--- /dev/null
+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
+update.
+
+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 both 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
+involves:
+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/ Spawns a manager per-subarray, when necessary
+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
+
+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
+sync_action
+
+ 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) 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...
+ 4a/ mdmon::process_update(): marks the array as reshaping
+ 4b/ mdmon::manage_member(): adds the spares (without assigning a slot)
+ 5/ mdadm::Grow_reshape(): Notes that mdmon has assigned spares and invokes
+ ->manage_reshape()
+ 5/ mdadm::<format>->manage_reshape(): (for each subarray) sets sync_max to
+ zero, starts the reshape, and pings mdmon
+ 5a/ mdmon::read_and_act(): notices that reshape has started and notifies
+ the metadata handler to record the slots chosen by the kernel
+ 6/ 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.
+ 6a/ mdmon::read_and_act(): records checkpoints
+ 7/ 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 TODO
+
+...
+
+[1]: Linux kernel design patterns - part 3, Neil Brown http://lwn.net/Articles/336262/
projects / thirdparty / mdadm.git / commitdiff
