem to happen. Instead, I see this on the standby:
LOG: streaming replication successfully connected to primary
FATAL: could not receive data from WAL stream: FATAL: requested WAL
segment 000000010000000000000000 has already been removed
This on the master:
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:
[mas...@pyramid pg_log]$ ls -l $PGDATA/pg_xlog
-rw------- 1 master master 16777216 Oct 31 16:29 000000010000000000000000
drwx------ 2 master master 4096 Oct 4 12:28 archive_status
[mas...@pyramid pg_log]$ 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? I'm not familiar enough with the SR
internals to reason out what's going wrong myself yet. Wanted to
validate that Matt's report wasn't a unique one though, with a bit more
detail included about the state the system gets into, and one potential
fix (increasing wal_keep_segments) already tried without improvement.
--
Greg Smith 2ndQuadrant US g...@2ndquadrant.com Baltimore, MD
PostgreSQL Training, Services and Support www.2ndQuadrant.us
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