otheus uibk wrote:
> The manual and in this mailing list, the claim is made that consistent,
> file-level backups may be made
> by bracketing the file-copy operation with the postgresql pg_start_backup and
> pg_stop_backup
> operations. Many people including myself have found that in some
> circumstances, using "tar" to copy
> these files will result in an error if one of the data files changes during
> the tar operation. The
> responses to those queries on this mailing list are unsatisfactory
> ("everything is fine, trust us").
Everything is fine, trust us.
>> bash-3.00# tar -cf 16aprilstandby.tar /db-data/
>> tar: Removing leading `/' from member names
>> tar: /db-data/base/24643/151667: file changed as we read it
>> tar: /db-data/base/24643/151764.2: file changed as we read it
>> tar: /db-data/base/24643/151766: file changed as we read it
>> tar: /db-data/base/24643/151768: file changed as we read it
>> tar: /db-data/base/66412/151969: file changed as we read it
> The above scenario is exactly what I saw, albeit with less frequency and
> severity.
> I decided to test this claim that these messages are "perfectly harmless" and
> "can be ignored":
[...]
> As you can see below, there were non-zero changes made to these files.
[...]
> Such changes occurred EVEN WHEN TAR DID NOT WARN of changed files. Further,
> when step 3 involved an
> actual backup, involving minutes, not milliseconds, dozens of differences to
> files in data/base/...
> are reported. To be clear, I excluded from consideration all files in
> pg_xlog, pg_clog, pg_subtrans,
> pg_stat_tmp.
>
> If these files are changing during the pg_start_backup() and pg_stop_backup,
> then exactly what is
> their purpose? Might they be changing during the tar, as tar thinks? How may
> an operator be assured
> the snapshot is consistent (unless one stops the databases)? Will the redo
> logs restore the files to
> a consistent state, no matter when these files are changed? I find it hard to
> believe that would be
> the case.
The files are indeed changing while they are backed up.
The tar archive is not a consistent backup.
Redo using the Write Ahead Log will indeed restore the files to a consistent
state,
astonishing as that may be.
Yours,
Laurenz Albe
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