Re: [opensuse] Curious about different software raid signatures in top (md & dm raid)

Hi there,

On Fri, 13 Feb 2009, 04:51:49 +0100, David C. Rankin wrote:
Listmates,

I am curious. I have several boxes that have regular software raid (mdraid)
and several boxes that make use of bios raid (dmraid). Looking at top on the
boxes with mdraid I notice that once every second or so the disks in the array
are 'talking?' to each other which produces and nice rhythmic signature in top
like:

1931 root 10 -5 0 0 0 S 0.7 0.0 4:59.63 md2_raid1
1090 root 10 -5 0 0 0 S 0.3 0.0 2:44.20 md1_raid1

Noticing this I checked the bios raid boxes to see if they had a similar
indication in top. They don't. I know both md and dm raid are different
software raid implementations for all practical purposes, but I was curious why
md raid would show up in tom and dmraid doesn't. Does anybody have a one or two
line synopsis they could share?

Next, when I see the mdraid make its fleeting appearance in top, what is
actually going on with the array? Guessing, it would appear that as part of the
normal operation, the disks in the array (md1_raid1 & md2_raid1, here) are
checking sync status and seeing if anything new has appeared that needs to be
duplicated from one disk to the other. Is that all that is going on or its it
something else?

It's a good thing I'm not a cat, it would have killed me a long time ago;-)

Looks like you're using a write-intent bitmap for your md-raid1's, which
is used to limit the amount of data to be re-synchronized in the case
something went wrong with one of the raid1 devices.

I'd suggest looking at the details of your md-raid1:

mdadm -Q -D /dev/md1

If it contains an indication about the use of an "Intent Bitmap", then
you know what's causing the traffic. If you don't want it to happen, you
can turn it off again with

mdadm --grow /dev/md1 --bitmap=none

which will cause a full resync in case a device of that md-raid1 device
is happening to fail. Full details in the mdadm manual page.

HTH, cheers.

l8er
manfred
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