Interesting large improvement in performance by putting the
'ext3' journal on a separate storage unit:
http://insights.oetiker.ch/linux/external-journal-on-ssd.html
Not sure that this is because the separate disk was SSD, perhaps
it was just because not on RAID6 (colossal RMW overheads for
small synchronous writes) or beause it avoided a lot of seeking
to/from the journal. I can imagine XFS journals also to benefit,
especially as they are transaction journals and not block
journals. But only with SSDs which handle well interleaved
read-write to the same erase block.
As to SSDs, I was impressed by the enormous peak latencies for
writing to some SSDs in a recent-ish test:
http://www.sabi.co.uk/blog/1104Apr.html#110417
I suspect that such long latencies are particularly unsuitable
for a journal log drive.
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