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From: "Mark Trumpold" <markt@netqa.com>
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Date: Tue, 21 May 2013 20:01:10 +0000
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Subject: [Qemu-devel] 'qemu-nbd' explicit flush
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To: qemu-devel@nongnu.org
Cc: markt@tachyon.net

Hello all,

I am using =E2=80=98qemu-nbd=E2=80=99 and =E2=80=98qemu-img=E2=80=99 from=
 the command line to provide =E2=80=98qcow2=E2=80=99 loop filesystems.  F=
or performance, I have '--cache=3Dwriteback' set for the qemu-nbd device.=


I am having trouble flushing all caches to disk at will; specifically bef=
ore snapshotting the underlying filesystem that hold the =E2=80=98qcow2=E2=
=80=99 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=3Dwriteback -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 sn=
apshotting.

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 calle=
d from the NBD socket interface.  Here I see 'bdrv_co_flush(exp->bs)' whi=
ch looks promising; however, I don't know how to setup the 'bs' pointer f=
or the call.

Ideally, I would like to add a command line parm to 'qemu-nbd.c' to expli=
citely do the flush, but so far no luck.

I've been struggling with this for some time.  Any guidance would be grea=
tly appreciated.

Thank you,
Mark Trumpold