On September 12, 2006 11:35:54 AM -0700 UNIX admin <[EMAIL PROTECTED]>
wrote:
There are also the speed enhancement provided by a HW
raid array, and
usually RAS too, compared to a native disk drive but
the numbers on
that are still coming in and being analyzed. (See
previous threads.)
It would be nice if you would attribute your quotes. Maybe this is a
limitation of the web interface?
Speed enhancements? What is the baseline of comparison?
Hardware RAIDs can be banalized to two features: cache which does data
reordering for optimal disk writes and parity calculation which is being
offloaded off of the server's CPU.
But HW calculations still take time, and the in-between, battery backed
cache serves to replace the individual disk caches, because of the
traditional file system approach which had to have some assurance that
the data made it to disk in one way or another.
With ZFS however the in-between cache is obsolete, as individual disk
caches can be used directly. I also openly question whether even the
dedicated RAID HW is faster than the newest CPUs in modern servers.
Unless there is something that I'm missing, I fail to see the benefit of
a HW RAID in tandem with ZFS. In my view, this holds especially true when
one gets into SAN storage like SE6920, EMC and Hitachi products.
I agree with your basic point, that the HW RAID cache is obsoleted by zfs
(which seems to be substantiated here by benchmark results), but I think
you slightly mischaracterize its use. The speed of the HW RAID CPU is
irrelevant; the parity is XOR which is extremely fast with any CPU when
compared to disk write speed.
What is relevant is, as Anton points out, the CPU cache on the host system.
Parity calculations kill the cache and will hurt memory-intensive apps.
So in this case, offloading it may help in the ufs case. (Not for zfs,
as I understand from reading here, since checksums still have to be done.
I would argue that this is *absolutely essential* [and zfs obsoletes all
other filesystems] and therefore the gain in the ufs on HW RAID-5 case is
worthless due to the correctness tradeoff.)
It would be interesting to have a zfs enabled HBA to offload the checksum
and parity calculations. How much of zfs would such an HBA have to
understand?
-frank
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