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Re: [PATCH v3 0/7] migration: bring improved savevm/loadvm/delvm to QMP
From: |
Dr. David Alan Gilbert |
Subject: |
Re: [PATCH v3 0/7] migration: bring improved savevm/loadvm/delvm to QMP |
Date: |
Fri, 11 Sep 2020 12:52:04 +0100 |
User-agent: |
Mutt/1.14.6 (2020-07-11) |
Kevin:
While we're still arguing about details of the last commit; can we get
the first few commits in - they seem to be generally nice cleanups/error
handling.
Dave
* Daniel P. Berrangé (berrange@redhat.com) wrote:
> v1: https://lists.gnu.org/archive/html/qemu-devel/2020-07/msg00866.html
> v2: https://lists.gnu.org/archive/html/qemu-devel/2020-07/msg07523.html
>
> When QMP was first introduced some 10+ years ago now, the snapshot
> related commands (savevm/loadvm/delvm) were not converted. This was
> primarily because their implementation causes blocking of the thread
> running the monitor commands. This was (and still is) considered
> undesirable behaviour both in HMP and QMP.
>
> In theory someone was supposed to fix this flaw at some point in the
> past 10 years and bring them into the QMP world. Sadly, thus far it
> hasn't happened as people always had more important things to work
> on. Enterprise apps were much more interested in external snapshots
> than internal snapshots as they have many more features.
>
> Meanwhile users still want to use internal snapshots as there is
> a certainly simplicity in having everything self-contained in one
> image, even though it has limitations. Thus the apps that end up
> executing the savevm/loadvm/delvm via the "human-monitor-command"
> QMP command.
>
> IOW, the problematic blocking behaviour that was one of the reasons
> for not having savevm/loadvm/delvm in QMP is experienced by applications
> regardless. By not portting the commands to QMP due to one design flaw,
> we've forced apps and users to suffer from other design flaws of HMP (
> bad error reporting, strong type checking of args, no introspection) for
> an additional 10 years. This feels rather sub-optimal :-(
>
> In practice users don't appear to care strongly about the fact that these
> commands block the VM while they run. I might have seen one bug report
> about it, but it certainly isn't something that comes up as a frequent
> topic except among us QEMU maintainers. Users do care about having
> access to the snapshot feature.
>
> Where I am seeing frequent complaints is wrt the use of OVMF combined
> with snapshots which has some serious pain points. This is getting worse
> as the push to ditch legacy BIOS in favour of UEFI gain momentum both
> across OS vendors and mgmt apps. Solving it requires new parameters to
> the commands, but doing this in HMP is super unappealing.
>
> After 10 years, I think it is time for us to be a little pragmatic about
> our handling of snapshots commands. My desire is that libvirt should never
> use "human-monitor-command" under any circumstances, because of the
> inherant flaws in HMP as a protocol for machine consumption.
>
> Thus in this series I'm proposing a fairly direct mapping of the existing
> HMP commands for savevm/loadvm/delvm into QMP as a first step. This does
> not solve the blocking thread problem, but it does put in a place a
> design using the jobs framework which can facilitate solving it later.
> It does also solve the error reporting, type checking and introspection
> problems inherant to HMP. So we're winning on 3 out of the 4 problems,
> and pushed apps to a QMP design that will let us solve the last
> remaining problem.
>
> With a QMP variant, we reasonably deal with the problems related to OVMF:
>
> - The logic to pick which disk to store the vmstate in is not
> satsifactory.
>
> The first block driver state cannot be assumed to be the root disk
> image, it might be OVMF varstore and we don't want to store vmstate
> in there.
>
> - The logic to decide which disks must be snapshotted is hardwired
> to all disks which are writable
>
> Again with OVMF there might be a writable varstore, but this can be
> raw rather than qcow2 format, and thus unable to be snapshotted.
> While users might wish to snapshot their varstore, in some/many/most
> cases it is entirely uneccessary. Users are blocked from snapshotting
> their VM though due to this varstore.
>
> These are solved by adding two parameters to the commands. The first is
> a block device node name that identifies the image to store vmstate in,
> and the second is a list of node names to include for the snapshots.
