Hi,
I've been looking at a performance issue on my system, its FreeBSD 8.1, zpool
version 14 (hope Im allowed to ask about FreeBSD on here?!). Anyway I think its
quite a general question. The pool contains a single 2 disk mirror, using 5k
SATA drives.
I am monitoring the percent busy, and the data in/out on the physical disks and
I am seeing close to 100% busy often. Thats ok, these things can happen! ;).
The thing I don't understand is why during these busy times that zpool iostat
shows very low IO. During a 10 second period the physical disks are close to
100% busy and show about 5MB/sec IO, during the same period zpool shows
something like this:
mx0 52.9G 691G 75 0 195K 0
mx0 52.9G 691G 178 0 430K 0
mx0 52.9G 691G 200 1 513K 35.9K
mx0 52.9G 691G 121 6 301K 688K
mx0 52.9G 691G 207 0 526K 0
mx0 52.9G 691G 281 0 708K 0
mx0 52.9G 691G 249 0 676K 0
mx0 52.9G 691G 191 0 518K 0
mx0 52.9G 691G 169 0 452K 128K
mx0 52.9G 691G 144 2 363K 383K
mx0 52.9G 691G 140 0 365K 0
mx0 52.9G 691G 151 0 395K 128K
So this is showing mostly reads, but nothing very taxing. Additionally my ARC
cache is not being fully utilised:
ARC Size:
Current Size: 28.75% 603.23M (arcsize)
Target Size: (Adaptive) 30.35% 636.68M (c)
Min Size (Hard Limit): 12.50% 262.26M (c_min)
Max Size (High Water): ~8:1 2098.08M (c_max)
So more ARC isn't going to help me.
My goal being, what is the best way to improve my system performance, more RAM,
more disks, dedicated LOG or ARC2 disks etc etc. The load appears in zpool
iostat to be mostly read, but my ARC isnt even fully utilised so that to be
seems an odd one.
I know I don't have high end hardware, and only 2 disks. But I'd like to
understand the apparent disparity between the physical disk IO and the zpool IO
output. That way I can better decide if I should, for example, add 2x SSDs for
ZIL (and L2ARC if it was needed, but seems not to be) or replace the existing
disks and put instead 2x 2 way mirror on slightly better 7k SATA drives.
Anyone able to throw some light on what might be going on under the hood?
thanks! Andy.
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