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On Wed, Jan 19, 2005 at 10:55:27AM +0200, Mihai CHELARU wrote:

> START disks
> /dev/wd0e
> /dev/wd1e
> /dev/wd2e
> /dev/wd3e
> START layout
> 32 1 1 5

A 4-unit raid-5 is almost always going to suck for filesystem
performance. There is virtually no layout that will easily map a
useful filesystem blocksize onto the underlying 3-unit stripe (plus
parity).

Therefore, for almost every write you do, you wind up doing a
read-modify-write cycle; that's what kills your performance.

If you have the chance, for the sake of demonstration, compare
performance with 3-way and 4-way raid-0.

To resolve this, if you can, add a fifth disk and choose your stripe
size and filesystem block size carefully so the filesystem blocks
exactly map to a full stripe write.  Otherwise, you'd be much better
off with stripe+mirror over the four disks.

--
Dan.

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