Hello All, I stumbled upon this link [1] which among other things contains the following:
"iSCSI, FC, or other forms of direct attached storage are only safe to use with live migration if you use cache=none." How valid is this assertion with current QEMU versions? I checked out the source code and was left with the impression that during migration and *before* handling control to the destination, a flush is performed on all disks of the VM. Since the VM is started on the destination only after the flush is done, its very first read will bring consistent data from disk. I can understand that on the corner case in which the storage device has already been mapped and perhaps has data in the page cache of the destination node, there is no way to invalidate them, so the VM will read stale data, despite the flushes which happened at the source node. In our case, we provision VMs using our custom storage layer, called Archipelago [2], which presents volumes as block devices in the host. We would like to run VMs in cache=writeback mode. If we guarantee externally that there will be no incoherent cached data on the destination host of the migration (e.g., by making sure the volume is not mapped on the destination node before the migration), would it be safe to do so? Can you comment on the aforementioned approach? Please let me know if there's something I have misunderstood. [1] http://wiki.qemu.org/Migration/Storage [2] http://www.synnefo.org/docs/archipelago/latest Kind Regards, -- Filippos <philipg...@grnet.gr>
