On 11.12.20 18:42, Vladimir Sementsov-Ogievskiy wrote:
11.12.2020 20:24, Max Reitz wrote:
On 11.12.20 17:50, Vladimir Sementsov-Ogievskiy wrote:
11.12.2020 19:05, Max Reitz wrote:
On 04.12.20 23:07, Vladimir Sementsov-Ogievskiy wrote:
The code already don't freeze base node and we try to make it prepared
for the situation when base node is changed during the operation. In
other words, block-stream doesn't own base node.
Let's introduce a new interface which should replace the current one,
which will in better relations with the code. Specifying bottom node
instead of base, and requiring it to be non-filter gives us the
following benefits:
- drop difference between above_base and base_overlay, which will be
renamed to just bottom, when old interface dropped
- clean way to work with parallel streams/commits on the same
backing
chain, which otherwise become a problem when we introduce a filter
for stream job
- cleaner interface. Nobody will surprised the fact that base
node may
disappear during block-stream, when there is no word about
"base" in
the interface.
Signed-off-by: Vladimir Sementsov-Ogievskiy <vsement...@virtuozzo.com>
---
qapi/block-core.json | 8 +++--
include/block/block_int.h | 1 +
block/monitor/block-hmp-cmds.c | 3 +-
block/stream.c | 50 +++++++++++++++++++---------
blockdev.c | 61
++++++++++++++++++++++++++++------
5 files changed, 94 insertions(+), 29 deletions(-)
diff --git a/qapi/block-core.json b/qapi/block-core.json
index 04055ef50c..5d6681a35d 100644
--- a/qapi/block-core.json
+++ b/qapi/block-core.json
@@ -2522,6 +2522,10 @@
# @base-node: the node name of the backing file.
# It cannot be set if @base is also set. (Since 2.8)
#
+# @bottom: the last node in the chain that should be streamed into
+# top. It cannot be set any of @base, @base-node or
@backing-file
s/set any/set if any/
But what’s the problem with backing-file? The fact that specifying
backing-file means that stream will look for that filename in the
backing chain when the job is done (so if you use @bottom, we
generally don’t want to rely on the presence of any nodes below it)?
I just wanted to deprecate 'backing-file' together with base and
base-node as a next step. If user wants to set backing file unrelated
to current backing-chain, is it correct at all? It's a direct
violation of what's going on, and I doubt that other parts of Qemu
working with backing-file are prepared for such situation. User can
do it by hand later.. Anyway, we'll have three releases deprecation
period for people to come and cry that this is a really needed
option, so we can support it later on demand.
(If so, I would have thought that we actually want the user to
specify backing-file so we don’t have to look down below @bottom to
look for a filename. Perhaps a @backing-fmt parameter would help.)
If we decide that 'backing-file' is really needed, than yes we should
require backing-fmt to be specified together with backing-file when
using new "bottom" interface.
Before I can agree on removing backing-file (or deprecating it), I
need to know what it’s actually used for. I actually don’t, though.
The only reason I could imagine was because the user wanted to write
some string into there that is different from base.filename.
(The original commit 13d8cc515df does mention cases like FD passing,
where qemu has no idea what an appropriate filename would be (it can
only see /dev/fd/*). From that, it does appear to me that it’ll be
needed even with @bottom.)
I should have checked it myself.. That's one more reason for my "RFC:
don't store backing filename in qcow2 image"..
OK, do you think we can require backing-fmt to be specified if
backing-file and bottom are specified?
Sure.
Or allow omitting it and
deprecate this thing? We actually already have deprecation message in
bdrv_change_backing_file(), and how we are trying to workaround it in
block-stream will not work with file descriptors anyway (hmm, and old
code works, so, actually 09 is a regression?)
I think requiring backing-fmt for bottom + backing-file would be the
most simple and clean way, hopefully saving us some headaches.
Max
