5:11 a.m.
Hello,
* virsh(1) offers multiple options to initiate Postcopy migration:
1) virsh migrate --postcopy --postcopy-after-precopy
2) virsh migrate --postcopy + virsh migrate-postcopy
3) virsh migrate --postcopy --timeout <N> --timeout-postcopy
When Postcopy migration is invoked via options (2) or (3) above, the migrated guest on the
destination host hangs sometimes. But such a hang is not reproducible with option (1)
above.
* When using option (1) above, libvirtd(8) waits for the first pass of pre-copy to finish
before enabling postcopy migration.
* Does the same waiting happen when using options (2) and (3) above?
===
2024-07-24 14:16:27.448+0000: msg={"execute":"migrate"
2024-07-24 14:16:29.318+0000: msg={"execute":"migrate-start-postcopy"
2024-07-24 14:28:39.737+0000: msg={"execute":"migrate"
2024-07-24 14:28:41.119+0000: msg={"execute":"migrate-start-postcopy"
2024-07-24 14:44:11.684+0000: msg={"execute":"migrate"
2024-07-24 14:44:12.835+0000: msg={"execute":"migrate-start-postcopy"
2024-07-24 14:48:00.675+0000: msg={"execute":"migrate"
2024-07-24 14:48:02.319+0000: msg={"execute":"migrate-start-postcopy"
2024-07-24 15:03:36.110+0000: msg={"execute":"migrate"
2024-07-24 15:03:37.341+0000: msg={"execute":"migrate-start-postcopy"
2024-07-24 16:05:25.602+0000: msg={"execute":"migrate"
2024-07-24 16:05:26.756+0000: msg={"execute":"migrate-start-postcopy"
===
* While running migration tests with options (2) and (3) above, switch to postcopy appears
to happen within 2 seconds of starting migration.
- Is that reasonable time to switch from pre-copy to postcopy?
- Is there an ideal time to wait before switching to postcopy?
* The feature page below suggests to wait until one cycle of RAM migration has completed
-> https://wiki.qemu.org/Features/PostCopyLiveMigration
* I'd much appreciate any clarification/confirmation about this.
Thank you.
---
-Prasad
6:28 a.m.
On Thu, Aug 29, 2024 at 10:11:05 +0000, Prasad Pandit wrote:
Hello,
* virsh(1) offers multiple options to initiate Postcopy migration:
1) virsh migrate --postcopy --postcopy-after-precopy
2) virsh migrate --postcopy + virsh migrate-postcopy
3) virsh migrate --postcopy --timeout <N> --timeout-postcopy
When Postcopy migration is invoked via options (2) or (3) above, the migrated guest on
the destination host hangs sometimes. But such a hang is not reproducible with option (1)
above.
* When using option (1) above, libvirtd(8) waits for the first pass of pre-copy to finish
before enabling postcopy migration.
Right.
* Does the same waiting happen when using options (2) and (3) above?
No. The explicit "virsh migrate-postcopy" request expects the user to
decide when to switch to post-copy by monitoring the migration.
===
2024-07-24 14:16:27.448+0000: msg={"execute":"migrate"
2024-07-24 14:16:29.318+0000:
msg={"execute":"migrate-start-postcopy"
2024-07-24 14:28:39.737+0000: msg={"execute":"migrate"
2024-07-24 14:28:41.119+0000:
msg={"execute":"migrate-start-postcopy"
2024-07-24 14:44:11.684+0000: msg={"execute":"migrate"
2024-07-24 14:44:12.835+0000:
msg={"execute":"migrate-start-postcopy"
2024-07-24 14:48:00.675+0000: msg={"execute":"migrate"
2024-07-24 14:48:02.319+0000:
msg={"execute":"migrate-start-postcopy"
2024-07-24 15:03:36.110+0000: msg={"execute":"migrate"
2024-07-24 15:03:37.341+0000:
msg={"execute":"migrate-start-postcopy"
2024-07-24 16:05:25.602+0000: msg={"execute":"migrate"
2024-07-24 16:05:26.756+0000:
msg={"execute":"migrate-start-postcopy"
===
* While running migration tests with options (2) and (3) above, switch to postcopy
appears to happen within 2 seconds of starting migration.
- Is that reasonable time to switch from pre-copy to postcopy?
No, that's not very reasonable. Basically every memory page access would
have to be delayed until the page is transferred from the source host.
- Is there an ideal time to wait before switching to postcopy?
Not really. As the name suggests this is meant as a timeout, i.e.,
switch to post-copy if pre-copy migration is taking too long and thus is
unlikely to ever converge. So logically the timeout should be long
enough to give pre-copy migration to do its job. In this case, switching
to post-copy is an alternative approach to CPU throttling for helping
migration to converge.
* The feature page below suggests to wait until one cycle of RAM
migration has completed
-> https://wiki.qemu.org/Features/PostCopyLiveMigration
Right, that's definitely a good approach as only memory pages that
changed during migration will have to be transferred from the source.
Jirka
9:01 a.m.
Hello Jiri,
On Thursday 29 August, 2024 at 04:58:39 pm IST, Jiri Denemark <jdenemar(a)redhat.com>
wrote:
> * Does the same waiting happen when using options (2) and (3)
above?
No. The explicit "virsh migrate-postcopy" request expects the user to
decide when to switch to post-copy by monitoring the migration.
> * While running migration tests switch to postcopy appears to
> happen within 2 seconds of starting migration.
> - Is that reasonable time to switch from pre-copy to postcopy?
No, that's not very reasonable. Basically every memory page access would
have to be delayed until the page is transferred from the source host.
> - Is there an ideal time to wait before switching to postcopy?
Not really. As the name suggests this is meant as a timeout, i.e.,
switch to post-copy if pre-copy migration is taking too long and thus is
unlikely to ever converge. So logically the timeout should be long
enough to give pre-copy migration to do its job. In this case, switching
to post-copy is an alternative approach to CPU throttling for helping
migration to converge.
> * The feature page below suggests to wait until one cycle of RAM migration has
completed
> -> https://wiki.qemu.org/Features/PostCopyLiveMigration
Right, that's definitely a good approach as only memory pages that
changed during migration will have to be transferred from the source.
* Okay, great! Thanks so much for these details, I appreciate it.
Thank you.
---
- Prasad
12:42 a.m.
Hello Jiri, David
[+cc dgilbert]
On Thursday 29 August, 2024 at 04:58:39 pm IST, Jiri Denemark <jdenemar(a)redhat.com>
wrote:
> - Is there an ideal time to wait before switching to postcopy?
Not really. As the name suggests this is meant as a timeout,
So logically the timeout should be long enough to give pre-copy
migration time to do its job.
> * The feature page below suggests to wait until one cycle of RAM migration has
completed
> -> https://wiki.qemu.org/Features/PostCopyLiveMigration
Right, that's definitely a good approach as only memory pages that
changed during migration will have to be transferred from the source.
Some follow-up questions:
* Why is it recommended to complete at least one cycle of RAM migration before switching
to Postcopy mode?
* What are the risks if we switch to postcopy before completing one cycle of RAM
migration?
* Is it plausible to start migration in postcopy mode from the beginning? ie. without
the first pre-copy pass?
* I'm asking because even when we switch to postcopy before completing the first
(pre-copy) pass, the migration seems to work fine most of the times. Errors occur
occasionally. It does not _always_ fail with errors.
- Could we file bugs for this occasional failure? OR that is expected because we did not
wait to complete first (pre-copy) round of migration.
* I'd appreciate if you could help to clarify this.
Thank you.
---
-Prasad
109
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3 comments
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participants (2)
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Jiri Denemark -
Prasad Pandit