On Tue, Feb 20, 2024 at 07:41:15PM -0300, Fabiano Rosas wrote: > 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> > --- > - update migration.json to 9.0 and improve wording > - move docs to a separate file and add use cases information > --- > docs/devel/migration/features.rst | 1 + > docs/devel/migration/fixed-ram.rst | 137 +++++++++++++++++++++++++++++ > migration/options.c | 34 +++++++ > migration/options.h | 1 + > migration/savevm.c | 1 + > qapi/migration.json | 6 +- > 6 files changed, 179 insertions(+), 1 deletion(-) > create mode 100644 docs/devel/migration/fixed-ram.rst > > diff --git a/docs/devel/migration/features.rst > b/docs/devel/migration/features.rst > index a9acaf618e..4c708b679a 100644 > --- a/docs/devel/migration/features.rst > +++ b/docs/devel/migration/features.rst > @@ -10,3 +10,4 @@ Migration has plenty of features to support different use > cases. > dirty-limit > vfio > virtio > + fixed-ram > diff --git a/docs/devel/migration/fixed-ram.rst > b/docs/devel/migration/fixed-ram.rst > new file mode 100644 > index 0000000000..a6c0e5a360 > --- /dev/null > +++ b/docs/devel/migration/fixed-ram.rst > @@ -0,0 +1,137 @@ > +Fixed-ram > +========= > + > +Fixed-ram is a new stream format for the RAM section designed to > +supplement the existing ``file:`` migration and make it compatible > +with ``multifd``. This enables parallel migration of a guest's RAM to > +a file. > + > +The core of the feature is to ensure that each RAM page has a specific > +offset in the resulting migration file. This enables the ``multifd`` > +threads to write exclusively to those offsets even if the guest is > +constantly dirtying pages (i.e. live migration). Another benefit is > +that 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. Having the pages at fixed offsets also allows the usage of > +O_DIRECT for save/restore of the migration stream as the pages are > +ensured to be written respecting O_DIRECT alignment restrictions. > + > +Usage > +----- > + > +On both source and destination, enable the ``multifd`` and > +``fixed-ram`` capabilities: > + > + ``migrate_set_capability multifd on`` > + > + ``migrate_set_capability fixed-ram on`` > + > +Use a ``file:`` URL for migration: > + > + ``migrate file:/path/to/migration/file`` > + > +Fixed-ram migration is best done non-live, i.e. by stopping the VM on > +the source side before migrating. > + > +For best performance enable the ``direct-io`` capability as well: > + > + ``migrate_set_capability direct-io on`` > + > +Use-cases > +--------- > + > +The fixed-ram feature was designed for use cases where the migration > +stream will be directed to a file in the filesystem and not > +immediately restored on the destination VM [#]_. These could be > +thought of as snapshots. We can further categorize them into live and > +non-live. > + > +- Non-live snapshot > + > +If the use case requires a VM to be stopped before taking a snapshot, > +that's the ideal scenario for fixed-ram migration. Not having to track > +dirty pages, the migration will write the RAM pages to the disk as > +fast as it can. > + > +Note: if a snapshot is taken of a running VM, but the VM will be > +stopped after the snapshot by the admin, then consider stopping it > +right before the snapshot to take benefit of the performance gains > +mentioned above. > + > +- Live snapshot > + > +If the use case requires that the VM keeps running during and after > +the snapshot operation, then fixed-ram migration can still be used, > +but will be less performant. Other strategies such as > +background-snapshot should be evaluated as well. One benefit of > +fixed-ram in this scenario is portability since background-snapshot > +depends on async dirty tracking (KVM_GET_DIRTY_LOG) which is not
Background snapshot uses userfaultfd-wp rather than KVM_GET_DIRTY_LOG. The statement is still correct though, that userfault is only supported on Linux in general (wp is one sub-feature, represents "write-protect mode") so this should help portability, as it removes the dependency on the OS. > +supported outside of Linux. > + > +.. [#] While this same effect could be obtained with the usage of > + snapshots or the ``file:`` migration alone, fixed-ram provides > + a performance increase for VMs with larger RAM sizes (10s to > + 100s of GiBs), specially if the VM has been stopped beforehand. > + > +RAM section format > +------------------ > + > +Instead of having a sequential stream of pages that follow the > +RAMBlock headers, the dirty pages for a RAMBlock follow its header > +instead. This ensures that each RAM page has a fixed offset in the > +resulting migration file. > + > +A bitmap is introduced to track which pages have been written in the > +migration file. Pages are written at a fixed location for every > +ramblock. Zero pages are ignored