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envelope-from=farosas@suse.de; helo=smtp-out2.suse.de X-Spam_score_int: -20 X-Spam_score: -2.1 X-Spam_bar: -- X-Spam_report: (-2.1 / 5.0 requ) BAYES_00=-1.9, DKIM_SIGNED=0.1, DKIM_VALID=-0.1, DKIM_VALID_AU=-0.1, DKIM_VALID_EF=-0.1, SPF_HELO_NONE=0.001, SPF_PASS=-0.001, T_SCC_BODY_TEXT_LINE=-0.01 autolearn=ham autolearn_force=no X-Spam_action: no action X-BeenThere: qemu-devel@nongnu.org X-Mailman-Version: 2.1.29 Precedence: list List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Errors-To: qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org Sender: qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org Daniel P. Berrang=C3=A9 writes: > On Wed, Feb 21, 2024 at 10:24:05AM -0300, Fabiano Rosas wrote: >> Markus Armbruster writes: >>=20 >> > Fabiano Rosas 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 >> > >> > [...] >> > >> >> 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'] } >> >>=20=20 >> >> ## >> >> # @MigrationCapabilityStatus: >> > >> > Can we find a better name than @fixed-ram? @fixed-ram-offsets? >> > @use-seek? >>=20 >> 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. >>=20 >> A little brainstorming on how fixed-ram is different from exiting >> migration: >>=20 >> Fixed-ram: >> uses a file, like the 'file:' migration; >>=20 >> needs a seeking medium, such as a file; >>=20 >> migrates ram by placing a page always in the same offset in the >> file, contrary to normal migration which streams the page changes >> continuously; >>=20 >> ensures a migration file of size bounded to VM RAM size, contrary to >> normal 'file:' migration which creates a file with unbounded size; >>=20 >> enables multi-threaded RAM migration, even though we only use it when >> multifd is enabled; >>=20 >> uses scatter-gatter APIs (pwritev, preadv); >>=20 >> So a few options: >>=20 >> (disconsidering use-seek, it might be even more generic/vague) >>=20 >> - 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) > > I could add 'mapped-ram', as an alternative to 'fixed-ram'. > > The key distinguishing & motivating feature here is that > RAM regions are mapped directly to file regions, instead > of just being streamed at arbitrary points. "map" is certainly a good shorthand for the various "placed at relative offsets" that I used throughout this series.