On 4/26/21 2:41 PM, Vladimir Sementsov-Ogievskiy wrote:
26.04.2021 21:30, John Snow wrote:
On 4/26/21 2:05 PM, Daniel P. Berrangé wrote:
On Mon, Apr 26, 2021 at 09:00:36PM +0300, Vladimir
Sementsov-Ogievskiy wrote:
26.04.2021 20:34, John Snow wrote:
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@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avoid problems with all that outdated option inheritance.
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.
(Here, I accidentally used the names of the argument objects instead of
the names of the commands. It's likely better to spell out the names of
the commands instead.)
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",
}
Good ideas. Hmm. Do you think that the whole explanation with
examples should be here, in docs/system/deprecated.rst ? Seems too
big in comparison with other deprecations
No, the deprecations entry should be short.
If the replacement is so complex that it requires us to provide
examples,
that's a sign that the replacement is insufficiently documented in its
own right. IOW, add all this docs info to a suitable place in the main
QEMU documentation, and just link to that from the deprecations page.
Agree with Dan, keep the deprec section short.
Perhaps we can add an example to the live block ops doc that shows an
example of how to transition and link to that example from here.
OK, will do
Thanks! I'm sure Eric and I can help copy-edit. It will be nice to get
rid of a confusing, mostly duplicated interface.
(Even if it is very convenient ...)
It's likely we'll want some iotests functions to help streamline the
process. Important things for me to consider while working on an API
helper library, too.
--js
