On 4/23/21 8:59 AM, 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.
No complaints from me, especially if Virtuozzo is on board. I would like to see
some documentation changes alongside this deprecation, though.
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avoid problems with all that outdated option inheritance.
docs/system/deprecated.rst | 5 +++++
qapi/block-core.json | 5 ++++-
2 files changed, 9 insertions(+), 1 deletion(-)
diff --git a/docs/system/deprecated.rst b/docs/system/deprecated.rst
index 80cae86252..b6f5766e17 100644
--- a/docs/system/deprecated.rst
+++ b/docs/system/deprecated.rst
@@ -186,6 +186,11 @@ 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 6.0)
+''''''''''''''''''''''''''''
+
+Use ``blockdev-backup`` in pair with ``blockdev-add`` instead.
+
1) Let's add a sphinx reference to
https://qemu-project.gitlab.io/qemu/interop/live-block-operations.html#live-disk-backup-drive-backup-and-blockdev-backup
2) Just a thought, not a request: We also may wish to update
https://qemu-project.gitlab.io/qemu/interop/bitmaps.html to use the new,
preferred method. However, this doc is a bit old and is in need of an overhaul
anyway (Especially to add the NBD pull workflow.) Since the doc is in need of
an overhaul anyway, can we ask Kashyap to help us here, if he has time?
3) Let's add a small explanation here that outlines the differences in using
these two commands. Here's a suggestion:
This change primarily separates the creation/opening process of the backup target with explicit, separate steps.
BlockdevBackup uses mostly the same arguments as DriveBackup, except the "format" and "mode"
options are removed in favor of using explicit "blockdev-create" and "blockdev-add" calls.
The "target" argument changes semantics. It no longer accepts filenames, and
will now additionally accept arbitrary node names in addition to device names.
4) Also not a request: If we want to go above and beyond, it might be nice to
spell out the exact steps required to transition from the old interface to the
new one. Here's a (hasty) suggestion for how that might look:
- The MODE argument is deprecated.
- "existing" is replaced by using "blockdev-add" commands.
- "absolute-paths" is replaced by using "blockdev-add" and
"blockdev-create" commands.
- The FORMAT argument is deprecated.
- Format information is given to "blockdev-add"/"blockdev-create".
- The TARGET argument has new semantics:
- Filenames are no longer supported, use blockdev-add/blockdev-create
as necessary instead.
- Device targets remain supported.
Example:
drive-backup $ARGS format=$FORMAT mode=$MODE target=$FILENAME becomes:
(taking some liberties with syntax to just illustrate the idea ...)
blockdev-create options={
"driver": "file",
"filename": $FILENAME,
"size": 0,
}
blockdev-add arguments={
"driver": "file",
"filename": $FILENAME,
"node-name": "Example_Filenode0"
}
blockdev-create options={
"driver": $FORMAT,
"file": "Example_Filenode0",
"size": $SIZE,
}
blockdev-add arguments={
"driver": $FORMAT,
"file": "Example_Filenode0",
"node-name": "Example_Formatnode0",
}
blockdev-backup arguments={
$ARGS ...,
"target": "Example_Formatnode0",
}