Thanks a lot. On Wed 11 Apr, 2018, 9:07 AM Michael Paquier, <mich...@paquier.xyz> wrote:
> On Tue, Apr 10, 2018 at 11:06:54PM +0530, Raghavendra Rao J S V wrote: > > I am not clear the difference between checkpoint_segments and > > wal_keep_segments . > > > > I would like to now below things. Please explain.Thanks in advance. > > > > - Difference between *checkpoint_segments *and *wal_keep_segments * > > value > > - Role of *checkpoint_segments *and *wal_keep_segments * > > - Which one should has higher value. > > Documentation is king here. For checkpoint_segments: > > https://www.postgresql.org/docs/9.4/static/runtime-config-wal.html#RUNTIME-CONFIG-WAL-CHECKPOINTS > > Maximum number of log file segments between automatic WAL > checkpoints (each segment is normally 16 megabytes). The default is > three segments. Increasing this parameter can increase the amount of > time needed for crash recovery. This parameter can only be set in > the postgresql.conf file or on the server command line. > > For wal_keep_segments: > > https://www.postgresql.org/docs/9.4/static/runtime-config-replication.html#RUNTIME-CONFIG-REPLICATION-SENDER > > 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 sending server falls behind by more than > wal_keep_segments segments, the sending server might remove a WAL > segment still needed by the standby, in which case the replication > connection will be terminated. Downstream connections will also > eventually fail as a result. (However, the standby server can recover by > fetching the segment from archive, if WAL archiving is in use.) > > Mentioning checkpoint_segments implies that you are using PostgreSQL 9.4 > or older versions as this has been removed and replaced by max_wal_size > in 9.5. You should consider upgrading to a newer version. > > Hence the first is used in the context of normal operations to decide > the frequency of checkpoints when those are triggered by volume. The > second can be used with streaming replication to give a standby a higher > catchup window. Giving value to one or the other depends on the > context, and both are usable in completely different circumstances. > -- > Michael >
