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From 3aa5bb0af1051432a83b2f7a9fd5c2763444c937 Mon Sep 17 00:00:00 2001
From: Mariusz Tkaczyk <mariusz.tkaczyk@linux.intel.com>
Date: Fri, 23 Feb 2024 15:51:46 +0100
Subject: [PATCH 19/41] mdadm: move documentation to folder
Move documentation text files to directory.
Signed-off-by: Mariusz Tkaczyk <mariusz.tkaczyk@linux.intel.com>
---
documentation/external-reshape-design.txt | 280 ++++++++++++++++++++++
documentation/mdadm.conf-example | 65 +++++
documentation/mdmon-design.txt | 146 +++++++++++
external-reshape-design.txt | 280 ----------------------
mdadm.conf-example | 65 -----
mdmon-design.txt | 146 -----------
6 files changed, 491 insertions(+), 491 deletions(-)
create mode 100644 documentation/external-reshape-design.txt
create mode 100644 documentation/mdadm.conf-example
create mode 100644 documentation/mdmon-design.txt
delete mode 100644 external-reshape-design.txt
delete mode 100644 mdadm.conf-example
delete mode 100644 mdmon-design.txt
diff --git a/documentation/external-reshape-design.txt b/documentation/external-reshape-design.txt
new file mode 100644
index 00000000..e4cf4e16
--- /dev/null
+++ b/documentation/external-reshape-design.txt
@@ -0,0 +1,280 @@
+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 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
+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/ 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
+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), 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 https://lwn.net/Articles/336262/
diff --git a/documentation/mdadm.conf-example b/documentation/mdadm.conf-example
new file mode 100644
index 00000000..35a75d12
--- /dev/null
+++ b/documentation/mdadm.conf-example
@@ -0,0 +1,65 @@
+# mdadm configuration file
+#
+# mdadm will function properly without the use of a configuration file,
+# but this file is useful for keeping track of arrays and member disks.
+# In general, a mdadm.conf file is created, and updated, after arrays
+# are created. This is the opposite behavior of /etc/raidtab which is
+# created prior to array construction.
+#
+#
+# the config file takes two types of lines:
+#
+# DEVICE lines specify a list of devices of where to look for
+# potential member disks
+#
+# ARRAY lines specify information about how to identify arrays so
+# so that they can be activated
+#
+# You can have more than one device line and use wild cards. The first
+# example includes SCSI the first partition of SCSI disks /dev/sdb,
+# /dev/sdc, /dev/sdd, /dev/sdj, /dev/sdk, and /dev/sdl. The second
+# line looks for array slices on IDE disks.
+#
+#DEVICE /dev/sd[bcdjkl]1
+#DEVICE /dev/hda1 /dev/hdb1
+#
+# If you mount devfs on /dev, then a suitable way to list all devices is:
+#DEVICE /dev/discs/*/*
+#
+#
+# The AUTO line can control which arrays get assembled by auto-assembly,
+# meaing either "mdadm -As" when there are no 'ARRAY' lines in this file,
+# or "mdadm --incremental" when the array found is not listed in this file.
+# By default, all arrays that are found are assembled.
+# If you want to ignore all DDF arrays (maybe they are managed by dmraid),
+# and only assemble 1.x arrays if which are marked for 'this' homehost,
+# but assemble all others, then use
+#AUTO -ddf homehost -1.x +all
+#
+# ARRAY lines specify an array to assemble and a method of identification.
+# Arrays can currently be identified by using a UUID, superblock minor number,
+# or a listing of devices.
