Hello all,
I am using ‘qemu-nbd’ and ‘qemu-img’ from the command line to provide ‘qcow2’
loop filesystems. For performance, I have '--cache=writeback' set for the
qemu-nbd device.
I am having trouble flushing all caches to disk at will; specifically before
snapshotting the underlying filesystem that hold the ‘qcow2’ images.
Environment:
Qemu 1.2.0
Debian 6.0.5
Linux kernel 3.3.1 with Qemu patch to enable kernel flushing:
http://thread.gmane.org/gmane.linux.drivers.nbd.general/1108
Usage example:
'qemu-nbd --cache=writeback -c /dev/nbd0 /images/my-qcow.img'
'mount /dev/nbd0 /my-mount-point'
Everything does flush correctly when I first unmount and then disconnect the
device; however, in my case I am not able to unmount things before snapshotting.
I tried several approaches externally to flush the device. For example:
'mount -o remount,ro /dev/nbd0'
'blockdev --flushbufs /dev/nbd0'
I have been looking at the Qemu source code and in user space 'nbd.c' in
routine 'nbd_trip' I see the case 'NBD_CMD_FLUSH' which looks to be called from
the NBD socket interface. Here I see 'bdrv_co_flush(exp->bs)' which looks
promising; however, I don't know how to setup the 'bs' pointer for the call.
Ideally, I would like to add a command line parm to 'qemu-nbd.c' to explicitely
do the flush, but so far no luck.
I've been struggling with this for some time. Any guidance would be greatly
appreciated.
Thank you,
Mark Trumpold
