Uneven load on drives in ZFS RAIDZ1

[Stefan Esser]

Am 19.12.2011 17:36, schrieb Michael Reifenberger:
> Hi,
> a quick test using `dd if=/dev/zero of=/test ...` shows:
> 
> dT: 10.004s  w: 10.000s  filter: ^a?da?.$
>  L(q)  ops/s    r/s   kBps   ms/r    w/s   kBps   ms/w   %busy Name
>     0    378      0      0   12.5    376  36414   11.9   60.6| ada0
>     0    380      0      0   12.2    378  36501   11.8   60.0| ada1
>     0    382      0      0    7.7    380  36847   11.6   59.2| ada2
>     0    375      0      0    7.4    374  36164    9.6   51.3| ada3
>     0    377      0      1   10.2    375  36325   10.1   53.3| ada4
>    10    391      0      0   39.3    389  38064   15.7   80.2| ada5
> 
> Seems to be sufficiently equally distributed for a life system...

Hi Michael,

in an earlier reply I mentioned the suspicious queue length and %busy of
ada5, which may be the result of other load (not caused by the dd
command) or of a hardware problem (I'd check drive health ...).

(Hmmm, the numbers look strange: ops/s is not the sum of r/s and w/s,
but misses that value by 2. I could understand a rounding difference of
1, but not 2 counts per second. But this is a different issue ...)

Anyway: The imbalance that I observe on my system is specific to reads,
not writes. Could you please check, whether sending a large (multi-GB)
file to /dev/null shows identical read load over all drives?

I suspect that 2 of the drives will see slightly (some 20%, perhaps)
less read requests than the rest.

Regards, STefan