I have setup streaming replication between a primary and secondary server. I have enabled archiving. In the Postgres log file I am seeing the below error.
< 2017-12-05 03:08:45.374 UTC > WARNING: archive_mode enabled, yet archive_command is not set
< 2017-12-05 03:08:46.668 UTC > ERROR: requested WAL segment 0000000100000000000000E3 has already been removed
< 2017-12-05 03:08:51.675 UTC > ERROR: requested WAL segment 0000000100000000000000E3 has already been removed
< 2017-12-05 03:08:56.682 UTC > ERROR: requested WAL segment 0000000100000000000000E3 has already been removed
Do we need to enable archive_mode = on for streaming replication? How can I avoid above error?
max_wal_senders = 3
wal_keep_segements = 32
https://www.postgresql.org/docs/current/static/warm-standby.html
If you use streaming replication without file-based continuous
archiving, the server might recycle old WAL segments before the
standby has received them. If this occurs, the standby will need to be
reinitialized from a new base backup. You can avoid this by setting
wal_keep_segments to a value large enough to ensure that WAL segments
are not recycled too early, or by configuring a replication slot for
the standby. If you set up a WAL archive that's accessible from the
standby, these solutions are not required, since the standby can
always use the archive to catch up provided it retains enough
segments.
emphasis mine.
so either increase wal_keep_segments to big enough (enough for your amount of block changes), or configure archive_command and set up some storage to keep removed wals from master to be available for slave. Or configuring a replication slot for the standby...
In my case I had to do reinit the replica in maintenance mode using below commands and it fixed the issue. This error was due to lag between leader and replica.
patronictl list
patronictl pause
patronictl reinit patroni
choose Replica pod
patronictl resume
Related
As per replication slot definition, it is a feature in PostgreSQL that ensure that the master server will retain the WAL logs that are needed by the replicas even when they are disconnected from server.
Is there any effect if I run pg_archivecleanup command every 15th day of month to free my storage. Does it has any effect on replication slot, since it is tracing WAL file which is required by standby server?
Because I run pg_archivecleanup removing WAL file from last checkpoint but I am not sure whether it is removing WAl file that is required for other replica.
If not removing then how it is actually tracing it?
I am looking for explanation from experts.
When you run pg_archivecleanup, PostgreSQL will delete all WAL segments that are older than the WAL segment you specify as argument. This will ignore replication slots, you you may end up removing WAL segments that may still be needed by standby servers to catch up (if they do that using restore_command).
Note that this normally not a problem, because pg_archivecleanup deletes WAL segments from the archive, while replication slots deal with WAL segments on the primary server (in the pg_wal directory), which are not affected by pg_archivecleanup. Now since the standby consumes WAL directly from the primary (as specified in primary_conninfo), it does not have to rely on the WAL archives.
Environment: Postgresql 13.x (dockerized)
I was trying to test the DR setup for PostgreSQL nodes.
pg_basebackup and wal_files archive was taken from the standby mode.
Done restoration on a new node by copying pg_basebackup and configured postgresql.conf to use restore_command pointing to walfiles archive.
#----------------------- RECOVERY CONFIGS -----------------------
restore_command = 'cp /db-restore/mydb/walfiles/%f "%p"'
recovery_target_timeline = 'latest'
recovery_target_action = promote
recovery seems to be fine. Some random select queries returning correct results.
But logfile is throwing below error frequently.
2022-04-19 10:19:53 UTC [291] rep_usr#[unknown] ERROR: replication slot "slot_name" does not exist
2022-04-19 10:19:58 UTC [296] rep_usr#[unknown] ERROR: replication slot "slot_name" does not exist
As I have taken backup from standby, is this restoration making new node as a standby and looking for the replication_slot it used in the previous generation?
How can I make new node as a Master (remove replication_slot info)
What are the proper steps to recover if the backup was taken from standby.
I have 1 master and 2 standby nodes. And planning to take a backup from a standby. So is there any specific changes required for archive_mode and archive_command when using this on a standby node? Current commands:
archive_mode = always
archive_level = logical
archive_command = 'test ! -f /db-archives/walfiles/%f && cp %p /db-archives/walfiles/%f'"
Could someone help with this? Any pointers?
I am sure, db-backup will have info about replication_slot and connection_info as the pg_basebackup itself is a clone of entire DB. To revert configs, I am manually removing postgresql.auto.conf in main directory which contains above parameters.
So how can I remove any other references of replication_slot if there are any in the DB backup?
These error messages don't seem to be thrown by recovery, but by some other tool that connects as database user rep_usr.
