Обсуждение: [GENERAL] pg_basebackup ----xlog-method=stream

On 04/21/2017 11:02 AM, Ian Harding wrote:
> I used this command to set up a streaming replica and it worked perfectly.

Was that the full command?

If not can you show the full command you used?

Postgres version?

>
> I tried to run it to create an online backup of the master on that
> replica for backup purposes and it seems not to have worked as well.
>
> I thought that streaming the WAL would eliminate the need to keep tons
> of WAL around, that since they were streamed concurrently with the file
> they would be in place.  However, I got
>
> pg_basebackup: could not get transaction log end position from server:
> ERROR:  requested WAL segment 000000010000496F000000BF has already been
> removed

Where was pg_basebackup running, on the master or somewhere else?

The pg_basebackup was from the same Postgres version as the master?

>
> The ...backup file in the archive logs folder of the master looks like this"
>
> # cat 000000010000496F000000BF.00578DF8.backup
> START WAL LOCATION: 496F/BF578DF8 (file 000000010000496F000000BF)
> STOP WAL LOCATION: 4971/11FC6528 (file 000000010000497100000011)
> CHECKPOINT LOCATION: 496F/C1ACE0D8
> BACKUP METHOD: streamed
> BACKUP FROM: master
> START TIME: 2017-04-21 01:08:38 PDT
> LABEL: full-20170421
> STOP TIME: 2017-04-21 06:31:28 PDT
>
> so the "...already been removed" message is referring to the very first
> file.
>
> The files were definitely being streamed, I restarted and on the master
> I see this:
>
> [root@db20 ~]# ps aux | grep stream
> postgres 113855  0.4  0.0 274955352 2932 ?      Ss   10:53   0:00
> postgres: wal sender process postgres 192.168.4.21(51292) streaming
> 4972/7FBC22B8
> root     115958  0.0  0.0 112652   956 pts/2    S+   10:56   0:00 grep
> --color=auto stream
> postgres 142072  0.3  0.0 274955348 3160 ?      Ss   Apr20   3:47
> postgres: wal sender process postgres 192.168.4.21(51240) streaming
> 4972/7FBC22B8
>
>
> although the --verbose output from pg_basebackup mentions starting the
> streaming, it doesn't mention the file names like it does for the
> datadir files.
>
> Am I misunderstanding how this works?  I have WAL archiving set up, so
> the files are available, but I wanted them included in the backup.
>
> Thanks!
>
> - Ian


--
Adrian Klaver
adrian.klaver@aklaver.com

On Sat, Apr 22, 2017 at 3:02 AM, Ian Harding <harding.ian@gmail.com> wrote:
> Am I misunderstanding how this works?  I have WAL archiving set up, so the
> files are available, but I wanted them included in the backup.

Please note that if you have a WAL archive available, you may not even
need to have --xlog-method=stream, which is useful to have fully
safe-contailed backups. For the error regarding the missing WAL
segment, it could happen if Postgres completes two checkpoints when
taking the backup. One thing that you could use is a replication slot
that gives the guarantee that segments are retained. Those have been
integrated in pg_basebackup with 9.6.
--
Michael

> On Apr 21, 2017, at 3:29 PM, Michael Paquier <michael.paquier@gmail.com> wrote:
>
>> On Sat, Apr 22, 2017 at 3:02 AM, Ian Harding <harding.ian@gmail.com> wrote:
>> Am I misunderstanding how this works?  I have WAL archiving set up, so the
>> files are available, but I wanted them included in the backup.
>
> Please note that if you have a WAL archive available, you may not even
> need to have --xlog-method=stream, which is useful to have fully
> safe-contailed backups. For the error regarding the missing WAL
> segment, it could happen if Postgres completes two checkpoints when
> taking the backup. One thing that you could use is a replication slot
> that gives the guarantee that segments are retained. Those have been
> integrated in pg_basebackup with 9.6.

Yeah. I really want to use replication slots but put the server into production without setting the appropriate config
settings.I need to schedule some downtime for a restart.  

> --
> Michael