Thread: [GENERAL] Interesting streaming replication issue

are you sure you're scp'ing from the archive, not from pg_xlog?

Regards,

Gunnar "Nick" Bluth

Am 27. Juli 2017 05:00:17 MESZ schrieb James Sewell <james.sewell@jirotech.com>:

Hi all,

I've got two servers (A,B) which are part of a streaming replication pair. A is the master, B is a hot standby. I'm sending archived WAL to a directory on A, B is reading it via SCP.

This all works fine normally. I'm on Redhat 7.3, running EDB 9.6.2 (I'm currently working to reproduce with standard 9.6)

We have recently seen a situation where B does not catch up when taken offline for maintenance. 

When B is started we see the following in the logs:

2017-07-27 11:55:57 AEST [21432]: [979-1] user=,db=,client=  (0:00000)LOG:  restored log file "0000000C0000005A000000AA" from archive
2017-07-27 11:55:58 AEST [21432]: [980-1] user=,db=,client=  (0:00000)LOG:  restored log file "0000000C0000005A000000AB" from archive
2017-07-27 11:55:58 AEST [21432]: [981-1] user=,db=,client=  (0:00000)LOG:  restored log file "0000000C0000005A000000AC" from archive
2017-07-27 11:55:59 AEST [21432]: [982-1] user=,db=,client=  (0:00000)LOG:  restored log file "0000000C0000005A000000AD" from archive
2017-07-27 11:55:59 AEST [21432]: [983-1] user=,db=,client=  (0:00000)LOG:  restored log file "0000000C0000005A000000AE" from archive
2017-07-27 11:56:00 AEST [21432]: [984-1] user=,db=,client=  (0:00000)LOG:  restored log file "0000000C0000005A000000AF" from archive
2017-07-27 11:56:00 AEST [21432]: [985-1] user=,db=,client=  (0:00000)LOG:  restored log file "0000000C0000005A000000B0" from archive
2017-07-27 11:56:01 AEST [21432]: [986-1] user=,db=,client=  (0:00000)LOG:  restored log file "0000000C0000005A000000B1" from archive
2017-07-27 11:56:01 AEST [21432]: [987-1] user=,db=,client=  (0:00000)LOG:  restored log file "0000000C0000005A000000B2" from archive
2017-07-27 11:56:02 AEST [21432]: [988-1] user=,db=,client=  (0:00000)LOG:  restored log file "0000000C0000005A000000B3" from archive
2017-07-27 11:56:02 AEST [21432]: [989-1] user=,db=,client=  (0:00000)LOG:  restored log file "0000000C0000005A000000B4" from archive
2017-07-27 11:56:03 AEST [21432]: [990-1] user=,db=,client=  (0:00000)LOG:  restored log file "0000000C0000005A000000B5" from archive
scp: /archive/xlog//0000000C0000005A000000B6: No such file or directory
2017-07-27 11:56:03 AEST [46191]: [1-1] user=,db=,client=  (0:00000)LOG:  started streaming WAL from primary at 5A/B5000000 on timeline 12
2017-07-27 11:56:03 AEST [46191]: [2-1] user=,db=,client=  (0:XX000)FATAL:  could not receive data from WAL stream: ERROR:  requested WAL segment 0000000C0000005A000000B5 has already been removed

scp: /archive/xlog//0000000D.history: No such file or directory
scp: /archive/xlog//0000000C0000005A000000B6: No such file or directory
2017-07-27 11:56:04 AEST [46203]: [1-1] user=,db=,client=  (0:00000)LOG:  started streaming WAL from primary at 5A/B5000000 on timeline 12
2017-07-27 11:56:04 AEST [46203]: [2-1] user=,db=,client=  (0:XX000)FATAL:  could not receive data from WAL stream: ERROR:  requested WAL segment 0000000C0000005A000000B5 has already been removed

This will loop indefinitely. At this stage the master reports no connected standbys in pg_stat_replication, and the standby has no running WAL receiver process.

This can be 'fixed' by running pg_switch_xlog() on the master, at which time a connection is seen from the standby and the logs show the following:

scp: /archive/xlog//0000000D.history: No such file or directory

2017-07-27 12:03:19 AEST [21432]: [1029-1] user=,db=,client=  (0:00000)LOG:  restored log file "0000000C0000005A000000B5" from archive

scp: /archive/xlog//0000000C0000005A000000B6: No such file or directory

2017-07-27 12:03:19 AEST [63141]: [1-1] user=,db=,client=  (0:00000)LOG:  started streaming WAL from primary at 5A/B5000000 on timeline 12

2017-07-27 12:03:19 AEST [63141]: [2-1] user=,db=,client=  (0:XX000)FATAL:  could not receive data from WAL stream: ERROR:  requested WAL segment 0000000C0000005A000000B5 has already been removed

scp: /archive/xlog//0000000D.history: No such file or directory

2017-07-27 12:03:24 AEST [21432]: [1030-1] user=,db=,client=  (0:00000)LOG:  restored log file "0000000C0000005A000000B5" from archive

2017-07-27 12:03:24 AEST [21432]: [1031-1] user=,db=,client=  (0:00000)LOG:  restored log file "0000000C0000005A000000B6" from archive

scp: /archive/xlog//0000000C0000005A000000B7: No such file or directory

2017-07-27 12:03:25 AEST [21432]: [1032-1] user=,db=,client=  (0:00000)LOG:  unexpected pageaddr 56/E7000000 in log segment 0000000C0000005A000000B7, offset 0

2017-07-27 12:03:25 AEST [63196]: [1-1] user=,db=,client=  (0:00000)LOG:  started streaming WAL from primary 

At this time replication is running as normal and all changes are streamed.

Am I missing something here, this seems very odd. One thing I have noticed is it only seems to be caused after a lot of WAL is produced and the pg_xlog directory is sitting at max_wal_size

James Sewell,

PostgreSQL Team Lead / Solutions Architect 

 

Suite 112, Jones Bay Wharf, 26-32 Pirrama Road, Pyrmont NSW 2009

P (+61) 2 8099 9000  W www.jirotech.com  F (+61) 2 8099 9099

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On Thu, Jul 27, 2017 at 4:41 PM, Gunnar "Nick" Bluth <gunnar.bluth@pro-open.de> wrote:

are you sure you're scp'ing from the archive, not from pg_xlog?

Yes:

restore_command = 'scp -o StrictHostKeyChecking=no 10.154.19.30:/archive/xlog//%f %p'

Although you are right - that would almost make sense if I had done that!