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migration/docs: Move docs for postcopy blocktime feature

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Move it out of vanilla postcopy session, but instead a standalone feature.
When at it, removing the NOTE because it's incorrect now after introduction
of max-postcopy-bandwidth, which can control the throughput even for
postcopy phase.

Reviewed-by: Juraj Marcin <jmarcin@redhat.com>
Link: https://lore.kernel.org/r/20250613140801.474264-4-peterx@redhat.com
Signed-off-by: Peter Xu <peterx@redhat.com>
Signed-off-by: Fabiano Rosas <farosas@suse.de>
This commit is contained in:
Peter Xu 2025-06-13 10:07:53 -04:00 • committed by Fabiano Rosas
parent 2862d6d4fb
commit 35290df01b
1 changed files with 17 additions and 19 deletions
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36
docs/devel/migration/postcopy.rst
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@ -33,25 +33,6 @@ will now cause the transition from precopy to postcopy.
It can be issued immediately after migration is started or any
time later on. Issuing it after the end of a migration is harmless.
Blocktime is a postcopy live migration metric, intended to show how
long the vCPU was in state of interruptible sleep due to pagefault.
That metric is calculated both for all vCPUs as overlapped value, and
separately for each vCPU. These values are calculated on destination
side. To enable postcopy blocktime calculation, enter following
command on destination monitor:
``migrate_set_capability postcopy-blocktime on``
Postcopy blocktime can be retrieved by query-migrate qmp command.
postcopy-blocktime value of qmp command will show overlapped blocking
time for all vCPU, postcopy-vcpu-blocktime will show list of blocking
time per vCPU.
.. note::
During the postcopy phase, the bandwidth limits set using
``migrate_set_parameter`` is ignored (to avoid delaying requested pages that
the destination is waiting for).
Postcopy internals
==================
@ -312,3 +293,20 @@ explicitly) to be sent in a separate preempt channel, rather than queued in
the background migration channel. Anyone who cares about latencies of page
faults during a postcopy migration should enable this feature. By default,
it's not enabled.
Postcopy blocktime statistics
-----------------------------
Blocktime is a postcopy live migration metric, intended to show how
long the vCPU was in state of interruptible sleep due to pagefault.
That metric is calculated both for all vCPUs as overlapped value, and
separately for each vCPU. These values are calculated on destination
side. To enable postcopy blocktime calculation, enter following
command on destination monitor:
``migrate_set_capability postcopy-blocktime on``
Postcopy blocktime can be retrieved by query-migrate qmp command.
postcopy-blocktime value of qmp command will show overlapped blocking
time for all vCPU, postcopy-vcpu-blocktime will show list of blocking
time per vCPU.