Re: idle array consuming cpu ??!!

[Bill Davidsen <davidsen@tmr.com>]

Carlos Carvalho wrote:
> Bill Davidsen (davidsen@tmr.com) wrote on 22 January 2008 17:53:
>  >Carlos Carvalho wrote:
>  >> Neil Brown (neilb@suse.de) wrote on 21 January 2008 12:15:
>  >>  >On Sunday January 20, carlos@fisica.ufpr.br wrote:
>  >>  >> A raid6 array with a spare and bitmap is idle: not mounted and with no
>  >>  >> IO to it or any of its disks (obviously), as shown by iostat. However
>  >>  >> it's consuming cpu: since reboot it used about 11min in 24h, which is quite
>  >>  >> a lot even for a busy array (the cpus are fast). The array was cleanly
>  >>  >> shutdown so there's been no reconstruction/check or anything else.
>  >>  >> 
>  >>  >> How can this be? Kernel is 2.6.22.16 with the two patches for the
>  >>  >> deadlock ("[PATCH 004 of 4] md: Fix an occasional deadlock in raid5 -
>  >>  >> FIX") and the previous one.
>  >>  >
>  >>  >Maybe the bitmap code is waking up regularly to do nothing.
>  >>  >
>  >>  >Would you be happy to experiment?  Remove the bitmap with
>  >>  >   mdadm --grow /dev/mdX --bitmap=none
>  >>  >
>  >>  >and see how that affects cpu usage?
>  >>
>  >> Confirmed, removing the bitmap stopped cpu consumption.
>  >
>  >Looks like quite a bit of CPU going into idle arrays here, too.
>
> I don't mind the cpu time (in the machines where we use it here), what
> worries me is that it shouldn't happen when the disks are completely
> idle. Looks like there's a bug somewhere.

That's my feeling, I have one array with an internal bitmap and one with 
no bitmap, and the internal bitmap uses CPU even when the machine is 
idle. I have *not* tried an external bitmap.

-- 
Bill Davidsen <davidsen@tmr.com>
  "Woe unto the statesman who makes war without a reason that will still
  be valid when the war is over..." Otto von Bismark