I am new to postgres and was trying to simulate a postgresql cluster so:
I have two nodes installed for postgres latest version and acting as active / hot standby and with master configuration :
archive_mode = on
archive_command = 'test ! -f /data/%f && cp %p /data/%f'
and slave configuration
primary_slot_name = 'standby_db2_slot'
hot_standby = on
and others default and related configuration
my question is if the standby was off for some time and the master crashes how to recover the data from my archived wal files also how to get the last wal file that the master was writing to before crashing?
You could copy the files from the archive (if it is still available) into the replica's pg_wal folder. Or more typically, you would set restore_command to copy each of them from the archive upon request.
how to get the last wal file that the master was writing to before crashing?
If it was a hard crash where the master's storage was irreparably destroyed, you likey can't get it. That is why streaming is great, it copies the data stream in near-real time to minimize loss. And if was a soft crash, why are you trying to promote the replica anyway, rather than just turning the master back on? If the master's storage was only partially destroyed, then just copy this last file to the archive manually.
I try to configure backuping database in postgresql with pg_basebackup and WAL logs.
For now I created full backup once a week and want to backup wal logs too. But, as I understand, posgresql writes them all the time. So, how can I copy them and be shure that they are not corrupted?
Thanks
You set archive_command to a shell command that copies the WAL file to a safe archive location, so that burden is mostly on you.
When PostgreSQL runs archive_command, it assumes that the WAL file is not corrupted. Only a PostgreSQL bug or a bug in the storage system could cause a corrupted WAL segment.
There is no better protection against PostgreSQL bugs than always running the latest bugfix release, and you can invest in storage hardware that will at least detect failure.
You can also write your archive_command with a certain amount of paranoia, e.g. by comparing the md5sum of the WAL segment and its archive copy.
Another idea is to write two copies of the WAL file to different storage systems.
I am not a DBA but i am using postgresql for production server and i am using postgresql 10 database. I am using Bigsql and i started replication of my production server to other server and on replication server everything is working but on my production server their is no space left. And after du command on my production server i am getting that pg_wal folder have 17 gb file and each file is of 16 mb size.
After some google search i change my postgresql.conf file as:
wal_level = logical
archive_mode = on
archive_command = 'cp -i %p /etc/bigsql/data/pg10/pg_wal/archive_status/%f'
i install postgresql 10 from Bigsql and did above changes.
After changes the dir /pg_wal/archive_status had 16 gb of log. So my question is that should i delete them manually or i have to wait for system delete them automatically.
And is that if i write archive_mode to on should that wal file getting removed automatically??
Thanks for your precious time.
This depends on how you do your backups and whether you'd ever need to restore the database to some point in time.
Only a full offline filesystem backup (offline meaning with database turned off) or an on-line logical backup with pg_dumpall will not need those files for a restore.
You'd need those files to restore a filesystem backup created while the database is running. Without them the backup will fail to restore. Though there exist backup solutions that copy needed WAL files automatically (like Barman).
You'd also need those files if your replica database will ever fall behind the master for some reason. Or you'd need to restore the database to some past point-in-time.
But these files compress pretty well - should be less than 10% size after compression - you can write your archive_command to compress them automatically instead of just copying.
And you should delete them eventually from the archive. I'd recommend to not delete them until they're at least a month old and also at least 2 full successful backups are done after creating them.
We have one master and two slaves and wal files are not getting deleted. I am new to the team and people are assuming they were getting deleted earlier and not any more.
But we found that we have wal file kept as long as Sep,2015.
Master:
archive_mode = off
archive_command = ''
slave1:
archive_cleanup_command -- not present on recovery_conf file
slave2:
archive_cleanup_command -- not present on recovery_conf file
wal logs are 600gb now and I have only couple of days before I delete those. Right now I am checking if basebackups are created so that I can delete wal files and resetxlog.
I am not understanding how wal files are generated when archive_mode is off. Can someone explain, what else to look for?
I will provide more information as needed.
Current situation
So I have WAL archiving set up to an independent internal harddrive on a data logging computer running Postgres. The harddrive containing the WAL archives is filling up and I'd like to remove and archive all the WAL archive files, including the initial base backup, to external backup drives.
