strange performance dip shown by iozone

[mi+mx at aldan.algebra.com]

Hello!

I'm trying to tune the amrd-based RAID5 and have made several iozone
runs on the array and -- for comparision -- on the single disk connected
to the Serial ATA controller directly.

The RAID-based FS was newfs-ed with ``-b 65536'', as it is intended to
store very large files. The single-disk FS was newfs-ed with defaults.

No softupdates were enabled on either, since those seem to degrade iozone
results slightly (iozone reads/writes a single file anyway).

The filesystems displayed different performance (reads are better with
RAID, writes -- with the single disk), but both have shown a notable dip
in writing (and re-writing) speed when iozone used the record lengthes
of 128 and 256. Can someone explain that? Is that a known fact? How can
that be avoided?

The machine is an amd64 running a fresh -current. The disks are 200Gb
SATAs. RAID5 consists of 6 of them.

Thanks!

	-mi