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Document restrictions regarding incremental backups and standbys.

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If you try to take an incremental backup on a standby and there hasn't
been much system activity, it might fail. Document why this happens.
Also add a hint to the error message you get, to make it more likely
that users will understand what has gone wrong.

Laurenz Albe and Robert Haas

Discussion: https://postgr.es/m/5468641ad821dad7aa3b2d65bf843146443a1b68.camel@cybertec.at
This commit is contained in:
Robert Haas
2024-07-25 15:45:06 -04:00
parent 580f8727ca
commit 744ddc6c6a
2 changed files with 13 additions and 1 deletions
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11
doc/src/sgml/backup.sgml
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@ -925,6 +925,17 @@ test ! -f /mnt/server/archivedir/00000001000000A900000065 && cp pg_wal/0
to manage. For a large database all of which is heavily modified, to manage. For a large database all of which is heavily modified,
incremental backups won't be much smaller than full backups. incremental backups won't be much smaller than full backups.
</para> </para>
<para>
An incremental backup is only possible if replay would begin from a later
checkpoint than for the previous backup upon which it depends. If you
take the incremental backup on the primary, this condition is always
satisfied, because each backup triggers a new checkpoint. On a standby,
replay begins from the most recent restartpoint. Therefore, an
incremental backup of a standby server can fail if there has been very
little activity since the previous backup, since no new restartpoint might
have been created.
</para>
</sect2> </sect2>
<sect2 id="backup-lowlevel-base-backup"> <sect2 id="backup-lowlevel-base-backup">

3
src/backend/backup/basebackup_incremental.c
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@ -441,7 +441,8 @@ PrepareForIncrementalBackup(IncrementalBackupInfo *ib,
errmsg("manifest requires WAL from final timeline %u ending at %X/%X, but this backup starts at %X/%X", errmsg("manifest requires WAL from final timeline %u ending at %X/%X, but this backup starts at %X/%X",
range->tli, range->tli,
LSN_FORMAT_ARGS(range->end_lsn), LSN_FORMAT_ARGS(range->end_lsn),
LSN_FORMAT_ARGS(backup_state->startpoint)))); LSN_FORMAT_ARGS(backup_state->startpoint)),
errhint("This can happen for incremental backups on a standby if there was little activity since the previous backup.")));
} }
else else
{ {