gmirror performance

[Lorenzo Perone]

Hi @ list, Hi Pawel,

just a question about gmirror performance. I have 2 15k SAS drives, 
mirrored by gmirror. the mirror was setup like this (like manpage example):

gmirror label -v -b split -s 2048 mirr0 da0 da1

on a partition of this drive, I make the following test:

# dd if=/dev/zero bs=1m count=2000 of=/mnt/2gigfile.dat
2000+0 records in
2000+0 records out
2097152000 bytes transferred in 11.203763 secs (187182824 bytes/sec)

# umount /mnt
# mount /dev/mirror/mirr0p4 /mnt

# dd if=/mnt/2gigfile.dat of=/dev/null bs=1m
2000+0 records in
2000+0 records out
2097152000 bytes transferred in 12.061197 secs (173875942 bytes/sec)

I'd expect read performance to be noticeably higher than write 
performance. Why is it not the case? Wrong expectation? :/

Further Details:

- FreeBSD 8.2-STABLE #0: Tue Mar 15 01:34:07 UTC 2011
- Underlying storage driver is the fresh, just MFC'd mps(4) for the DELL 
PERC H200A controller (so it could be related to that, as well).
- Using bs=8k gets better results (180783854 bytes/sec), but this may be 
caused by other factors.
- The filesystem is UFS with soft-updates (newfs -U).

Thanx for listening, and for all the nice GEOMs we have @ FreeBSD land :)

Regards,

Lorenzo