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pg_combinebackup: Detect checksum mismatches and document limitation.

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If not all backups have the same checksum status, but the final backup
has checksums enabled, then the output directory may include pages
with invalid checksums. Document this limitation and explain how to
work around it.

In a future release, we may want to teach pg_combinebackup to
recompute page checksums when required, but as feature freeze has come
and gone, it seems a bit too late to do that for this release.

Patch by me, reviewed by Daniel Gustafsson

Discussion: http://postgr.es/m/CA+TgmoZugzOSmgkx97u3pc0M7U8LycWvugqoyWBv6j15a4hE5g@mail.gmail.com
This commit is contained in:
Robert Haas 2024-04-25 14:58:59 -04:00
parent 6d4bc96eb7
commit 205db0114e
3 changed files with 47 additions and 1 deletions
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6
doc/src/sgml/backup.sgml
View File

@ -892,7 +892,11 @@ test ! -f /mnt/server/archivedir/00000001000000A900000065 && cp pg_wal/0
only the incremental backup itself but also all earlier backups that
are required to supply the blocks omitted from the incremental backup.
See <xref linkend="app-pgcombinebackup"/> for further information about
this requirement.
this requirement. Note that there are restrictions on the use of
<literal>pg_combinebackup</literal> when the checksum status of the
cluster has been changed; see
<link linkend="app-pgcombinebackup-limitations">pg_combinebackup
limitations</link>.
</para>
<para>

20
doc/src/sgml/ref/pg_combinebackup.sgml
View File

@ -261,6 +261,26 @@ PostgreSQL documentation
</refsect1>
<refsect1 id="app-pgcombinebackup-limitations">
<title>Limitations</title>
<para>
<literal>pg_combinebackup</literal> does not recompute page checksums when
writing the output directory. Therefore, if any of the backups used for
reconstruction were taken with checksums disabled, but the final backup was
taken with checksums enabled, the resulting directory may contain pages
with invalid checksums.
</para>
<para>
To avoid this problem, taking a new full backup after changing the checksum
state of the cluster using <xref linkend="app-pgchecksums "/> is
recommended. Otherwise, you can disable and then optionally reenable
checksums on the directory produced by <literal>pg_combinebackup</literal>
in order to correct the problem.
</para>
</refsect1>
<refsect1>
<title>Environment</title>

22
src/bin/pg_combinebackup/pg_combinebackup.c
View File

@ -583,6 +583,8 @@ check_control_files(int n_backups, char **backup_dirs)
{
int i;
uint64 system_identifier = 0; /* placate compiler */
uint32 data_checksum_version = 0; /* placate compiler */
bool data_checksum_mismatch = false;
/* Try to read each control file in turn, last to first. */
for (i = n_backups - 1; i >= 0; --i)
@ -612,6 +614,16 @@ check_control_files(int n_backups, char **backup_dirs)
controlpath, (unsigned long long) system_identifier,
(unsigned long long) control_file->system_identifier);
/*
* Detect checksum mismatches, but only if the last backup in the
* chain has checksums enabled.
*/
if (i == n_backups - 1)
data_checksum_version = control_file->data_checksum_version;
else if (data_checksum_version != 0 &&
data_checksum_version != control_file->data_checksum_version)
data_checksum_mismatch = true;
/* Release memory. */
pfree(control_file);
pfree(controlpath);
@ -624,6 +636,16 @@ check_control_files(int n_backups, char **backup_dirs)
pg_log_debug("system identifier is %llu",
(unsigned long long) system_identifier);
/*
* Warn the user if not all backups are in the same state with regards to
* checksums.
*/
if (data_checksum_mismatch)
{
pg_log_warning("only some backups have checksums enabled");
pg_log_warning_hint("disable, and optionally reenable, checksums on the output directory to avoid failures");
}
return system_identifier;
}