[PATCH v2 3/3] qapi: deprecate drive-backup

Wed May 5 13:58:03 UTC 2021

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 at 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 ++-
 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.
 
 
 .. _`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``
+-----------------------------------------------------------------------
 
-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.
+
 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``
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+``blockdev-backup`` differs in a way of specifying 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
+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
+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.
 
 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.
+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.
+
 System accelerators
 -------------------
 
diff --git a/qapi/block-core.json b/qapi/block-core.json
index 6d227924d0..8e2c6e1622 100644
--- a/qapi/block-core.json
+++ b/qapi/block-core.json
@@ -1642,6 +1642,9 @@
 # The operation can be stopped before it has completed using the
 # block-job-cancel command.
 #
+# Features:
+# @deprecated: This command is deprecated. Use @blockdev-backup instead.
+#
 # Returns: - nothing on success
 #          - If @device is not a valid block device, GenericError
 #
@@ -1657,7 +1660,7 @@
 #
 ##
 { 'command': 'drive-backup', 'boxed': true,
-  'data': 'DriveBackup' }
+  'data': 'DriveBackup', 'features': ['deprecated'] }
 
 ##
 # @blockdev-backup:
-- 
2.29.2



