On Fri, Apr 23, 2021 at 15:59:00 +0300, Vladimir Sementsov-Ogievskiy wrote: > Modern way is using blockdev-add + blockdev-backup, which provides a > lot more control on how target is opened. > > As example of drive-backup problems consider the following: > > User of drive-backup expects that target will be opened in the same > cache and aio mode as source. Corresponding logic is in > drive_backup_prepare(), where we take bs->open_flags of source. > > It works rather bad if source was added by blockdev-add. Assume source > is qcow2 image. On blockdev-add we should specify aio and cache options > for file child of qcow2 node. What happens next: > > drive_backup_prepare() looks at bs->open_flags of qcow2 source node. > But there no BDRV_O_NOCAHE neither BDRV_O_NATIVE_AIO: BDRV_O_NOCAHE is > places in bs->file->bs->open_flags, and BDRV_O_NATIVE_AIO is nowhere, > as file-posix parse options and simply set s->use_linux_aio. > > Signed-off-by: Vladimir Sementsov-Ogievskiy <vsementsov@xxxxxxxxxxxxx> > --- > > Hi all! I remember, I suggested to deprecate drive-backup some time ago, > and nobody complain.. But that old patch was inside the series with > other more questionable deprecations and it did not landed. > > Let's finally deprecate what should be deprecated long ago. > > We now faced a problem in our downstream, described in commit message. > In downstream I've fixed it by simply enabling O_DIRECT and linux_aio > unconditionally for drive_backup target. But actually this just shows > that using drive-backup in blockdev era is a bad idea. So let's motivate > everyone (including Virtuozzo of course) to move to new interfaces and > avoid problems with all that outdated option inheritance. libvirt never used 'drive-backup' thus Reviewed-by: Peter Krempa <pkrempa@xxxxxxxxxx>
