slow zvol performance compared to files on the same pool

[Pete French]

I was experimenting with iscsi earlier, using both a flat file as the
backing store and also a zvol. I noticed that the zvol was giving me
dreadful performance - reading at about 20 meg/second and writing at
about 12. the fklat file gives about 45 meg/second both ways.

i thouht it was to do wuth the iscsi layer, but I then tried it using
dd on the machinbe itself and got the same results. it seems very
curious - I am creating both the filesystem for the iscsi file and the
zvol on the same pool, so the underlying discs (4 x 15k SCSI drives on
U320) are the same in both places, as is the pool.

anybody got any opinions ? this is on 7.1-RC2, but I have nothing else
to compare it to. 

-pete.