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+External Reshape
+1 Problem statement
+External (third-party metadata) reshape differs from native-metadata
+reshape in three key ways:
+1.1 Format specific constraints
+In the native case reshape is limited by what is implemented in the
+generic reshape routine (Grow_reshape()) and what is supported by the
+kernel. There are exceptional cases where Grow_reshape() may block
+operations when it knows that the kernel implementation is broken, but
+otherwise the kernel is relied upon to be the final arbiter of what
+reshape operations are supported.
+In the external case the kernel, and the generic checks in
+Grow_reshape(), become the super-set of what reshapes are possible. The
+metadata format may not support, or have yet to implement a given
+reshape type. The implication for Grow_reshape() is that it must query
+the metadata handler and effect changes in the metadata before the new
+geometry is posted to the kernel. The ->reshape_super method allows
+Grow_reshape() to validate the requested operation and post the metadata
+1.2 Scope of reshape
+Native metadata reshape is always performed at the array scope (no
+metadata relationship with sibling arrays on the same disks). External
+reshape, depending on the format, may not allow the number of member
+disks to be changed in a subarray unless the change is simultaneously
+applied to all subarrays in the container. For example the imsm format
+requires all member disks to be a member of all subarrays, so a 4-disk
+raid5 in a container that also houses a 4-disk raid10 array could not be
+reshaped to 5 disks as the imsm format does not support a 5-disk raid10
+representation. This requires the ->reshape_super method to check the
+contents of the array and ask the user to run the reshape at container
+scope (if all subarrays are agreeable to the change), or report an
+error in the case where one subarray cannot support the change.
+1.3 Monitoring / checkpointing
+Reshape, unlike rebuild/resync, requires strict checkpointing to survive
+interrupted reshape operations. For example when expanding a raid5
+array the first few stripes of the array will be overwritten in a
+destructive manner. When restarting the reshape process we need to know
+the exact location of the last successfully written stripe, and we need
+to restore the data in any partially overwritten stripe. Native
+metadata stores this backup data in the unused portion of spares that
+are being promoted to array members, or in an external backup file
+(located on a non-involved block device).
+The kernel is in charge of recording checkpoints of reshape progress,
+but mdadm is delegated the task of managing the backup space which
+1/ Identifying what data will be overwritten in the next unit of reshape
+ operation
+2/ Suspending access to that region so that a snapshot of the data can
+ be transferred to the backup space.
+3/ Allowing the kernel to reshape the saved region and setting the
+ boundary for the next backup.
+In the external reshape case we want to preserve this mdadm
+'reshape-manager' arrangement, but have a third actor, mdmon, to
+consider. It is tempting to give the role of managing reshape to mdmon,
+but that is counter to its role as a monitor, and conflicts with the
+existing capabilities and role of mdadm to manage the progress of
+reshape. For clarity the external reshape implementation maintains the
+role of mdmon as a (mostly) passive recorder of raid events, and mdadm
+treats it as it would the kernel in the native reshape case (modulo
+needing to send explicit metadata update messages and checking that
+mdmon took the expected action).
+External reshape can use the generic md backup file as a fallback, but in the
+optimal/firmware-compatible case the reshape-manager will use the metadata
+specific areas for managing reshape. The implementation also needs to spawn a
+reshape-manager per subarray when the reshape is being carried out at the
+container level. For these two reasons the ->manage_reshape() method is
+introduced. This method in addition to base tasks mentioned above:
+1/ Processed each subarray one at a time in series - where appropriate.
+2/ Uses either generic routines in Grow.c for md-style backup file
+ support, or uses the metadata-format specific location for storing
+ recovery data.
+This aims to avoid a "midlayer mistake"[1] and lets the metadata handler
+optionally take advantage of generic infrastructure in Grow.c
+2 Details for specific reshape requests
+There are quite a few moving pieces spread out across md, mdadm, and mdmon for
+the support of external reshape, and there are several different types of
+reshape that need to be comprehended by the implementation. A rundown of
+these details follows.