+#
+# super-minor is usually the minor number of the metadevice
+# UUID is the Universally Unique Identifier for the array
+# Each can be obtained using
+#
+# mdadm -D <md>
+#
+#ARRAY /dev/md0 UUID=3aaa0122:29827cfa:5331ad66:ca767371
+#ARRAY /dev/md1 super-minor=1
+#ARRAY /dev/md2 devices=/dev/hda1,/dev/hdb1
+#
+# ARRAY lines can also specify a "spare-group" for each array. mdadm --monitor
+# will then move a spare between arrays in a spare-group if one array has a failed
+# drive but no spare
+#ARRAY /dev/md4 uuid=b23f3c6d:aec43a9f:fd65db85:369432df spare-group=group1
+#ARRAY /dev/md5 uuid=19464854:03f71b1b:e0df2edd:246cc977 spare-group=group1
+#
+# When used in --follow (aka --monitor) mode, mdadm needs a
+# mail address and/or a program. This can be given with "mailaddr"
+# and "program" lines to that monitoring can be started using
+# mdadm --follow --scan & echo $! > /run/mdadm/mon.pid
+# If the lines are not found, mdadm will exit quietly
+#MAILADDR root@mydomain.tld
+#PROGRAM /usr/sbin/handle-mdadm-events
diff --git a/documentation/mdmon-design.txt b/documentation/mdmon-design.txt
new file mode 100644
index 00000000..f09184a9
--- /dev/null
+++ b/documentation/mdmon-design.txt
@@ -0,0 +1,146 @@
+
+When managing a RAID1 array which uses metadata other than the
+"native" metadata understood by the kernel, mdadm makes use of a
+partner program named 'mdmon' to manage some aspects of updating
+that metadata and synchronising the metadata with the array state.
+
+This document provides some details on how mdmon works.
+
+Containers
+----------
+
+As background: mdadm makes a distinction between an 'array' and a
+'container'. Other sources sometimes use the term 'volume' or
+'device' for an 'array', and may use the term 'array' for a
+'container'.
+
+For our purposes:
+ - a 'container' is a collection of devices which are described by a
+ single set of metadata. The metadata may be stored equally
+ on all devices, or different devices may have quite different
+ subsets of the total metadata. But there is conceptually one set
+ of metadata that unifies the devices.
+
+ - an 'array' is a set of datablock from various devices which
+ together are used to present the abstraction of a single linear
+ sequence of block, which may provide data redundancy or enhanced
+ performance.
+
+So a container has some metadata and provides a number of arrays which
+are described by that metadata.
+
+Sometimes this model doesn't work perfectly. For example, global
+spares may have their own metadata which is quite different from the
+metadata from any device that participates in one or more arrays.
+Such a global spare might still need to belong to some container so
+that it is available to be used should a failure arise. In that case
+we consider the 'metadata' to be the union of the metadata on the
+active devices which describes the arrays, and the metadata on the
+global spares which only describes the spares. In this case different
+devices in the one container will have quite different metadata.
+
+
+Purpose
+-------
+
+The main purpose of mdmon is to update the metadata in response to
+changes to the array which need to be reflected in the metadata before
+futures writes to the array can safely be performed.
+These include:
+ - transitions from 'clean' to 'dirty'.
+ - recording the devices have failed.
+ - recording the progress of a 'reshape'
+
+This requires mdmon to be running at any time that the array is
+writable (a read-only array does not require mdmon to be running).
+
+Because mdmon must be able to process these metadata updates at any
+time, it must (when running) have exclusive write access to the
+metadata. Any other changes (e.g. reconfiguration of the array) must
+go through mdmon.
+
+A secondary role for mdmon is to activate spares when a device fails.
+This role is much less time-critical than the other metadata updates,
+so it could be performed by a separate process, possibly
+"mdadm --monitor" which has a related role of moving devices between
+arrays. A main reason for including this functionality in mdmon is
+that in the native-metadata case this function is handled in the
+kernel, and mdmon's reason for existence to provide functionality
+which is otherwise handled by the kernel.
+
+
+Design overview
+---------------
+
+mdmon is structured as two threads with a common address space and
+common data structures. These threads are know as the 'monitor' and
+the 'manager'.
+
+The 'monitor' has the primary role of monitoring the array for
+important state changes and updating the metadata accordingly. As
+writes to the array can be blocked until 'monitor' completes and
+acknowledges the update, it much be very careful not to block itself.
+In particular it must not block waiting for any write to complete else
+it could deadlock. This means that it must not allocate memory as
+doing this can require dirty memory to be written out and if the
+system choose to write to the array that mdmon is monitoring, the
+memory allocation could deadlock.
