Re: Beating a dead horse

[mlelstv%serpens.de@localhost (Michael van Elst)]

wam%hiwaay.net@localhost ("William A. Mahaffey III") writes:

>Hmmmm .... I thought that the RAID5 would write 1 parity byte & 4 data 
>bytes in parallel, i.e. no '1 drive bottleneck'.

That only happens when the "4 data bytes" (actually the whole stripe)
gets written in one operation.

Unfortunately this currently requires that the filesystem sends
a whole stripe to the RAID driver which is only possible for
a few configurations because of the MAXPHYS limit of 64kbyte.

The limit, despite its name, also applies to the RAID driver.
Additionally it helps when the block size (or even the fragment
size) is as large as the stripe.

So, with a 4+1 RAID5:

sectPerSU = 32              -> 4*32 = 128 blocks = 64KByte stripe
newfs -b 65536 -f 65536     -> format filesystem with 64k blocks and frags

To overcome this issue either MAXPHYS needs to be larger or
the raid driver needs to coalesce multiple writes.

-- 
-- 
                                Michael van Elst
Internet: mlelstv%serpens.de@localhost
                                "A potential Snark may lurk in every tree."