[zfs-discuss] Lower latency ZIL Option?: SSD behind Controller BB Write Cache

Christopher George
Wed, 26 Jan 2011 16:29:52 -0000

> ZIL OPTIONS: Obviously a DDRdrive is the ideal (36k 4k random
> IOPS***) but for the same budget I can get 2x Vertex 2 EX 50GB
> drives and put each behind it?s own P410 512MB BBWC controller.

The Vertex 2 EX goes for approximately $900 each online, while the
P410/512 BBWC is listed at HP for $449 each.  Cost wise you should
contact us for a quote, as we are price competitive with just a single
SSD/HBA combination.  Especially, as one obtains 4GB instead of
512MB of ZIL accelerator capacity.

> Assuming the SSDs can do 6300 4k random IOPS*** and that the
> controller cache confirms those writes in the same latency as the

For 4KB random writes you need to look closely at slides 47/48 of the
referenced presentation (http://www.ddrdrive.com/zil_accelerator).

The 6443 IOPS is obtained after testing for *only* 2 hours post
unpackaging or secure erase.  The slope of both curves gives a hint, as
the Vertex 2 EX does not level off and will continue to decrease.  I am
working on a new presentation focusing on this very fact for random
write IOPS performance over time (life of the device).  Suffice to say,
6443 IOPS is *not* worst case performance for random writes on the
Vertex 2 EX.

> DDRdrive (both PCIe attached RAM?****) then we should have
> DDRdrive type latency up to 6300 sustained IOPS.

All tests used a QD (Queue Depth) of 32 which will hide the device
latency of a single IO.  Very meaningful, as real life workloads can
be bound by even a single outstanding IO.  Let''s trace the latency to
determine which has the advantage.  For the SSD/HBA combination
an IO has to run the gauntlet through two controllers (HBA and SSD)
and propagate over a SATA cable.  The DDRdrive X1 has a single
unified controller and no extraneous SATA cable, see slides 15-17.

Best regards,

Christopher George
Founder/CTO