"High" cpu usage when using ZFS cache device

[Christer Solskogen]

On Tue, Nov 16, 2010 at 12:47 PM, Ivan Voras <ivoras at freebsd.org> wrote:

> You can easily test it - use the stick as a simple disk device with UFS and
> see how much CPU does it take simply to talk to the device.

See, that is why I think it is a ZFS issue. Because I did that.
I created a UFS filesystem on the same usb stick. Mounted it and did a
"dd if=/dev/zero of=/mnt/file".
The systemload goes +0.6 instead if +10.3.

See:
CPU:  0.0% user,  0.0% nice,  0.6% system,  0.0% interrupt, 99.3% idle
Mem: 832M Active, 960M Inact, 7017M Wired, 2600K Cache, 1237M Buf, 3063M Free
Swap: 8192M Total, 8192M Free

  PID USERNAME    THR PRI NICE   SIZE    RES STATE   C   TIME   WCPU COMMAND
38261 root          1  46    0  5776K  1112K wdrain  7   0:07  4.98% dd

But when using it as cache device for zfs:

CPU:  0.0% user,  0.0% nice, 11.9% system,  0.0% interrupt, 88.1% idle
Mem: 832M Active, 193M Inact, 5782M Wired, 2592K Cache, 1237M Buf, 5066M Free
Swap: 8192M Total, 8192M Free

The funny thing is that when I add the device (and some cache is added
to it) the load is normal. But the load goes up when nothing is
written to it (or beeing read from it)

-- 
chs,