Create the replication slot if your application needs it!
I removed all configs and started with fresh.
removed main/postgresql.auto.conf which was present in the backup.
main/postgresql.auto.conf is present in standby nodes when we take pg_basebackup. contains the configs used for pg_basebackup in standby nodes. (slot_name, and connect_info).
As I was restoring backup from standby to a Master, I don't need that postgresql.auto.conf.
Since upgrading to Postgres 11 I cannot get my production standby server to catch up. In the logs things look fine eventually:
2019-02-06 19:23:53.659 UTC [14021] LOG: consistent recovery state reached at 3C772/8912C508
2019-02-06 19:23:53.660 UTC [13820] LOG: database system is ready to accept read only connections
2019-02-06 19:23:53.680 UTC [24261] LOG: started streaming WAL from primary at 3C772/8A000000 on timeline 1
But the following queries show everything is not fine:
warehouse=# SELECT coalesce(abs(pg_wal_lsn_diff(pg_last_wal_receive_lsn(), pg_last_wal_replay_lsn())), -1) / 1024 / 1024 / 1024 AS replication_delay_gbytes;
replication_delay_gbytes
-------------------------
208.2317776754498486
(1 row)
warehouse=# select now() - pg_last_xact_replay_timestamp() AS replication_delay;
replication_delay
-------------------
01:54:19.150381
(1 row)
After a while (a couple hours) replication_delay stays about the same but replication_delay_gbytes grows, although note replication_delay is behind from the beginning and replication_delay_gbytes starts near 0. During startup there were a number of these messages:
2019-02-06 18:24:36.867 UTC [14036] WARNING: xlog min recovery request 3C734/FA802AA8 is past current point 3C700/371ED080
2019-02-06 18:24:36.867 UTC [14036] CONTEXT: writing block 0 of relation base/16436/2106308310_vm
but Googling suggests these are fine.
Replica was created using repmgr by running pg_basebackup to perform the clone and then starting up the replica and seeing it catch up. This previously was working with Postgres 10.
Any thoughts on why this replica comes up but is perpetually lagging?
I'm still not sure what the issue is/was, but I was able to get the standby caught up with these two changes:
set use_replication_slots=true in the repmgr config
set wal_compression=on in the postgres config
Use replication slots didn't seem to change anything other than to cause replication_delay_gbytes to stay roughly flat. Turing on WAL compression did help, somehow, although I'm not entirely sure how. Yes, in theory it made it possible to ship WAL files to the standby faster, but reviewing network logs I see a drop in sent/received bytes that matches the effects of compression, so it seems to be shipping WAL files at the same speed just using less network.
It still seems like there is some underlying issue at play here, though, because for example when I do pg_basebackup to create the standby it generates roughly 500 MB/s of network traffic, but then when it is streaming WALs after the standby finishes recovery it drops to ~250 MB/s without WAL compression and ~100 MB/s with WAL compression, but there is no decrease in network traffic after it caught up with WAL compression, so I'm not sure what's going on there that allowed it to catch up.
im trying to set up streaming replication, but for some reason when i update the database on the master, the changes are not reflected on the standby server UNTIL i restart the postgresql service on the master. (i see new xlog files in master server but these do not get synced to the standby server). when i restart the service on master, i finally see new files added to my shared wal_archive folder
the only way I can make it sync automatically is if i set the archive_timeout.
Master:
wal_level = 'hot_standby' # minimal, archive, hot_standby, or logical
archive_mode = on # allows archiving to be done
# (change requires restart)
archive_command = 'copy "%p" "\\\\VBOXSVR\\wal_archive\\%f"'
max_wal_senders = 3 # max number of walsender processes
# (change requires restart)
wal_keep_segments = 10 # in logfile segments, 16MB each; 0 disables
pb_hba.conf
host replication postgres slaveip/32 trust
It sounds like you're using archive-based replication without streaming. So it's only replicating when a WAL archive is finished and a new one is opened, which happens:
When the server does a checkpoint before a clean shutdown
When a WAL archive is filled by write activity and a new one is needed
at archive_timeout time
If you want continuous replication you need to use streaming replication. See the manual for details. This involves setting a connection string in your downstream server's recovery.conf so it can connect directly to the upstream master to receive new writes in near-real-time.
You should still leave archive based replication enabled, because this allows the replica to recover if it's disconnected for a while. It's also useful for point-in-time recovery.
We have a master-slave replication configuration as follows.
