8.0-RELEASE/amd64 - full ZFS install - low read and write disk performance

Dan Naumov dan.naumov at gmail.com
Sun Jan 24 18:40:37 UTC 2010

On Sun, Jan 24, 2010 at 8:34 PM, Jason Edwards <sub.mesa at gmail.com> wrote:
>> ZFS writes to a mirror pair
>> requires two independent writes.  If these writes go down independent I/O
>> paths, then there is hardly any overhead from the 2nd write.  If the
>> writes
>> go through a bandwidth-limited shared path then they will contend for that
>> bandwidth and you will see much less write performance.
> What he said may confirm my suspicion on PCI. So if you could try the same
> with "real" Serial ATA via chipset or PCI-e controller you can confirm this
> story. I would be very interested. :P
> Kind regards,
> Jason

This wouldn't explain why ZFS mirror on 2 disks directly, on the exact
same controller (with the OS running off a separate disks) results in
"expected" performance, while having the OS run off/on a ZFS mirror
running on top of MBR-partitioned disks, on the same controller,
results in very low speed.

- Dan

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