Re: [PATCH] block/mirror: add 'write-blocking-after-ready' copy mode

[Fiona Ebner]

Am 07.12.22 um 14:27 schrieb Fiona Ebner:
> The new copy mode starts out in 'background' mode and switches to
> 'write-blocking' mode once the job transitions to ready.
> 
> Before switching to active mode and indicating that the drives are
> actively synced, it is necessary to have seen and handled all guest
> I/O. This is done by checking the dirty bitmap inside a drained
> section. Transitioning to ready is also only done at the same time.
> 
> The new mode is useful for management applications using drive-mirror
> in combination with migration. Currently, migration doesn't check on
> mirror jobs before inactivating the blockdrives, so it's necessary to
> either:
> 1) use the 'pause-before-switchover' migration capability and complete
>    mirror jobs before actually switching over.
> 2) use 'write-blocking' copy mode for the drive mirrors.
> 
> The downside with 1) is longer downtime for the guest, while the
> downside with 2) is that guest write speed is limited by the
> synchronous writes to the mirror target. The newly introduced copy
> mode reduces the time that limit is in effect.
> 
> Signed-off-by: Fiona Ebner <f.ebner@proxmox.com>
> ---
> 
> See [0] for a bit more context. While the new copy mode doesn't
> fundamentally improve the downside of 2) (especially when multiple
> drives are mirrored), it would still improve things a little. And I
> guess when trying to keep downtime short, guest write speed needs to
> be limited at /some/ point (always in the context of migration with
> drive-mirror of course). Ideally, that could go hand-in-hand with
> migration convergence, but that would require some larger changes to
> implement and introduce more coupling.
> 
> [0] https://lists.nongnu.org/archive/html/qemu-devel/2022-09/msg04886.html
> 
Ping. If there is no interest from upstream in such a feature, it would
still be nice to know.