Re: - PostgreSQL Replication Types - Mailing list pgsql-general

On 12/17/2015 07:56 AM, Will McCormick wrote:
> Thanks a ton for the prompt response.
>
> I've read most of this but some it was not clear until we discussed.
>
> Updated with WLM:
>
> On 12/17/2015 07:17 AM, Will McCormick wrote:
>
>     I inherited a 9.1 replication environment
>
>     Few basic questions that I can't find clear answers / clarifications for
>     if possible:
>
>     3 types of replication in 9.1 I've read about from the offical docs:
>
>     1) warm standby
>     2) hot standby
>     3) streaming replication
>
>     I'm using streaming replication I believe,  the only indication I have
>     is that  there is the primary_conninfo on the standby. Is this the only
>     indication?
>
>
> WLM: I also see streaming replication in the logs.
>
> On standby:
>
> http://www.postgresql.org/docs/9.1/interactive/functions-admin.html
> "
> pg_last_xlog_receive_location()
>
> Get last transaction log location received and synced to disk by
> streaming replication. While streaming replication is in progress this
> will increase monotonically. If recovery has completed this will remain
> static at the value of the last WAL record received and synced to disk
> during recovery. If streaming replication is disabled, or if it has not
> yet started, the function returns NULL."
>
> WLM: When I do this on the standby I get an error:
>
>    ERROR:  recovery is in progress
>
>    HINT:  WAL control functions cannot be ...

What is the full hint message?
The functions are supposed to be able to be run while the server is in
recovery.

>
>
>
>     Is it possible to get if using streaming replication under normal
>     operations?
>
>     /cp: cannot stat `/opt/postgres/9.1/archive/000000070000000F00000057':
>     No such file or directory/
>
>     /cp: cannot stat `/opt/postgres/9.1/archive/000000070000000F00000057':
>     No such file or directory/
>
>     /LOG:  streaming replication successfully connected to primary/
>
>     /FATAL:  could not receive data from WAL stream: FATAL:  requested WAL
>     segment 000000070000000F00000057 has already been removed/
>
>
> Assuming above is from standby log, correct? WLM: yes
>
> The cp lines would seem to indicate a restore_command in the standby
> recovery.conf, is that the case?:
>
> http://www.postgresql.org/docs/9.1/static/archive-recovery-settings.html
>
> restore_command (string) WLM: Correct
>
>
> The FATAL indicates that the WAL file has already been recycled on the
> master.
>
> WLM: I had read this what confuses me a bit is:
>
>    /cp: cannot stat `/opt/postgres/9.1/archive/000000070000000F00000057':
>    No such file or directory/
>
>     Does Streaming replication automatically use Archived WAL files when
> WAL XLOG files don't contain a transaction?
>
>     We did have wal_keep_segments set to 0. I changed this to 50 but
> want to better understand this. Especially the correlation between the
> Archived WALs and the XLOG WALs. My guess is the difference between
> Streaming replication and the others is very simply that Streaming
> replication can read the XLOG WALs as well? So if all the Archived WALs
> have been shipped and processed to the Standby then the XLOGs are
> processed but not shipped? This meaning at a transaction level "kindof"?

Not really see the section below(#STREAMING-REPLICATION) I posted
previously. It is either or, if streaming is set up and the standby can
reach the master xlog directory then it will stream the WAL files from
there. If the standby cannot access the xlog directory and if you have
WAL archiving set up on the master and archive restore setup on the
standby it will switch to full WAL log shipping from the archived WAL
directory, assuming the files it needs are there.

>
> See:
>
> http://www.postgresql.org/docs/9.1/interactive/runtime-config-replication.html#RUNTIME-CONFIG-REPLICATION-MASTER
>
> "wal_keep_segments (integer)
>
>      Specifies the minimum number of past log file segments kept in the
> pg_xlog directory, in case a standby server needs to fetch them for
> streaming replication. Each segment is normally 16 megabytes. If a
> standby server connected to the primary falls behind by more than
> wal_keep_segments segments, the primary might remove a WAL segment still
> needed by the standby, in which case the replication connection will be
> terminated. (However, the standby server can recover by fetching the
> segment from archive, if WAL archiving is in use.)
>
>      This sets only the minimum number of segments retained in pg_xlog;
> the system might need to retain more segments for WAL archival or to
> recover from a checkpoint. If wal_keep_segments is zero (the default),
> the system doesn't keep any extra segments for standby purposes, so the
> number of old WAL segments available to standby servers is a function of
> the location of the previous checkpoint and status of WAL archiving.
> This parameter has no effect on restartpoints. This parameter can only
> be set in the postgresql.conf file or on the server command line.
> "
>
> http://www.postgresql.org/docs/9.1/interactive/warm-standby.html#STREAMING-REPLICATION
>
> "If you use streaming replication without file-based continuous
> archiving, you have to set wal_keep_segments in the master to a value
> high enough to ensure that old WAL segments are not recycled too early,
> while the standby might still need them to catch up. If the standby
> falls behind too much, it needs to be reinitialized from a new base
> backup. If you set up a WAL archive that's accessible from the standby,
> wal_keep_segments is not required as the standby can always use the
> archive to catch up."
>
>
>     /
>     /
>
>     My understanding is that warm standby and hot standby do log shipping
>     and there is a greater window for transactions not to be send to the
>     standby because WAL XLOG must be filled.
>
>
> Hot versus warm standby refer to whether it is possible to run read only
> queries on the standby in the first case or not in the second case.
>
>
>
>     Whereas Streaming replication basically sends at the transaction level?
>
>
> The difference you are looking for is log shipping versus streaming,
> where log shipping moves complete WAL files and streaming streams the
> same files.
>
> WLM: I still am having trouble with distinction. By the same files do
> you mean XLOG WALs?

