John Levon wrote:
On Thu, Oct 15, 2009 at 02:35:21AM -0700, Brian McKerr wrote:
The plan is to have the VM files synced across to the 'secondary' node
via ZFS send/receive so that should one fail I can simply restart the
VMs on the second node. I may even be able to spread the VM load
across the two nodes and sync each bunch of VMs to the 'other' node.
I'm not clear on your setup. So you have box A that is presumably
exporting ZFS volumes for each VM storage across iSCSI, and you want to
sync those volumes with send/receive across to a backup box.
Where are the VMs running? How often do you need this sync to happen?
Note that you can't sync when guests are running as the storage may not
be in a stable state filesystem-wise.
Concur with John, you won't be able to guarantee consistency of the
filesystem within the ZVOL. Your device/filesystem chain is probably
going to look like
guest FS (ZFS/ext3/etc) -> virtual disk -> iSCSI LU -> ZVOL
While ZFS will take care that anything written to the iSCSI target is
consistent, there are cases where the guest may have written only
partial filesystem data at the time you snapshot/send/receive. ZFS
should reduce that possibility but may not entirely eliminate it.
One idea that I had been thinking about but have not had a chance to
test is to export two iSCSI LUs from the target, one from each storage
server, and use them to create a mirrored filesystem on the guest.
Theoretically this should provide better redundancy if either storage
node goes down but I'm not sure how the iSCSI initiator will handle a
dead node (TCP timeouts, etc...)
--joe
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