Peter Rabbitson wrote:
H. Peter Anvin wrote:
Peter Rabbitson wrote:
Hi,
I am experimenting with raid6 on 4 drives on 2.6.27.11. The problem I am
having is that no matter what chunk size I use, the write benchmark
always comes out at single drive speed, although I should be seeing
double drive speed (read speed is at near 4x as expected).
I have no idea why you "should" be seeing double drive speed. All
drives have to be written, so you'd logically see single drive speed.
Because with properly adjusted elevators and chunk sizes it is reasonable
to expect N * S write speed from _any_ raid, where N is the number of
different data bearing disks in a stripe, and S is the speed of a hard
drive (assuming the drive speeds are equal). So for raid5 we have N =
numdisks-1, for raid6 numdisks-2, for raid10 -n4 -pf3 we get 4-(3-1) and
so on. I have personally verified the write behavior for raid10 and raid5,
don't see why it should/would be different for raid6.
That's a lovely theory, but in practice I have to say I have never
measured any such thing, using benchmarks intended to match real world,
or even heavy disk writes of a dumb nature like dd. I have tested
through the raw device, and through filesystems, tuned stripe-cache-size
and buffers, tried setting "stride" in ext3, all to conclude that with
raid5 I see essentially write speed of 1x a single drive and read speed
of (N-1)x as you suggest. Actually, looking at results for arrays with
more drives I can see a trend to write at (N/3)x speed, being a
seek-write for full chunk data and seef-read-write for XOR. But even on
six drive arrays I don't get near (N-1)x in measurable.
--
Bill Davidsen <davidsen@xxxxxxx>
"Woe unto the statesman who makes war without a reason that will still
be valid when the war is over..." Otto von Bismark
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