On Wed, May 05, 2021 at 04:58:03PM +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.
The documentation is updated in a minimal way, so that drive-backup is noted only as a deprecated command, and blockdev-backup used in most of places.
Signed-off-by: Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com> ---
TODO: We also need to deprecate drive-backup transaction action.. But union members in QAPI doesn't support 'deprecated' feature. I tried to dig a bit, but failed :/ Markus, could you please help with it? At least by advice?
docs/interop/live-block-operations.rst | 47 +++++++++++++++++--------- docs/system/deprecated.rst | 11 ++++++ qapi/block-core.json | 5 ++-
The core changes itself looks good; I have some minor nit-picks below, hope that's not annoying. :-) With those addressed: Reviewed-by: Kashyap Chamarthy <kchamart@redhat.com>
3 files changed, 46 insertions(+), 17 deletions(-)
diff --git a/docs/interop/live-block-operations.rst b/docs/interop/live-block-operations.rst index 1073b930dc..f71f79ae2a 100644 --- a/docs/interop/live-block-operations.rst +++ b/docs/interop/live-block-operations.rst @@ -116,8 +116,8 @@ QEMU block layer supports. (3) ``drive-mirror`` (and ``blockdev-mirror``): Synchronize a running disk to another image.
-(4) ``drive-backup`` (and ``blockdev-backup``): Point-in-time (live) copy - of a block device to a destination. +(4) ``blockdev-backup`` (and deprecated ``drive-backup``): Point-in-time + (live) copy of a block device to a destination.
nit: s/deprecated ``drive-backup``/the deprecated ``drive-backup``/
.. _`Interacting with a QEMU instance`: @@ -553,13 +553,14 @@ Currently, there are four different kinds:
(3) ``none`` -- Synchronize only the new writes from this point on.
- .. note:: In the case of ``drive-backup`` (or ``blockdev-backup``), - the behavior of ``none`` synchronization mode is different. - Normally, a ``backup`` job consists of two parts: Anything - that is overwritten by the guest is first copied out to - the backup, and in the background the whole image is - copied from start to end. With ``sync=none``, it's only - the first part. + .. note:: In the case of ``blockdev-backup`` (or deprecated + ``drive-backup``), the behavior of ``none`` + synchronization mode is different. Normally, a + ``backup`` job consists of two parts: Anything that is + overwritten by the guest is first copied out to the + backup, and in the background the whole image is copied + from start to end. With ``sync=none``, it's only the + first part.
(4) ``incremental`` -- Synchronize content that is described by the dirty bitmap @@ -924,19 +925,22 @@ Shutdown the guest, by issuing the ``quit`` QMP command:: }
-Live disk backup --- ``drive-backup`` and ``blockdev-backup`` -------------------------------------------------------------- +Live disk backup --- ``blockdev-backup`` and deprecated``drive-backup`` +-----------------------------------------------------------------------
Here too, missing the article "the": "the deprecated".
-The ``drive-backup`` (and its newer equivalent ``blockdev-backup``) allows +The ``blockdev-backup`` (and deprecated ``drive-backup``) allows you to create a point-in-time snapshot.
-In this case, the point-in-time is when you *start* the ``drive-backup`` -(or its newer equivalent ``blockdev-backup``) command. +In this case, the point-in-time is when you *start* the +``blockdev-backup`` (or deprecated ``drive-backup``) command.
QMP invocation for ``drive-backup`` ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+Note that ``drive-backup`` command is deprecated since Qemu 6.1 and +will be removed in future.
nit: Let's consistently spell QEMU in all caps, please: s/Qemu/QEMU/
Yet again, starting afresh with our example disk image chain::
[A] <-- [B] <-- [C] <-- [D] @@ -961,11 +965,22 @@ will be issued, indicating the live block device job operation has completed, and no further action is required.
+Moving from deprecated ``drive-backup`` to newer ``blockdev-backup`` +~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
s/from/from the/
+``blockdev-backup`` differs in a way of specifying backup target.
I might slightly rephrase it this way: ``blockdev-backup`` differs from ``drive-backup`` in how you specify the backup target.
+With ``blockdev-backup`` you can't specify filename as a target. +Instead you use node-name of existing block node, which you may add
Can use literals also for node-name: s/node-name/``node-name``
+by ``blockdev-add`` or ``blockdev-create`` commands. Correspondingly, +``blockdev-backup`` doesn't have ``mode`` and ``format`` arguments +which don't apply to existing block node. See following sections for
s/to/to an/
+details and examples. + + Notes on ``blockdev-backup`` ~~~~~~~~~~~~~~~~~~~~~~~~~~~~
-The ``blockdev-backup`` command is equivalent in functionality to -``drive-backup``, except that it operates at node-level in a Block Driver +The ``blockdev-backup`` operates at node-level in a Block Driver State (BDS) graph.
s/``blockdev-backup``/``blockdev-backup`` command/
E.g. the sequence of actions to create a point-in-time backup diff --git a/docs/system/deprecated.rst b/docs/system/deprecated.rst index 80cae86252..676d72a1ed 100644 --- a/docs/system/deprecated.rst +++ b/docs/system/deprecated.rst @@ -186,6 +186,17 @@ Use the more generic commands ``block-export-add`` and ``block-export-del`` instead. As part of this deprecation, where ``nbd-server-add`` used a single ``bitmap``, the new ``block-export-add`` uses a list of ``bitmaps``.
+``drive-backup`` (since 6.1) +'''''''''''''''''''''''''''' + +Use ``blockdev-backup`` in pair with ``blockdev-add`` instead.
nit: s/in pair/in combination/
+This change primarily separates the creation/opening process of the backup +target with explicit, separate steps. ``blockdev-backup`` uses mostly the +same arguments as ``drive-backup``, except the ``format`` and ``mode`` +options are removed in favor of using explicit ``blockdev-create`` and +``blockdev-add`` calls. See :doc:`/interop/live-block-operations` for +details.
The rest reads good to me. Thanks for fixing this. [...] -- /kashyap