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Re: [PATCH] qapi: deprecate drive-backup
From: |
Peter Krempa |
Subject: |
Re: [PATCH] qapi: deprecate drive-backup |
Date: |
Mon, 26 Apr 2021 09:39:31 +0200 |
User-agent: |
Mutt/2.0.5 (2021-01-21) |
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@virtuozzo.com>
> ---
>
> 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 avoid problems with all that outdated option inheritance.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@redhat.com>
Re: [PATCH] qapi: deprecate drive-backup, Kashyap Chamarthy, 2021/04/27