I'm having a discussion with a coworker concerning the cost of md's
raid5 implementation versus hardware raid5 implementations.
Specifically, he states:
> The performance [of raid5 in hardware] is so much better with the
> write-back caching on the card and the offload of the parity, it
> seems to me that the minor increase in work of having to upgrade the
> firmware if there's a buggy one is a highly acceptable trade-off to
> the increased performance. The md driver still commits you to
> longer run queues since IO calls to disk, parity calculator and the
> subsequent kflushd operations are non-interruptible in the CPU. A
> RAID card with write-back cache releases the IO operation virtually
> instantaneously.
It would seem that his comments have merit, as there appears to be
work underway to move stripe operations outside of the spinlock:
http://lwn.net/Articles/184102/
What I'm curious about is this: for real-world situations, how much
does this matter? In other words, how hard do you have to push md
raid5 before doing dedicated hardware raid5 becomes a real win?
James
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