+2.0 General provisions:
+Obtain an exclusive open on the container to make sure we are not
+running concurrently with a Create() event.
+2.1 Freezing sync_action
+ Before making any attempt at a reshape we 'freeze' every array in
+ the container to ensure no spare assignment or recovery happens.
+ This involves writing 'frozen' to sync_action and changing the '/'
+ after 'external:' in metadata_version to a '-'. mdmon knows that
+ this means not to perform any management.
+ Before doing this we check that all sync_actions are 'idle', which
+ is racy but still useful.
+ Afterwards we check that all member arrays have no spares
+ or partial spares (recovery_start != 'none') which would indicate a
+ race. If they do, we unfreeze again.
+ Once this completes we know all the arrays are stable. They may
+ still have failed devices as devices can fail at any time. However
+ we treat those like failures that happen during the reshape.
+2.2 Reshape size
+ 1/ mdadm::Grow_reshape(): checks if mdmon is running and optionally
+ initializes st->update_tail
+ 2/ mdadm::Grow_reshape() calls ->reshape_super() to check that the size change
+ is allowed (being performed at subarray scope / enough room) prepares a
+ metadata update
+ 3/ mdadm::Grow_reshape(): flushes the metadata update (via
+ flush_metadata_update(), or ->sync_metadata())
+ 4/ mdadm::Grow_reshape(): post the new size to the kernel
+2.3 Reshape level (simple-takeover)
+"simple-takeover" implies the level change can be satisfied without touching
+ 1/ mdadm::Grow_reshape(): checks if mdmon is running and optionally
+ initializes st->update_tail
+ 2/ mdadm::Grow_reshape() calls ->reshape_super() to check that the level change
+ is allowed (being performed at subarray scope) prepares a
+ metadata update
+ 2a/ raid10 --> raid0: degrade all mirror legs prior to calling
+ ->reshape_super
+ 3/ mdadm::Grow_reshape(): flushes the metadata update (via
+ flush_metadata_update(), or ->sync_metadata())
+ 4/ mdadm::Grow_reshape(): post the new level to the kernel
+2.4 Reshape chunk, layout
+2.5 Reshape raid disks (grow)
+ 1/ mdadm::Grow_reshape(): unconditionally initializes st->update_tail
+ because only redundant raid levels can modify the number of raid disks
+ 2/ mdadm::Grow_reshape(): calls ->reshape_super() to check that the level
+ change is allowed (being performed at proper scope / permissible
+ geometry / proper spares available in the container), chooses
+ the spares to use, and prepares a metadata update.
+ 3/ mdadm::Grow_reshape(): Converts each subarray in the container to the
+ raid level that can perform the reshape and starts mdmon.
+ 4/ mdadm::Grow_reshape(): Pushes the update to mdmon.
+ 5/ mdadm::Grow_reshape(): uses container_content to find details of
+ the spares and passes them to the kernel.
+ 6/ mdadm::Grow_reshape(): gives raid_disks update to the kernel,
+ sets sync_max, sync_min, suspend_lo, suspend_hi all to zero,
+ and starts the reshape by writing 'reshape' to sync_action.
+ 7/ mdmon::monitor notices the sync_action change and tells
+ managemon to check for new devices. managemon notices the new
+ devices, opens relevant sysfs file, and passes them all to
+ monitor.
+ 8/ mdadm::Grow_reshape() calls ->manage_reshape to oversee the
+ rest of the reshape.
+ 9/ mdadm::<format>->manage_reshape(): saves data that will be overwritten by
+ the kernel to either the backup file or the metadata specific location,
+ advances sync_max, waits for reshape, ping mdmon, repeat.
+ Meanwhile mdmon::read_and_act(): records checkpoints.
+ Specifically.
+ 9a/ if the 'next' stripe to be reshaped will over-write
+ itself during reshape then:
+ 9a.1/ increase suspend_hi to cover a suitable number of
+ stripes.
+ 9a.2/ backup those stripes safely.