On the master:
postgresql.conf has replication configured as follows (commented line taken out for brevity):
max_wal_senders = 1
wal_keep_segments = 8
On the slave:
Same postgresql.conf as on the master. recovery.conf looks like this:
standby_mode = 'on'
primary_conninfo = 'host=master1 port=5432 user=replication password=replication'
trigger_file = '/tmp/postgresql.trigger.5432'
When this was initially setup, we performed some simple tests and confirmed the replication was working. However, when we did the initial data load, only some of the data made it to the slave.
Slave's log is now filled with messages that look like this:
< 2015-01-23 23:59:47.241 EST >LOG: started streaming WAL from primary at F/52000000 on timeline 1
< 2015-01-23 23:59:47.241 EST >FATAL: could not receive data from WAL stream: ERROR: requested WAL segment 000000010000000F00000052 has already been removed
< 2015-01-23 23:59:52.259 EST >LOG: started streaming WAL from primary at F/52000000 on timeline 1
< 2015-01-23 23:59:52.260 EST >FATAL: could not receive data from WAL stream: ERROR: requested WAL segment 000000010000000F00000052 has already been removed
< 2015-01-23 23:59:57.270 EST >LOG: started streaming WAL from primary at F/52000000 on timeline 1
< 2015-01-23 23:59:57.270 EST >FATAL: could not receive data from WAL stream: ERROR: requested WAL segment 000000010000000F00000052 has already been removed
After some analysis and help on the #postgresql IRC channel, I've come to the conclusion that the slave cannot keep up with the master. My proposed solution is as follows.
On the master:
Set max_wal_senders=5
Set wal_keep_segments=4000 . Yes I know it is very high, but I'd like to monitor the situation and see what happens. I have room on the master.
On the slave:
Save configuration files in the data directory (i.e. pg_hba.conf pg_ident.conf postgresql.conf recovery.conf)
Clear out the data directory (rm -rf /var/lib/pgsql/9.3/data/*) . This seems to be required by pg_basebackup.
Run the following command:
pg_basebackup -h master -D /var/lib/pgsql/9.3/data --username=replication --password
Am I missing anything ? Is there a better way to bring the slave up-to-date w/o having to reload all the data ?
Any help is greatly appreciated.
The two important options for dealing with the WAL for streaming replication:
wal_keep_segments should be set high enough to allow a slave to catch up after a reasonable lag (e.g. high update volume, slave being offline, etc...).
archive_mode enables WAL archiving which can be used to recover files older than wal_keep_segments provides. The slave servers simply need a method to retrieve the WAL segments. NFS is the simplest method, but anything from scp to http to tapes will work so long as it can be scripted.
# on master
archive_mode = on
archive_command = 'cp %p /path_to/archive/%f'
# on slave
restore_command = 'cp /path_to/archive/%f "%p"'
When the slave can't pull the WAL segment directly from the master, it will attempt to use the restore_command to load it. You can configure the slave to automatically remove segments using the archive_cleanup_commandsetting.
If the slave comes to a situation where the next WAL segment it needs is missing from both the master and the archive, there will be no way to consistently recover the database. The only reasonable option then is to scrub the server and start again from a fresh pg_basebackup.
You can configure replication slots for postgress to keep WAL segments for replica mentioned in such slot.
Read more at https://www.percona.com/blog/2018/11/30/postgresql-streaming-physical-replication-with-slots/
On master server run
SELECT pg_create_physical_replication_slot('standby_slot');
On slave server add next line to recovery.conf
primary_slot_name = 'standby_slot'
actually to recover, you don't have to drop the whole DB and start from scratch. since master has up-to-date binary, you can do following to recover the slave and bring them back to in-sync:
psql -c "select pg_start_backup('initial_backup');"
rsync -cva --inplace --exclude=*pg_xlog* <data_dir> slave_IP_address:<data_dir>
psql -c "select pg_stop_backup();"
Note:
1. slave has to be turned down by service stop
2. master will turn to read-only due to query pg_start_backup
3. master can continue serving read only queries
4. bring back slave at the end of the steps
I did this in prod, it works perfect for me.
slave and master are in sync and there is no data loss.
You will get that error if keep_wal_segments setting is too low.
When you set the value for keep_wal_segments consider that "How long is the pg_basebackup taking?"
Remember that segments are generated about every 5 minutes, so if the backup takes an hour, you need at least 12 segments saved. At 2 hours, you need 24, etc. I would set the value to about 12.2 segments/hour of backup.
As Ben Grimm suggested in the comments, this is a question of making sure to set segments to the maximum possible value to allow the slave to catch up.