ZFS zvol write speed incredibly slow

[Alex Frangis]

I just wanted to check in and say that I myself am encountering the  
same problem. I just got 7.0-Release online and working and I am  
seeing similar numbers to yours on a box that performed quite well  
running solaris.

Configuration:
AMD Semperon 3100+ 64bit Enabled
2gb Ram
3x 750GB Drives in a single Raid-Z

Thanks,
Alex

Hi guys!

I just got 7.0-RELEASE installed and running on my home server, and I've
noticed incredibly slow write speeds to a ZFS zvol. Reading speeds seem
normal. It may be related to the OpenSolaris bug #6428639:
http://bugs.opensolaris.org/view_bug.do?bug_id=6428639

  >> To another zpool:
# dd if=junk of=/tank2/junk bs=65536
1024+0 records in
1024+0 records out
67108864 bytes transferred in 0.331802 secs (202255676 bytes/sec)

  >> To a zvol in the same pool:
dd if=junk of=/dev/zvol/tank/tempvol bs=65536
1024+0 records in
1024+0 records out
67108864 bytes transferred in 11.575167 secs (5797658 bytes/sec)

Using smaller block sizes exacerbates the problem (444606 bytes/sec with
4096-byte blocks!)