Re: high cpu load with sw raid1

[David Hajek]

Inlined.

On Thu, Sep 20, 2001, Jakob Østergaard wrote:
> On Thu, Sep 20, 2001 at 10:26:16AM +0200, David Hajek wrote:
> > Hi,
> > 
> > I have linux box with 70GB SW Raid1. This box runs for half
> > a year without problems but now I meet the high cpu load 
> > problems. I suspect that it can be caused by not enough 
> > free disk space on this md device. I see following:
> > 
> > 1 GB free  - load > 5
> > 5 GB free  - load < 1
> 
> RAID does not know about "free space" (that is a filesystem thing), so that
> would be either some strange interaction between the filesystem and lower
> layers, or a measurement error - I guess.
> 
> High fragmentation could lead to extra filesystem activity, but that's not
> really something the RAID can influence.

Why not? I do not say that it is cause be the RAID. It can be caused
by filesystem. Do you think that fsck helps to defrag filesystem?

> 
> Please check that your disks are using DMA (hdparm /dev/hdX). You should
> see something like:
> 
> [root@eagle /root]# hdparm /dev/hda
> 
> /dev/hda:
>  multcount    = 16 (on)
>  I/O support  =  0 (default 16-bit)
>  unmaskirq    =  0 (off)
>  using_dma    =  1 (on)                <=========== INDICATES DMA
>  keepsettings =  0 (off)
>  nowerr       =  0 (off)
>  readonly     =  0 (off)
>  readahead    =  8 (on)
>  geometry     = 782/128/63, sectors = 6306048, start = 0
> 
> Without DMA you will see high CPU load from accessing the disks, regardless of
> free space.

I use SCSI disks for this raid. Here is the output:

[root@marvin /root]# hdparm /dev/sda

/dev/sda:
 readonly     =  0 (off)
 geometry     = 8924/255/63, sectors = 143374738, start = 0
[root@marvin /root]# hdparm -t /dev/sda

  /dev/sda:
   Timing buffered disk reads:  64 MB in  1.87 seconds = 34.22 MB/sec
[root@marvin /root]#   

I think disk speeds are ok - it works for really fine for half a year.

-- 
David Hajek
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