Re: XFS File systems and PostgreSQL

> Bruce Momjian <pgman(at)candle(dot)pha(dot)pa(dot)us> writes:
>
> > Yes, the irony is that a journaling file system is being used to have
> > fast, reliable restore after crash bootup, but with no fsync, the db is
> > probably hosed.
>
> It just struck me--is it necessarily true that we get the big
> performance hit?
>
> On a non-data-journaling FS (like ext3), since WAL files are
> preallocated (right?), a WAL sync shouldn't involve any metadata
> updates. So we just write the WAL data to a (hopefully contiguous)
> chunk of data blocks.
>
> On an FS that journals data AND metadata, fsync() can return once the
> updates are committed to the log--it doesn't have to wait until the
> log is back-flushed (or whatever you call it) to the main filesystem.
>
> The above is theoretical, and I don't know enough about Reiser or XFS
> to know how they behave.

Theoretically, yes, all these log-based file system just log metadata
changes, not user data, so it should not affect it. I just don't know
how well the fsync's are implemented on these things.

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