The directory structure is like:
D:/WALBACKUP/ which is the parent folder for all the WAL files (00000110000.CA00000004 etc)
D:/WALBACKUP/BASEBACKUP/ which holds the .tar of the initial base backup
The question I have then is:
Can I safely move literally every single WAL file except the current WAL archive file, (000000000001.CA0000.. and so on), including the base backup, and move them to another hdd. (Note that the database is live and receiving data)
cheers!
WAL archives
You can use the pg_archivecleanup command to remove WAL from an archive (not pg_xlog) that's not required by a given base backup.
In general I suggest using PgBarman or a similar tool to automate your base backups and WAL retention though. It's easier and less error prone.
pg_xlog
Never remove WAL from pg_xlog manually. If you have too much WAL then:
your wal_keep_segments setting is keeping WAL around;
you have archive_mode on and archive_command set but it isn't working correctly (check the logs);
your checkpoint_segments is ridiculously high so you're just generating too much WAL; or
you have a replication slot (see the pg_replication_slots view) that's preventing the removal of WAL.
You should fix the problem that's causing WAL to be retained. If nothing seems to have happened after changing a setting run a manual CHECKPOINT command.
If you have an offline server and need to remove WAL to start it you can use pg_archivecleanup if you must. It knows how to remove only WAL that isn't needed by the server its self ... but it might break your archive-based backups, streaming replicas, etc. So don't use it unless you must.
WAL files are incremental, so the simple answer is: You cannot throw any files out. The solution is to make a new base backup and then all previous WALs can be deleted.
The WAL files contain individual statements that modify tables so if you throw some older WALs out, then the recovery process will fail (it will not silently skip missing WAL files) because the state of the database cannot be restored reliably. You can move the WAL files to some other location without upsetting the WAL process but then you'd have to make all WAL files available again from a single location if you ever need to recover your database from some point in the past; if you are running out of disk space then that may mean recovering from some location where you have enough space to store the base backup and all WAL files. The main issue here is if you can do that fast enough to restore a full database after an incident.
Another issue is that if you cannot identify where/when a problem occurred that needs to be corrected your only option is to start with the base backup and then replay all the WAL files. This procedure is not difficult, but if you have an old base backup and many WAL files to process, this simply takes a lot of time.
The best approach for your case, in general, is to make a new base backup every x months and collect WALs with that base backup. After every new base backup you can delete the old base backup and its subsequent WALs or move them to cheap offline storage (DVD, tape, etc). In the case of a major incident you can quickly restore the database to a known correct state from the recent base backup and the relatively few WAL files collected since then.
A solution that we went for, is executing pg_basebackup every night. This would create a base backup and later on we can use pg_archivecleanup to clean up all the "old" WAL files before that base using something like
"%POSTGRES_INSTALLDIR%\bin\pg_archivecleanup" -d %WAL_backup_dir% %newestBaseFile%
Fortunately, we never had to recover yet, but it should work in theory.
In case someone found this by searching how to safely cleanup the WAL directory under a replication architecture, consider the scenario where there might be left overs from offline replicas, in this case, unused replica slots waiting for the replica to come back online and thus keeping a lot of WAL archives on the Master DB.
In our case we had an issue with a replica going down due to hardware failure, we had to recreate it along with its replica_slot on the Master DB but forgot to get rid of the previous used one. Once we cleared that out PSQL got rid of unused WALs and all was good.
You can add the script to automatically clean or remove pg_wal files. This will work in pg-11 version. If you want to use other psql version the you can simply replace the command "/usr/pgsql-11/bin/pg_archivecleanup" to /usr/pgsql-12/bin/pg_archivecleanup or 13 as per your wish.
#!/bin/bash
/usr/pgsql-11/bin/pg_controldata -D /var/lib/pgsql/11/data/ > pgwalfile.txt
/usr/pgsql-11/bin/pg_archivecleanup -d /var/lib/pgsql/11/data/pg_wal $(cat pgwalfile.txt | grep "Latest checkpoint's REDO WAL file" | awk '{print $6}')