Execute pre-freeze and post-thaw
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No, it's not using the same mechanism. It's a PV call (if I remember, but I'm not sure). You cannot write a xenstore key in live to a VM (only on boot).
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And beyond that, I'm not sure we actually want this behaviour from a security perspective. But it's another topic.
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@olivierlambert ok, but it wouldn't have to. The guest tools always run the scripts (if they exist) when it sees a snap happening and then reports out the status the same way it says that the block commits are complete.
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They aren't meant for that, I have no idea how difficult it would be to make the PV drivers to actually "do something" (what exactly? write a value in the xenstore?). Maybe @teddyastie could provide an opinion on that.
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You can read key/values from the xenstore, and write some (from VM to outside), but you cannot write values "in live" from outside the VM to the inside.
It is, but XAPI doesn't provide a interface for it.
do the guest tools quiesce the filesystems before snapshotting?
Tools are aware of a snapshot so you don't have blocks in flight.
do the guest tools quiesce the filesystems before snapshotting?
Guests kernel are aware, as it is them that are performing a "suspend" on toolstack request (thus quiece filesystems); although "tools" can only observe that the system has been suspended after the fact by measuring side effects, and not orchestrate it.
It's because suspend/resume operation doesn't come from "guest tools" actually, but instead from the kernel drivers. So userland tools has no say on it.
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There used to be quiescent snapshot capabilities in older versions (mainly for Windows VSS support), but it has since been removed. I'd say @Team-XAPI-Network knows more about the reason.
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@dinhngtu It's possible to run hooks inside the control domain on different stages of VM lifecycle operations, but these need to be installed beforehand, see https://xapi-project.github.io/new-docs/xenopsd/design/hooks/index.html
I think the vm-pre-suspend is the name of the hook the does this, but I can't confirm it. The docuementation is missing the hook, but it exists in the source code: https://github.com/xapi-project/xen-api/blob/master/ocaml/xenopsd/lib/xenops_hooks.ml#L37
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@psafont Looks like these hooks are for suspend-resume rather than snapshot/backups? I couldn't find any mention of hook calls during the snapshot process
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@dinhngtu said in Execute pre-freeze and post-thaw:
There used to be quiescent snapshot capabilities in older versions (mainly for Windows VSS support), but it has since been removed. I'd say @Team-XAPI-Network knows more about the reason.
We have vmware with ppdm backups and the vss part is actually causing some annoying issues such as quite long io stalls. But I do understand the reason for vss, especially for applications that aren't crash safe.
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@dinhngtu Xenopsd does not have a concept of snapshot. Because I understand that "freezing" the VM means suspending it, I think that the suspend hook will get triggered in any snapshot with memory
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@psafont Freezing/thawing doesn't actually suspend a VM, but it's just a part of the backup process, where I/O operations are quiesced to create a stable view of the on-disk data.