+
+So 'monitor' must never allocate memory and must limit the number of
+other system call it performs. It may:
+ - use select (or poll) to wait for activity on a file descriptor
+ - read from a sysfs file descriptor
+ - write to a sysfs file descriptor
+ - write the metadata out to the block devices using O_DIRECT
+ - send a signal (kill) to the manager thread
+
+It must not e.g. open files or do anything similar that might allocate
+resources.
+
+The 'manager' thread does everything else that is needed. If any
+files are to be opened (e.g. because a device has been added to the
+array), the manager does that. If any memory needs to be allocated
+(e.g. to hold data about a new array as can happen when one set of
+metadata describes several arrays), the manager performs that
+allocation.
+
+The 'manager' is also responsible for communicating with mdadm and
+assigning spares to replace failed devices.
+
+
+Handling metadata updates
+-------------------------
+
+There are a number of cases in which mdadm needs to update the
+metdata which mdmon is managing. These include:
+ - creating a new array in an active container
+ - adding a device to a container
+ - reconfiguring an array
+etc.
+
+To complete these updates, mdadm must send a message to mdmon which
+will merge the update into the metadata as it is at that moment.
+
+To achieve this, mdmon creates a Unix Domain Socket which the manager
+thread listens on. mdadm sends a message over this socket. The
+manager thread examines the message to see if it will require
+allocating any memory and allocates it. This is done in the
+'prepare_update' metadata method.
+
+The update message is then queued for handling by the monitor thread
+which it will do when convenient. The monitor thread calls
+->process_update which should atomically make the required changes to
+the metadata, making use of the pre-allocate memory as required. Any
+memory the is no-longer needed can be placed back in the request and
+the manager thread will free it.
+
+The exact format of a metadata update is up to the implementer of the
+metadata handlers. It will simply describe a change that needs to be
+made. It will sometimes contain fragments of the metadata to be
+copied in to place. However the ->process_update routine must make
+sure not to over-write any field that the monitor thread might have
+updated, such as a 'device failed' or 'array is dirty' state.
+
+When the monitor thread has completed the update and written it to the
+devices, an acknowledgement message is sent back over the socket so
+that mdadm knows it is complete.
diff --git a/external-reshape-design.txt b/external-reshape-design.txt
deleted file mode 100644
index e4cf4e16..00000000
--- a/external-reshape-design.txt
+++ /dev/null
@@ -1,280 +0,0 @@
-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 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
-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/ 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
-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), 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 https://lwn.net/Articles/336262/
diff --git a/mdadm.conf-example b/mdadm.conf-example
deleted file mode 100644
index 35a75d12..00000000
--- a/mdadm.conf-example
+++ /dev/null
@@ -1,65 +0,0 @@
-# mdadm configuration file
-#
-# mdadm will function properly without the use of a configuration file,
-# but this file is useful for keeping track of arrays and member disks.
-# In general, a mdadm.conf file is created, and updated, after arrays
-# are created. This is the opposite behavior of /etc/raidtab which is
-# created prior to array construction.
-#
-#
-# the config file takes two types of lines:
-#
-# DEVICE lines specify a list of devices of where to look for
-# potential member disks
-#
-# ARRAY lines specify information about how to identify arrays so
-# so that they can be activated
-#
-# You can have more than one device line and use wild cards. The first
-# example includes SCSI the first partition of SCSI disks /dev/sdb,
-# /dev/sdc, /dev/sdd, /dev/sdj, /dev/sdk, and /dev/sdl. The second
-# line looks for array slices on IDE disks.
-#
-#DEVICE /dev/sd[bcdjkl]1
-#DEVICE /dev/hda1 /dev/hdb1
-#
-# If you mount devfs on /dev, then a suitable way to list all devices is:
-#DEVICE /dev/discs/*/*
-#
-#
-# The AUTO line can control which arrays get assembled by auto-assembly,
-# meaing either "mdadm -As" when there are no 'ARRAY' lines in this file,
-# or "mdadm --incremental" when the array found is not listed in this file.
-# By default, all arrays that are found are assembled.
-# If you want to ignore all DDF arrays (maybe they are managed by dmraid),
-# and only assemble 1.x arrays if which are marked for 'this' homehost,
-# but assemble all others, then use
-#AUTO -ddf homehost -1.x +all
-#
-# ARRAY lines specify an array to assemble and a method of identification.
