Using linux software raid (mdadm) in a shared-disk cluster.
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:
Source node Destination node
mdadm --assemble /dev/md0 ....
Start migrate
domU suspended
call migration script
\--------------------> mdadm --assemble /dev/md0 ...
domU starts running
...
domU destroyed
mdadm --stop /dev/md0
I seems to me that the source node could still be messing with the bitmap and resyncing between the moment the destination node starts the RAID10 and the source node stops it[*].
Am I right? Is there a window?
If there is a window it could be closed if there was some kind of mdadm --freeze command which would stop the sync activity, which could be run on the source node before doing the assemble on the destination node.
([*] - imagine some block is marked unsynced in the bitmap. The destination node does the assemble, so now it's in-memory bitmap has the block marked. The source node syncs the block, updates the on disk bitmap. Now the destination node happens to write that block, it thinks the block is marked unsynced on the disk so it doesn't bother updating the bitmnap. If the destination node crashes at this point there is a block on the disk that is unsyced, but the bitmap claims it's in sync.)
Quoted 14 lines · John Hughes
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.
Quoted 3 lines
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?
Quoted 13 lines
Source node Destination node
mdadm --assemble
/dev/md0....
Start migrate
domU suspended
call migration script
\--------------------> mdadm --assemble/dev/md0...
domU starts running
...
domU destroyed
mdadm --stop/dev/md0
Quoted 5 lines
I seems to me that the source node could still be messing with the bitmap and resyncing between the moment the destination node starts the RAID10 and the source node stops it[*].
Am I right? Is there a window?
Certainly.
Quoted 5 lines
If there is a window it could be closed if there was some kind of mdadm --freeze command which would stop the sync activity, which could be run on the source node before doing the assemble on the destination node.
Quoted 8 lines
([*] - imagine some block is marked unsynced in the bitmap. The destination node does the assemble, so now it's in-memory bitmap has the block marked. The source node syncs the block, updates the on
disk bitmap. Now the destination node happens to write that block,
it thinks the block is marked unsynced on the disk so it doesn't bother updating the bitmnap. If the destination node crashes at this point there is a block on the disk that is unsyced, but the bitmap claims it's in sync.)
Source node Destination node
read block X for sync
Write block X Write mirror of block X
write mirror of block X
Now block X and its mirror have different content while being marked in sync.
I'm not even sure putting a raid in read-only mode will stop background syncing.
As an alternative approach how about running the raid10 inside the domU?
MfG
Goswin
Quoted 16 lines · Goswin von Brederlow
Quoted 14 lines · John Hughes
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.
Quoted 7 lines
Quoted 4 lines
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"[*].
Quoted 3 lines
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 chunkNo, it seems to keep on syncing. Bummer.
Quoted 3 lines
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).
Quoted 58 lines · John Hughes
Quoted 16 lines · Goswin von Brederlow
Quoted 14 lines · John Hughes
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.
Quoted 7 lines
Quoted 4 lines
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"[*].
Quoted 3 lines
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 chunkNo, it seems to keep on syncing. Bummer.
Quoted 8 lines
Quoted 3 lines
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