Re: tape backups

Ah, got it. Thanks!

On Dec 23, 2006, at 5:59 PM, Tom Lane wrote:

> Ben <bench(at)silentmedia(dot)com> writes:
>> But, this page confuses me when it talks about pg_start_backup and
>> pg_stop_backup. What do these functions do? It seems like they do
>> nothing more than let me know which wal files were in use over the
>> duration of the backup, which is certainly useful. But they do NOT
>> seem to freeze the actual data files, and it seems to me that because
>> the data files won't be archived atomically while they may be
>> changing, that I might end up with corrupted data files that a replay
>> of wal files wouldn't correct. Is my fear groundless?
>
> Yes. The reason we don't have to freeze the data files during a
> backup
> is that any page that changes within that interval will be rewritten
> anyway when the WAL log is replayed during recovery. This is why the
> WAL sequence has to start before the pg_start_backup rather than at
> some
> later point --- that overlap is exactly what makes it safe to not
> freeze
> the data files.
>
> regards, tom lane
>
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