From: Fiona Ebner <f.ebner@proxmox.com>
To: pve-devel@lists.proxmox.com
Subject: Re: [pve-devel] [PATCH-SERIES qemu 0/6] async snapshot improvements
Date: Mon, 31 Mar 2025 17:06:59 +0200 [thread overview]
Message-ID: <8b3982fe-9944-4cc2-8028-e449e1bc0895@proxmox.com> (raw)
In-Reply-To: <20250331145507.196208-1-f.ebner@proxmox.com>
Am 31.03.25 um 16:55 schrieb Fiona Ebner:
> Most importantly, start using a dedicated IO thread for the state
> file when doing a live snapshot.
>
> Having the state file be in the iohandler context means that a
> blk_drain_all() call in the main thread or vCPU thread that happens
> while the snapshot is running will result in a deadlock.
Forgot to mention, this is easily reproducible by shutting down or
rebooting a Linux VM with SCSI disk (from within the guest) while a
snapshot is being taken. Patch 5/6 contains technical details.
Note that shutting down the VM will still result in failure after these
patches, but it's much better, because it's not a deadlock anymore and I
guess it's kinda philosophically okay, because there can be no VM state
of a shutdown VM. Can still be changed later if we want to do something
differently there.
Much more interesting is the reboot scenario of course (think in the
context of time-based automated snapshots where the guest might be doing
whatever at the time a snapshot is taken, in particular reboot), which
now should work seamlessly and not result in a deadlock either anymore.
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next prev parent reply other threads:[~2025-03-31 15:07 UTC|newest]
Thread overview: 9+ messages / expand[flat|nested] mbox.gz Atom feed top
2025-03-31 14:55 Fiona Ebner
2025-03-31 14:55 ` [pve-devel] [PATCH qemu 1/6] savevm-async: improve setting state of snapshot operation in savevm-end handler Fiona Ebner
2025-03-31 14:55 ` [pve-devel] [PATCH qemu 2/6] savevm-async: rename saved_vm_running to vm_needs_start Fiona Ebner
2025-03-31 14:55 ` [pve-devel] [PATCH qemu 3/6] savevm-async: improve runstate preservation Fiona Ebner
2025-03-31 14:55 ` [pve-devel] [PATCH qemu 4/6] savevm-async: cleanup error handling in savevm_start Fiona Ebner
2025-03-31 14:55 ` [pve-devel] [PATCH qemu 5/6] savevm-async: use dedicated iothread for state file Fiona Ebner
2025-03-31 14:55 ` [pve-devel] [PATCH qemu 6/6] savevm-async: treat failure to set iothread context as a hard failure Fiona Ebner
2025-03-31 15:06 ` Fiona Ebner [this message]
2025-04-02 15:35 ` [pve-devel] applied-series: [PATCH-SERIES qemu 0/6] async snapshot improvements Wolfgang Bumiller
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