Markus Armbruster <arm...@redhat.com> writes:
> Fabiano Rosas <faro...@suse.de> writes:
>
>> Add a new migration capability 'fixed-ram'.
>>
>> The core of the feature is to ensure that each RAM page has a specific
>> offset in the resulting migration stream. The reasons why we'd want
>> such behavior are:
>>
>> - The resulting file will have a bounded size, since pages which are
>> dirtied multiple times will always go to a fixed location in the
>> file, rather than constantly being added to a sequential
>> stream. This eliminates cases where a VM with, say, 1G of RAM can
>> result in a migration file that's 10s of GBs, provided that the
>> workload constantly redirties memory.
>>
>> - It paves the way to implement O_DIRECT-enabled save/restore of the
>> migration stream as the pages are ensured to be written at aligned
>> offsets.
>>
>> - It allows the usage of multifd so we can write RAM pages to the
>> migration file in parallel.
>>
>> For now, enabling the capability has no effect. The next couple of
>> patches implement the core functionality.
>>
>> Signed-off-by: Fabiano Rosas <faro...@suse.de>
>
> [...]
>
>> diff --git a/qapi/migration.json b/qapi/migration.json
>> index 5a565d9b8d..3fce5fe53e 100644
>> --- a/qapi/migration.json
>> +++ b/qapi/migration.json
>> @@ -531,6 +531,10 @@
>> # and can result in more stable read performance. Requires KVM
>> # with accelerator property "dirty-ring-size" set. (Since 8.1)
>> #
>> +# @fixed-ram: Migrate using fixed offsets in the migration file for
>> +# each RAM page. Requires a migration URI that supports seeking,
>> +# such as a file. (since 9.0)
>> +#
>> # Features:
>> #
>> # @deprecated: Member @block is deprecated. Use blockdev-mirror with
>> @@ -555,7 +559,7 @@
>> { 'name': 'x-ignore-shared', 'features': [ 'unstable' ] },
>> 'validate-uuid', 'background-snapshot',
>> 'zero-copy-send', 'postcopy-preempt', 'switchover-ack',
>> - 'dirty-limit'] }
>> + 'dirty-limit', 'fixed-ram'] }
>>
>> ##
>> # @MigrationCapabilityStatus:
>
> Can we find a better name than @fixed-ram? @fixed-ram-offsets?
> @use-seek?
I have no idea how we came to fixed-ram. The archives don't provide any
clarification. I find it confusing at first glance as well.
A little brainstorming on how fixed-ram is different from exiting
migration:
Fixed-ram:
uses a file, like the 'file:' migration;
needs a seeking medium, such as a file;
migrates ram by placing a page always in the same offset in the
file, contrary to normal migration which streams the page changes
continuously;
ensures a migration file of size bounded to VM RAM size, contrary to
normal 'file:' migration which creates a file with unbounded size;
enables multi-threaded RAM migration, even though we only use it when
multifd is enabled;
uses scatter-gatter APIs (pwritev, preadv);
So a few options:
(disconsidering use-seek, it might be even more generic/vague)
- fixed-ram-offsets
- non-streaming (or streaming: false)
- ram-scatter-gather (ram-sg)
- parallel-ram (even with the slight inaccuracy that we sometimes do it
single-threaded)
Remember we also use this name internally, so I think a broader
"feature" name is better that a super specific one.
Does anyone have a strong preference? Other suggestions?
> Apart from that, QAPI schema
> Acked-by: Markus Armbruster <arm...@redhat.com>
Thanks!
