Kevin Wolf <kw...@redhat.com> writes:
> Am 02.10.2019 um 13:57 hat Markus Armbruster geschrieben:
>> Eric Blake <ebl...@redhat.com> writes:
>>
>> > On 10/1/19 2:34 PM, Markus Armbruster wrote:
>> >> Peter Krempa <pkre...@redhat.com> writes:
>> >>
>> >>> savevm was buggy as it considered all monitor owned block device nodes
>> >>
>> >> Recommend "monitor-owned block device nodes" or "block device nodes
>> >> owned by a monitor"
>> >>
>> >>> for snapshot. With introduction of -blockdev the common usage made all
>> >>> nodes including protocol nodes monitor owned and thus considered for
>> >>> snapshot.
>> >>
>> >> What exactly is / was the problem?
>> >
>> >
>> > Old way: using QMP add_device, you create a drive backend with two BDS
>> > (format and protocol) assigned to it; the drive backend has your given
>> > name, and both BDS have a generated name (beginning with '#'). The
>> > two BDS are not monitor-owned, rather, the drive is.
>> >
>> > New way: using QMP blockdev_add, you create the two BDS manually with
>> > names of your choice, then plug that blockdev into an unnamed
>> > blockbackend (the drive no longer needs a name, because you can get at
>> > everything through the BDS name). You _could_ do this in one step
>> > (the QAPI allows self-recursion where you can define both the format
>> > and protocol in one step), but it is easier to do in two steps (define
>> > the protocol BDS first, then define the format BDS using a "string"
>> > name of the protocol BDS instead of a { "driver":..., args... } object
>> > of the protocol layer. But by making two calls, now both BDS are
>> > monitor-owned.
>> >
>> > At snapshot-time, the code currently looks for all monitor-owned nodes
>> > when deciding what to snapshot. In the old way, this finds the named
>> > drive, picks up its associated top-most node, and snapshots the format
>> > layer. In the new way, the drive is unnamed so it is skipped, while
>> > there are two named BDS, but we don't want a snapshot of the protocol
>> > layer.
>>
>> So the problem is certain (common & sane) -blockdev use makes savevm
>> create additional, unwanted snapshots.
>
> Actually, the most common protocol driver is file-posix, which doesn't
> support snapshots, so usually the result was that savevm just fails
> because it can't snapshot something that it (incorrectly) thinks it
> should snapshot.
v3's commit message:
qapi: Allow introspecting fix for savevm's cooperation with blockdev
'savevm' was buggy as it considered all monitor-owned block device nodes
for snapshot. With introduction of -blockdev the common usage made all
nodes including protocol and backing file nodes monitor-owned and thus
considered for snapshot.
This is a problem since the 'file' protocol nodes can't have internal
snapshots and it does not make sense to take snapshot of nodes
representing backing files.
This was fixed by commit 05f4aced658a02b02 clients need to be able to
detect whether this fix is present.
Since savevm does not have an QMP alternative, add the feature for the
'human-monitor-command' backdoor which is used to call this command in
modern use.
Signed-off-by: Peter Krempa <pkre...@redhat.com>
Kevin, is this explanation sufficiently correct & complete?