> If the list of nodes isn't given, it falls back to the historical
> behaviour of using all disks matching some undocumented criteria.
>
> In the block code I've only dealt with node names for block devices, as
> IIUC, this is all that libvirt should need in the -blockdev world it now
> lives in. IOW, I've made not attempt to cope with people wanting to use
> these QMP commands in combination with -drive args, as libvirt will
> never use -drive with a QEMU new enough to have these new commands.
>
> The main limitations of this current impl
>
> - The snapshot process runs serialized in the main thread. ie QEMU
> guest execution is blocked for the duration. The job framework
> lets us fix this in future without changing the QMP semantics
> exposed to the apps.
>
> - Most vmstate loading errors just go to stderr, as they are not
> using Error **errp reporting. Thus the job framework just
> reports a fairly generic message
>
> "Error -22 while loading VM state"
>
> Again this can be fixed later without changing the QMP semantics
> exposed to apps.
>
> I've done some minimal work in libvirt to start to make use of the new
> commands to validate their functionality, but this isn't finished yet.
>
> My ultimate goal is to make the GNOME Boxes maintainer happy again by
> having internal snapshots work with OVMF:
>
> https://gitlab.gnome.org/GNOME/gnome-boxes/-/commit/c486da262f6566326fbcb5e=
> f45c5f64048f16a6e
>
> Changed in v3:
>
> - Schedule a bottom half to escape from coroutine context in
> the jobs. This is needed because the locking in the snapshot
> code goes horribly wrong when run from a background coroutine
> instead of the main event thread.
>
> - Re-factor way we iterate over devices, so that we correctly
> report non-existant devices passed by the user over QMP.
>
> - Add QAPI docs notes about limitations wrt vmstate error
> reporting (it all goes to stderr not an Error **errp)
> so QMP only gets a fairly generic error message currently.
>
> - Add I/O test to validate many usage scenarios / errors
>
> - Add I/O test helpers to handle QMP events with a deterministic
> ordering
>
> - Ensure 'delete-snapshot' reports an error if requesting
> delete from devices that don't support snapshot, instead of
> silently succeeding with no erro.
>
> Changed in v2:
>
> - Use new command names "snapshot-{load,save,delete}" to make it
> clear that these are different from the "savevm|loadvm|delvm"
> as they use the Job framework
>
> - Use an include list for block devs, not an exclude list
>
> Daniel P. Berrang=C3=A9 (7):
> migration: improve error reporting of block driver state name
> block: push error reporting into bdrv_all_*_snapshot functions
> migration: stop returning errno from load_snapshot()
> block: add ability to specify list of blockdevs during snapshot
> block: allow specifying name of block device for vmstate storage
> iotests: add support for capturing and matching QMP events
> migration: introduce snapshot-{save,load,delete} QMP commands
>
> block/monitor/block-hmp-cmds.c | 7 +-
> block/snapshot.c | 233 ++++++++++++++-------
> include/block/snapshot.h | 19 +-
> include/migration/snapshot.h | 10 +-
> migration/savevm.c | 258 +++++++++++++++++++----
> monitor/hmp-cmds.c | 11 +-
> qapi/job.json | 9 +-
> qapi/migration.json | 135 ++++++++++++
> replay/replay-snapshot.c | 4 +-
> softmmu/vl.c | 2 +-
> tests/qemu-iotests/267.out | 14 +-
> tests/qemu-iotests/310 | 255 +++++++++++++++++++++++
> tests/qemu-iotests/310.out | 369 +++++++++++++++++++++++++++++++++
> tests/qemu-iotests/common.qemu | 107 +++++++++-
> tests/qemu-iotests/group | 1 +
> 15 files changed, 1289 insertions(+), 145 deletions(-)
> create mode 100755 tests/qemu-iotests/310
> create mode 100644 tests/qemu-iotests/310.out
>
> --=20
> 2.26.2
>
--
Dr. David Alan Gilbert / dgilbert@redhat.com / Manchester, UK
- Re: [PATCH v3 0/7] migration: bring improved savevm/loadvm/delvm to QMP,
Dr. David Alan Gilbert <=