Re: "High" cpu usage when using ZFS cache device

On 16 November 2010 13:15, Christer Solskogen
<[e-mail]> wrote:
> On Tue, Nov 16, 2010 at 12:47 PM, Ivan Voras <[e-mail]> 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=3D/dev/zero of=3D/mnt/file".
> The systemload goes +0.6 instead if +10.3.
>
> See:
> CPU: =C2=A00.0% user, =C2=A00.0% nice, =C2=A00.6% system, =C2=A00.0% inte=
rrupt, 99.3% idle
> Mem: 832M Active, 960M Inact, 7017M Wired, 2600K Cache, 1237M Buf, 3063M =
Free
> Swap: 8192M Total, 8192M Free
>
> =C2=A0PID USERNAME =C2=A0 =C2=A0THR PRI NICE =C2=A0 SIZE =C2=A0 =C2=A0RES=
 STATE =C2=A0 C =C2=A0 TIME =C2=A0 WCPU COMMAND
> 38261 root =C2=A0 =C2=A0 =C2=A0 =C2=A0 =C2=A01 =C2=A046 =C2=A0 =C2=A00 =
=C2=A05776K =C2=A01112K wdrain =C2=A07 =C2=A0 0:07 =C2=A04.98% dd
>
> But when using it as cache device for zfs:
>
> CPU: =C2=A00.0% user, =C2=A00.0% nice, 11.9% system, =C2=A00.0% 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)

You mean you have system load on an otherwise idle system?

Try this:

1) start "top" with parameters -H -S, see if anything is using the CPU time

2) start "gstat", see if anything is using IO, and if it's
particularly slow or busying the device too much
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