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
