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SubjectRe: high cpu load with sw raid1
Inlined.

On Thu, Sep 20, 2001, Jakob Řstergaard 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
hajek@idoox.com GSM: +420 604 352968
- You can't cheat the phone company.

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