> From: zfs-discuss-boun...@opensolaris.org [mailto:zfs-discuss-
> boun...@opensolaris.org] On Behalf Of deg...@free.fr
> I'm not familiar with ZFS stuff, so I'll try to give you as much as info I
can get
> with our environment
> We are using a ZFS pool as a VLS for a backup server (Sun V445 Solaris
> and we are faced with very low read performance (whilst write performance
> is much better, i.e : up to 40GB/h to migrate data onto LTO-3 tape from
> and up to 100GB/h to unstage data from LTO-3 tape to disk, either with
> Navigator 4.2 software or directly using dd commands)
> We have tunned ZFS parameters for ARC and disabled preftech but
> performance is poor. If we dd from disk to RAM or tape, it's very slow,
but if
> we dd from tape or RAM to disk, it's faster. I can't figure out why. I've
> other posts related to this, but I'm not sure what can of tuning can be
> For disks concern, I have no idea on how our System team created the ZFS
> volume.
> Can you help ?

Normally, even a single cheap disk in the dumbest configuration should
vastly outperform an LTO3 tape device.  And 100 GB/h is nowhere near what
you should expect, unless you're using highly fragmented or scattered small
files.  In the optimal configuration, you'll read/write something like
1Gbit/sec per disk, until you saturate your controller, let's just pick
rough numbers and say 6Gbit/sec = 2.7 TB per hour.  So there's a ballpark to
think about.

Next things next.  I am highly skeptical of dd.  I constantly get weird
performance problems when using dd.  Especially if you're reading/writing
tapes.  Instead, this is a good benchmark for how fast your disks can
actually go in the present configuration:  zfs send somefilesystem@somesnap
| pv -i 30 > /dev/null    (You might have to install pv, for example using
opencsw or blastwave.  If you don't have pv and don't want to install it,
you might want to time zfs send | wc > /dev/null, so you can get the total
size and the total time.)  Expect the performance to go up and down...  So
watch it a while.  Or wait for it to complete and then you'll have the

Also...  In what way are you using dd?  dd is not really an appropriate tool
for backing up a ZFS filesystem.  Well, there are some corner cases where it
might be ok, but generally speaking, no.  So the *very* first question you
should be asking is probably not about the bad performance you're seeing,
but verifying the validity of your backup technique.

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