as they'd be zero in the destination > +migration as well. > + > +:: > + > + Without fixed-ram: With fixed-ram: > + > + --------------------- -------------------------------- > + | ramblock 1 header | | ramblock 1 header | > + --------------------- -------------------------------- > + | ramblock 2 header | | ramblock 1 fixed-ram header | > + --------------------- -------------------------------- > + | ... | | padding to next 1MB boundary | > + --------------------- | ... | > + | ramblock n header | -------------------------------- > + --------------------- | ramblock 1 pages | > + | RAM_SAVE_FLAG_EOS | | ... | > + --------------------- -------------------------------- > + | stream of pages | | ramblock 2 header | > + | (iter 1) | -------------------------------- > + | ... | | ramblock 2 fixed-ram header | > + --------------------- -------------------------------- > + | RAM_SAVE_FLAG_EOS | | padding to next 1MB boundary | > + --------------------- | ... | > + | stream of pages | -------------------------------- > + | (iter 2) | | ramblock 2 pages | > + | ... | | ... | > + --------------------- -------------------------------- > + | ... | | ... | > + --------------------- -------------------------------- > + | RAM_SAVE_FLAG_EOS | > + -------------------------------- > + | ... | > + -------------------------------- > + > + where: > + - ramblock header: the generic information for a ramblock, such as > + idstr, used_len, etc. > + > + - ramblock fixed-ram header: the information added by this feature: > + bitmap of pages written, bitmap size and offset of pages in the > + migration file. > + > +Restrictions > +------------ > + > +Since pages are written to their relative offsets and out of order > +(due to the memory dirtying patterns), streaming channels such as > +sockets are not supported. A seekable channel such as a file is > +required. This can be verified in the QIOChannel by the presence of > +the QIO_CHANNEL_FEATURE_SEEKABLE. > diff --git a/migration/options.c b/migration/options.c > index 3e3e0b93b4..4909e5c72a 100644 > --- a/migration/options.c > +++ b/migration/options.c > @@ -204,6 +204,7 @@ Property migration_properties[] = { > DEFINE_PROP_MIG_CAP("x-switchover-ack", > MIGRATION_CAPABILITY_SWITCHOVER_ACK), > DEFINE_PROP_MIG_CAP("x-dirty-limit", MIGRATION_CAPABILITY_DIRTY_LIMIT), > + DEFINE_PROP_MIG_CAP("x-fixed-ram", MIGRATION_CAPABILITY_FIXED_RAM), Let's directly use "fixed-ram" (or "mapped-ram", or whatever new name we decide to use, as long as without "x-")? migration_properties is not documented anywhere, mostly yet for debugging purpose. We could have dropped all the "x-"s, IMHO. > DEFINE_PROP_END_OF_LIST(), > }; > > @@ -263,6 +264,13 @@ bool migrate_events(void) > return s->capabilities[MIGRATION_CAPABILITY_EVENTS]; > } > > +bool migrate_fixed_ram(void) > +{ > + MigrationState *s = migrate_get_current(); > + > + return s->capabilities[MIGRATION_CAPABILITY_FIXED_RAM]; > +} > + > bool migrate_ignore_shared(void) > { > MigrationState *s = migrate_get_current(); > @@ -645,6 +653,32 @@ bool migrate_caps_check(bool *old_caps, bool *new_caps, > Error **errp) > } > } > > + if (new_caps[MIGRATION_CAPABILITY_FIXED_RAM]) { > + if (new_caps[MIGRATION_CAPABILITY_MULTIFD]) { > + error_setg(errp, > + "Fixed-ram migration is incompatible with multifd"); > + return false; > + } > + > + if (new_caps[MIGRATION_CAPABILITY_XBZRLE]) { > + error_setg(errp, > + "Fixed-ram migration is incompatible with xbzrle"); > + return false; > + } > + > + if (new_caps[MIGRATION_CAPABILITY_COMPRESS]) { > + error_setg(errp, > + "Fixed-ram migration is incompatible with > compression"); > + return false; > + } > + > + if (new_caps[MIGRATION_CAPABILITY_POSTCOPY_RAM]) { > + error_setg(errp, > + "Fixed-ram migration is incompatible with postcopy > ram"); > + return false; > + } > + } > + > return true; > } > > diff --git a/migration/options.h b/migration/options.h > index 246c160aee..8680a10b79 100644 > --- a/migration/options.h > +++ b/migration/options.h > @@ -31,6 +31,7 @@ bool migrate_compress(void); > bool migrate_dirty_bitmaps(void); > bool migrate_dirty_limit(void); > bool migrate_events(void); > +bool migrate_fixed_ram(void); > bool migrate_ignore_shared(void); > bool migrate_late_block_activate(void); > bool migrate_multifd(void); > diff --git a/migration/savevm.c b/migration/savevm.c > index d612c8a902..4b928dd6bb 100644 > --- a/migration/savevm.c > +++ b/migration/savevm.c > @@ -245,6 +245,7 @@ static bool should_validate_capability(int capability) > /* Validate only new capabilities to keep compatibility. */ > switch (capability) { > case MIGRATION_CAPABILITY_X_IGNORE_SHARED: > + case MIGRATION_CAPABILITY_FIXED_RAM: > return true; > default: > return false; > 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: > -- > 2.35.3 > -- Peter Xu