On 07 Dec 1999 10:00:08 -0800, Scott Henry <scotth@sgi.com> said:
> The experience of XFS on IRIX is that unless you put the journal on
> a REALLY fast device, like a battery-backed SSD, then the difference
> in performance between an internal journal and an external one is
> noise, even on very large, active, and performance-critical
> filesystems. Ie: writing to the journal is NOT a bottleneck.
Mostly correct, but there is one important exception: data journaling.
NFS servers (at least in NFSv2) need ultra-fast commit to disk of all
data writes in order to provide low-latency responses. To do that,
one option is to journal both the data and the metadata, so that
random data writes can be acknowledged by the NFS server rapidly: all
of the updates, including data, are hard on disk as soon as you have
streamed out the transaction to the log.
If you do that, support for an external log disk becomes much more
important.
--Stephen
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