freebsd stable - Jan 2009 - slow zvol performance compared to files on the same pool

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.

Pete French wrote:
> 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. 
On 7.x (where ZFS is really quite broken for server use - don't waste 
too much time on it) the ZVOL code did an "fsync" after every single 
block write.  Its a testament to your fast disks that you got as high as 
12mb/s.  I don't know why your read speed was so bad, but you should try 
again on 8-current as numerous fixes and improvements have happened.


- Andrew