Uneven load on drives in ZFS RAIDZ1

[Dan Nelson]

In the last episode (Dec 19), Stefan Esser said:
> for quite some time I have observed an uneven distribution of load between
> drives in a 4 * 2TB RAIDZ1 pool.  The following is an excerpt of a longer
> log of 10 second averages logged with gstat:
> 
> dT: 10.001s  w: 10.000s  filter: ^a?da?.$
>  L(q)  ops/s    r/s   kBps   ms/r    w/s   kBps   ms/w   %busy Name
>     0    130    106   4134    4.5     23   1033    5.2   48.8| ada0
>     0    131    111   3784    4.2     19   1007    4.0   47.6| ada1
>     0     90     66   2219    4.5     24   1031    5.1   31.7| ada2
>     1     81     58   2007    4.6     22   1023    2.3   28.1| ada3
[...]
> zpool status -v
>   pool: raid1
>  state: ONLINE
>   scan: none requested
> config:
> 
>         NAME        STATE     READ WRITE CKSUM
>         raid1       ONLINE       0     0     0
>           raidz1-0  ONLINE       0     0     0
>             ada0p2  ONLINE       0     0     0
>             ada1p2  ONLINE       0     0     0
>             ada2p2  ONLINE       0     0     0
>             ada3p2  ONLINE       0     0     0

Any read from your raidz device will hit three disks (the checksum is
applied across the stripe, not on each block, so a full stripe is always
read) so I think your extra IOs are coming from somewhere else.

What's on p1 on these disks?  Could that be the cause of your extra I/Os? 
Does "zpool iostat -v 10" give you even numbers across all disks?

-- 
	Dan Nelson
	dnelson at allantgroup.com