On Mon, Feb 07, 2005 at 05:30:15PM -0800, David Sparks wrote:
> Nobody commented on this question though, I'll put it up again. Most of
> the information I've gathered on external logs dates back to the IRIX
> days -- I hope there isn't anything out of date there.
>
> > Regarding external logs, I've never formatted a XFS with an external
> > log. From what I can gather (google, man pages), doing so is supposed
> > to reduce disk head seeking hence performance is improved. Is anyone
> > aware of any benchmarks that explore internal/external logs and sizes?
Anything that is metadata intensive - dbench, postmark, etc. Run the
benchmark, recreate the fs with different parameters, run it again and
observe the difference. Easily scriptable....
> Is there any advantage to an external log on a separate partition on the
> same device, as opposed to an external log on a separate device?
No. Often this is a disadvantage because the log is usually placed
in the center of the filesystem to minimise _average_ disk seek time for
log writes. If it's in a different partition on the same disk, then
the seek penalty for log writes on average is going to be greater than
if it was internal to the filesystem because seek distances will be
greater.
> Basically is an external log only for performance benefits or are there
> robustness benefits also?
With external logs you can use different redundancy strategies to
the rest of the filesystem (e.g. mirror instead of RAID5, different
hw, etc) that can increase robustness or performance of the log.
External logs provide flexibility; how you use that is up to you....
Cheers,
Dave.
--
Dave Chinner
R&D Software Engineer
SGI Australian Software Group
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