Yes. The WALs created in the xlog directory are the basis for
replication. Leaving out logical replication, not available in 9.1, the
only way to do replication using the Postgres core tools is to move
those files from the master to the standby. You can either ship them
whole which is log shipping or you can stream them, which is streaming.
It also possible, which is what you are seeing, to do the belt and
suspenders approach. That is to use streaming for the responsiveness,
but have a parallel process that ships the whole logs to a archive
directory where they can be accessed also. Having an archive directory
also allows for PITR(Point In Time Recovery), but that is another topic:

http://www.postgresql.org/docs/9.1/static/continuous-archiving.html

>
> See here for more detail: WLM: Reading now :)
>
> http://www.postgresql.org/docs/9.1/interactive/high-availability.html
>
> On Thu, Dec 17, 2015 at 10:37 AM, Adrian Klaver
> <adrian.klaver@aklaver.com <mailto:adrian.klaver@aklaver.com>> wrote:
>
>     On 12/17/2015 07:17 AM, Will McCormick wrote:
>
>         I inherited a 9.1 replication environment
>
>         Few basic questions that I can't find clear answers /
>         clarifications for
>         if possible:
>
>         3 types of replication in 9.1 I've read about from the offical docs:
>
>         1) warm standby
>         2) hot standby
>         3) streaming replication
>
>         I'm using streaming replication I believe,  the only indication
>         I have
>         is that  there is the primary_conninfo on the standby. Is this
>         the only
>         indication?
>
>
>     On standby:
>
>     http://www.postgresql.org/docs/9.1/interactive/functions-admin.html
>     "
>     pg_last_xlog_receive_location()
>
>     Get last transaction log location received and synced to disk by
>     streaming replication. While streaming replication is in progress
>     this will increase monotonically. If recovery has completed this
>     will remain static at the value of the last WAL record received and
>     synced to disk during recovery. If streaming replication is
>     disabled, or if it has not yet started, the function returns NULL."
>
>
>         Is it possible to get if using streaming replication under normal
>         operations?
>
>         /cp: cannot stat
>         `/opt/postgres/9.1/archive/000000070000000F00000057':
>         No such file or directory/
>
>         /cp: cannot stat
>         `/opt/postgres/9.1/archive/000000070000000F00000057':
>         No such file or directory/
>
>         /LOG:  streaming replication successfully connected to primary/
>
>         /FATAL:  could not receive data from WAL stream: FATAL:
>         requested WAL
>         segment 000000070000000F00000057 has already been removed/
>
>
>     Assuming above is from standby log, correct?
>
>     The cp lines would seem to indicate a restore_command in the standby
>     recovery.conf, is that the case?:
>
>     http://www.postgresql.org/docs/9.1/static/archive-recovery-settings.html
>
>     restore_command (string)
>
>
>     The FATAL indicates that the WAL file has already been recycled on
>     the master.
>
>     See:
>
>     http://www.postgresql.org/docs/9.1/interactive/runtime-config-replication.html#RUNTIME-CONFIG-REPLICATION-MASTER
>
>     "wal_keep_segments (integer)
>
>          Specifies the minimum number of past log file segments kept in
>     the pg_xlog directory, in case a standby server needs to fetch them
>     for streaming replication. Each segment is normally 16 megabytes. If
>     a standby server connected to the primary falls behind by more than
>     wal_keep_segments segments, the primary might remove a WAL segment
>     still needed by the standby, in which case the replication
>     connection will be terminated. (However, the standby server can
>     recover by fetching the segment from archive, if WAL archiving is in
>     use.)
>
>          This sets only the minimum number of segments retained in
>     pg_xlog; the system might need to retain more segments for WAL
>     archival or to recover from a checkpoint. If wal_keep_segments is
>     zero (the default), the system doesn't keep any extra segments for
>     standby purposes, so the number of old WAL segments available to
>     standby servers is a function of the location of the previous
>     checkpoint and status of WAL archiving. This parameter has no effect
>     on restartpoints. This parameter can only be set in the
>     postgresql.conf file or on the server command line.
>     "
>
>     http://www.postgresql.org/docs/9.1/interactive/warm-standby.html#STREAMING-REPLICATION
>
>     "If you use streaming replication without file-based continuous
>     archiving, you have to set wal_keep_segments in the master to a
>     value high enough to ensure that old WAL segments are not recycled
>     too early, while the standby might still need them to catch up. If
>     the standby falls behind too much, it needs to be reinitialized from
>     a new base backup. If you set up a WAL archive that's accessible
>     from the standby, wal_keep_segments is not required as the standby
>     can always use the archive to catch up."
>
>
>         /
>         /
>
>         My understanding is that warm standby and hot standby do log
>         shipping
>         and there is a greater window for transactions not to be send to the
>         standby because WAL XLOG must be filled.
>
>
>     Hot versus warm standby refer to whether it is possible to run read
>     only queries on the standby in the first case or not in the second case.
>
>
>
>         Whereas Streaming replication basically sends at the transaction
>         level?
>
>
>     The difference you are looking for is log shipping versus streaming,
>     where log shipping moves complete WAL files and streaming streams
>     the same files.
>
>     See here for more detail:
>
>     http://www.postgresql.org/docs/9.1/interactive/high-availability.html
>
>
>
>
>         I'm sure this is somewhat misinformed!
>
>
>         Thanks,
>
>
>         Will
>
>
>
>     --
>     Adrian Klaver
>     adrian.klaver@aklaver.com <mailto:adrian.klaver@aklaver.com>
>
>


--
Adrian Klaver
adrian.klaver@aklaver.com