[pve-devel] applied-series: [PATCH v3 qemu-server 1/2] migration: avoid crash with heavy IO on local VM disk

[Thomas Lamprecht]

Am 04/07/2024 um 11:32 schrieb Fiona Ebner:
> There is a possibility that the drive-mirror job is not yet done when
> the migration wants to inactivate the source's blockdrives:
> 
>> bdrv_co_write_req_prepare: Assertion `!(bs->open_flags & BDRV_O_INACTIVE)' failed.
> 
> This can be prevented by using the 'write-blocking' copy mode (also
> called active mode) for the mirror. However, with active mode, the
> guest write speed is limited by the synchronous writes to the mirror
> target. For this reason, a way to start out in the faster 'background'
> mode and later switch to active mode was introduced in QEMU 8.2.
> 
> The switch is done once the mirror job for all drives is ready to be
> completed to reduce the time spent where guest IO is limited.
> 
> The loop waiting for actively-synced to become true is not an endless
> loop: Once the remaining dirty parts have been mirrored by the
> background iteration, the actively-synced flag will be set. Because
> the 'block-job-change' QMP command already succeeded, new writes will
> be done synchronously to the target and thus not lead to new dirty
> parts. If the job fails or vanishes (shouldn't actually happen,
> because auto-dismiss is false), the loop will be exited and the error
> propagated.
> 
> Reported rarely, but steadily over the years:
> https://forum.proxmox.com/threads/78954/post-353651
> https://forum.proxmox.com/threads/78954/post-380015
> https://forum.proxmox.com/threads/100020/post-431660
> https://forum.proxmox.com/threads/111831/post-482425
> https://forum.proxmox.com/threads/111831/post-499807
> https://forum.proxmox.com/threads/137849/
> 
> Signed-off-by: Fiona Ebner <f.ebner at proxmox.com>
> ---
> 
> Changes in v3:
>     * avoid endless loop when job fails while switching to active mode
>     * mention rationale why loop is not and endless loop in commit
>       message
> 
>  PVE/QemuMigrate.pm                    |  8 +++++
>  PVE/QemuServer.pm                     | 51 +++++++++++++++++++++++++++
>  test/MigrationTest/QemuMigrateMock.pm |  6 ++++
>  3 files changed, 65 insertions(+)
> 
>

applied both patches, thanks!

Albeit I'm a bit wondering if we would be able to mock at a deeper level than, e.g.,
qemu_drive_mirror_switch_to_active_mode, maybe by mocking mon_cmd and then also
checking if they monitor commands are triggered as expected and in the correct order.
But that might affect a few more methods and definitively orthogonal to this.