Koichi Suzuki <suzuki(dot)koichi(at)oss(dot)ntt(dot)co(dot)jp> writes:
> Year, I agree we should carefully follow how Done really did a backup.
> My point is PostgreSQL may have to extend the file during the hot backup
> to write to the new block. It is slightly different from Oracle's case.
> Oracle allocates all the database space in advance so that there could
> be no risk to modify the metadata on the fly. In our case, because SAN
> based storage snapshot is device level, not file system level, even a
> file system does not know that the snapshot is being taken and we might
> encounter the case where metadata and/or user data are not consistent.
> Such snapshot (whole filesystem) might be corrupted and cause file
> system level error.
Surely a hot-backup technique that cannot even produce a consistent
state of filesystem metadata is too broken to be considered a backup
technique at all.
AFAIK, actually workable methods of this type depend on filesystem
cooperation, and are able to produce coherent snapshots of the logical
(not necessarily physical) filesystem content at a specific instant.
regards, tom lane
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