You are correct. The disk image(s) must be available to both the sending and
receiving qemu/kvm processes. Otherwise, we would have to copy (or rsync if a
base copy exists) the disk image. This copy operation would usually take a long
time as the image is very big.
Also, writing to temporary local files (e.g. -snapshot) would fail the
migration process.
Before transferring the VM state, the migration code makes sure all IO
operations to disk have been completed (waiting for aio/BH operations to
complete, which does not mean all the way to the disk yet), and calls fsync on
the disk image file descriptor (bdrv_flush). It is assumed that the file system
flushes all data to disk when fsync is requested.
Theoretically, I should add a close/open synchronization too.
________________________________
From: [EMAIL PROTECTED] on behalf of Troy Benjegerdes
Sent: Fri 20/04/2007 00:39
To: kvm-devel@lists.sourceforge.net
Subject: [kvm-devel] Migration and disk-image filesystem semantics?
The kvm migration wiki seems to imply (but not directly state) that the
disk image file must be on a shared filesystem for both the sending and
the receiving kvm/qemu processes.
Is this correct? And if so, what are the semantics that kvm/qemu
requires of the filesystem hosting the image? Is there any
locking/synchronization/flushing that goes on?
--
--------------------------------------------------------------------------
Troy Benjegerdes 'da hozer' [EMAIL PROTECTED]
Somone asked me why I work on this free (http://www.fsf.org/philosophy/)
software stuff and not get a real job. Charles Shultz had the best answer:
"Why do musicians compose symphonies and poets write poems? They do it
because life wouldn't have any meaning for them if they didn't. That's why
I draw cartoons. It's my life." -- Charles Shultz
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