+ 9a.3/ advance sync_max to allow those stripes to be backed up
+ 9a.4/ when sync_completed indicates that those stripes have
+ been reshaped, manage_reshape must ping_manager
+ 9a.5/ when mdmon notices that sync_completed has been updated,
+ it records the new checkpoint in the metadata
+ 9a.6/ after the ping_manager, manage_reshape will increase
+ suspend_lo to allow access to those stripes again
+ 9b/ if the 'next' stripe to be reshaped will over-write unused
+ space during reshape then we apply same process as above,
+ except that there is no need to back anything up.
+ Note that we *do* need to keep suspend_hi progressing as
+ it is not safe to write to the area-under-reshape. For
+ kernel-managed-metadata this protection is provided by
+ ->reshape_safe, but that does not protect us in the case
+ of user-space-managed-metadata.
+ 10/ mdadm::<format>->manage_reshape(): Once reshape completes changes the raid
+ level back to the nominal raid level (if necessary)
+ FIXME: native metadata does not have the capability to record the original
+ raid level in reshape-restart case because the kernel always records current
+ raid level to the metadata, whereas external metadata can masquerade at an
+ alternate level based on the reshape state.
+2.6 Reshape raid disks (shrink)
+3 Interaction with metadata handle.
+ The following calls are made into the metadata handler to assist
+ with initiating and monitoring a 'reshape'.
+ 1/ ->reshape_super is called quite early (after only minimial
+ checks) to make sure that the metadata can record the new shape
+ and any necessary transitions. It may be passed a 'container'
+ or an individual array within a container, and it should notice
+ the difference and act accordingly.
+ When a reshape is requested against a container it is expected
+ that it should be applied to every array in the container,
+ however it is up to the metadata handler to determine final
+ policy.
+ If the reshape is supportable, the internal copy of the metadata
+ should be updated, and a metadata update suitable for sending
+ to mdmon should be queued.
+ If the reshape will involve converting spares into array members,
+ this must be recorded in the metadata too.
+ 2/ ->container_content will be called to find out the new state
+ of all the array, or all arrays in the container. Any newly
+ added devices (with state==0 and raid_disk >= 0) will be added
+ to the array as spares with the relevant slot number.
+ It is likely that the info returned by ->container_content will
+ have ->reshape_active set, ->reshape_progress set to e.g. 0, and
+ new_* set appropriately. mdadm will use this information to
+ cause the correct reshape to start at an appropriate time.
+ 3/ ->set_array_state will be called by mdmon when reshape has
+ started and again periodically as it progresses. This should
+ record the ->last_checkpoint as the point where reshape has
+ progressed to. When the reshape finished this will be called
+ again and it should notice that ->curr_action is no longer
+ 'reshape' and so should record that the reshape has finished
+ providing 'last_checkpoint' has progressed suitably.
+ 4/ ->manage_reshape will be called once the reshape has been set
+ up in the kernel but before sync_max has been moved from 0, so
+ no actual reshape will have happened.
+ ->manage_reshape should call progress_reshape() to allow the
+ reshape to progress, and should back-up any data as indicated
+ by the return value. See the documentation of that function
+ for more details.
+ ->manage_reshape will be called multiple times when a
+ container is being reshaped, once for each member array in
+ the container.
+ The progress of the metadata is as follows:
+ 1/ mdadm sends a metadata update to mdmon which marks the array
+ as undergoing a reshape. This is set up by
+ ->reshape_super and applied by ->process_update
+ For container-wide reshape, this happens once for the whole
+ container.
+ 2/ mdmon notices progress via the sysfs files and calls
+ ->set_array_state to update the state periodically
+ For container-wide reshape, this happens repeatedly for
+ one array, then repeatedly for the next, etc.
+ 3/ mdmon notices when reshape has finished and call
+ ->set_array_state to record the the reshape is complete.
+ For container-wide reshape, this happens once for each
+ member array.
+[1]: Linux kernel design patterns - part 3, Neil Brown