Re: Using linux software raid (mdadm) in a shared-disk cluster.

[Goswin von Brederlow <goswin-v-b@web.de>]

John Hughes <john@Calva.COM> writes:

> Goswin von Brederlow wrote:
>> John Hughes <john@Calva.COM> writes:
>>
>>> I've got a little shared disk cluster (parallel SCSI, external DELL
>>> PV210 disk cabinet).
>>>
>>> I've used linux raid to make a nice RAID10 on the external disks.
>>>
>>> I can access this from either machine in the cluster, only one at a
>>> time of course, it works very well and I'm happy.
>>>
>>> Now I'm running XEN and I want to be able to migrate a XEN domU from
>>> one machine to the other while the domU is using the RAID10 device.  I
>>> can make this "work" using XEN's migration hooks - it calls a script
>>> when it has stopped the running domU and I can start the raid device
>>> on the destination node, ready for the arrival of the domU.
>>>
>>> There is one small problem - I can't stop the RAID10 on the source
>>> node until the domU has finished, so it seems to me there is a window
>>> that could lead to data corruption:
>>>
>>
>> Can you put it into read-only mode?
>>
>
> How do I do that?
>
> Ah, by "mdadm --readonly"[*].
>
>> I'm not even sure putting a raid in read-only mode will stop
>> background syncing.
>>
>
> Hang on a sec, I'll try it.
>
> ... run bonnie on mounted /dev/md0, kill it, umount the device ...
>
> # cat /proc/mstat
> Personalities : [raid10] md0 : active raid10 sda3[0] sde2[4](S)
> sdd3[3] sdc3[2] sdb3[1]
>      32067456 blocks 64K chunks 2 near-copies [4/4] [UUUU]
>      bitmap: 66/123 pages [264KB], 128KB chunk
>
> # mdadm --readonly /dev/md0
> # cat /proc/mdstat Personalities : [raid10] md0 : active (read-only)
> raid10 sda3[0] sde2[4](S) sdd3[3] sdc3[2] sdb3[1]
>      32067456 blocks 64K chunks 2 near-copies [4/4] [UUUU]
>      bitmap: 66/123 pages [264KB], 128KB chunk
>
> ... wait a bit
> # cat /proc/mdstat
> Personalities : [raid10] md0 : active (read-only) raid10 sda3[0]
> sde2[4](S) sdd3[3] sdc3[2] sdb3[1]
>      32067456 blocks 64K chunks 2 near-copies [4/4] [UUUU]
>      bitmap: 0/123 pages [0KB], 128KB chunk
>
> No, it seems to keep on syncing.  Bummer.
>
>> As an alternative approach how about running the raid10 inside the
>> domU?
>>
>
> Didn't want to do that as it requires exporting loadsa devices to the
> domU instead of one.
>
> But maybe it's the only way (without hacking the md code).

You could wait for the syncs to complete after the domU is
suspended. But that could mean hours if there is a resync or repair
running. For those you realy need a "pause" state.

MfG
        Goswin