On Thu, April 9, 2009 10:00 am, Iustin Pop wrote:
> On Wed, Apr 08, 2009 at 05:50:46PM -0400, Bill Davidsen wrote:
>> Steven Ellis wrote:
>>> I've resolved most of my raid issues by re-housing the affected system
>>> and replacing the motherboard, but across the 3 boards I've tried I
>>> always have an issue with my /dev/md1 array producing mismatch_count
>>> of 128 or 256.
>>>
>>> System is running Centos 5.2 with a Xen Dom0 kernel
>>>
>>> This md1 volume is a pair of 40GB HDs raid1 on an IDE controller which
>>> I them have a bunch of LVM's that are my Xen guests.
>>>
>>> Is there any chance that these mismatch_count values are due to swap
>>> partitions for the Xen guests?
>>
>> That's the cause, and since md code doesn't currently have a clue which
>> copy is "right" it's always a problem if you do something like suspend
>> to disk. You probably don't do that with xen images, but swap and raid1
>> almost always have a mismatch.
>
> But only because (for non-xen guests) the raid1 code and the swap code /
> data live in the same address space, and could be changed in between the
> two writes.
>
> I would be surprised if this happens for xen guests, where the address
> space is not shared; once the xen guest initiates a write, dom0 gets the
> data and writes it from its internal buffer, not the domU's one which
> could be modified.
>
> At least, that's how I think things happen.
The box has 5 Raid arrays
# cat /proc/mdstat
Personalities : [raid1] [raid6] [raid5] [raid4]
md3 : active raid1 sdb1[0] sda1[1]
976759936 blocks [2/2] [UU]
md0 : active raid1 hdb1[0] hda1[1]
128384 blocks [2/2] [UU]
md4 : active raid1 hdb2[1] hda2[0]
522048 blocks [2/2] [UU]
md2 : active raid5 hdh1[3] hdg1[2] hdf1[1] hde1[0]
732587712 blocks level 5, 64k chunk, algorithm 2 [4/4] [UUUU]
md1 : active raid1 hdb3[0] hda3[1]
38427392 blocks [2/2] [UU]
unused devices: <none>
md4 is the swap partition for the Xen Server, and md0 is the boot
partition. Neither of these have any issues.
md2 is a raid5 set that also hasn't been reporting any issues.
md3 is a new SATA based raid1 set that I had some issues with when using a
different motherboard, but doesn't produce any errors no even under
serious load.
md1 contains the root file system for my Xen server, plus the root + swap
partitions for my various Xen guests. This is the volume that is
generating the mismatch_count errors.
Now most of my Xen guests are presented with two LVM allocated partitions
out of md1, eg guest_root and guest_swap.
I do have an exception to this for one guest where I present a single LVM
partition as a virtual HD to the guest which then manages the
swap/root/home partitions itself.
I'm wondering if this presentation of a partition as a disk is the issue.
Steve
--------------------------------------------
Steven Ellis - Technical Director
OpenMedia Limited - The Home of myPVR
email - steven@xxxxxxxxxxxxxxx
website - http://www.openmedia.co.nz
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