-# Arrays can currently be identified by using a UUID, superblock minor number,
-# or a listing of devices.
-#
-# super-minor is usually the minor number of the metadevice
-# UUID is the Universally Unique Identifier for the array
-# Each can be obtained using
-#
-# mdadm -D <md>
-#
-#ARRAY /dev/md0 UUID=3aaa0122:29827cfa:5331ad66:ca767371
-#ARRAY /dev/md1 super-minor=1
-#ARRAY /dev/md2 devices=/dev/hda1,/dev/hdb1
-#
-# ARRAY lines can also specify a "spare-group" for each array. mdadm --monitor
-# will then move a spare between arrays in a spare-group if one array has a failed
-# drive but no spare
-#ARRAY /dev/md4 uuid=b23f3c6d:aec43a9f:fd65db85:369432df spare-group=group1
-#ARRAY /dev/md5 uuid=19464854:03f71b1b:e0df2edd:246cc977 spare-group=group1
-#
-# When used in --follow (aka --monitor) mode, mdadm needs a
-# mail address and/or a program. This can be given with "mailaddr"
-# and "program" lines to that monitoring can be started using
-# mdadm --follow --scan & echo $! > /run/mdadm/mon.pid
-# If the lines are not found, mdadm will exit quietly
-#MAILADDR root@mydomain.tld
-#PROGRAM /usr/sbin/handle-mdadm-events
diff --git a/mdmon-design.txt b/mdmon-design.txt
deleted file mode 100644
index f09184a9..00000000
--- a/mdmon-design.txt
+++ /dev/null
@@ -1,146 +0,0 @@
-
-When managing a RAID1 array which uses metadata other than the
-"native" metadata understood by the kernel, mdadm makes use of a
-partner program named 'mdmon' to manage some aspects of updating
-that metadata and synchronising the metadata with the array state.
-
-This document provides some details on how mdmon works.
-
-Containers
-----------
-
-As background: mdadm makes a distinction between an 'array' and a
-'container'. Other sources sometimes use the term 'volume' or
-'device' for an 'array', and may use the term 'array' for a
-'container'.
-
-For our purposes:
- - a 'container' is a collection of devices which are described by a
- single set of metadata. The metadata may be stored equally
- on all devices, or different devices may have quite different
- subsets of the total metadata. But there is conceptually one set
- of metadata that unifies the devices.
-
- - an 'array' is a set of datablock from various devices which
- together are used to present the abstraction of a single linear
- sequence of block, which may provide data redundancy or enhanced
- performance.
-
-So a container has some metadata and provides a number of arrays which
-are described by that metadata.
-
-Sometimes this model doesn't work perfectly. For example, global
-spares may have their own metadata which is quite different from the
-metadata from any device that participates in one or more arrays.
-Such a global spare might still need to belong to some container so
-that it is available to be used should a failure arise. In that case
-we consider the 'metadata' to be the union of the metadata on the
-active devices which describes the arrays, and the metadata on the
-global spares which only describes the spares. In this case different
-devices in the one container will have quite different metadata.
-
-
-Purpose
--------
-
-The main purpose of mdmon is to update the metadata in response to
-changes to the array which need to be reflected in the metadata before
-futures writes to the array can safely be performed.
-These include:
- - transitions from 'clean' to 'dirty'.
- - recording the devices have failed.
- - recording the progress of a 'reshape'
-
-This requires mdmon to be running at any time that the array is
-writable (a read-only array does not require mdmon to be running).
-
-Because mdmon must be able to process these metadata updates at any
-time, it must (when running) have exclusive write access to the
-metadata. Any other changes (e.g. reconfiguration of the array) must
-go through mdmon.
-
-A secondary role for mdmon is to activate spares when a device fails.
-This role is much less time-critical than the other metadata updates,
-so it could be performed by a separate process, possibly
-"mdadm --monitor" which has a related role of moving devices between
-arrays. A main reason for including this functionality in mdmon is
-that in the native-metadata case this function is handled in the
-kernel, and mdmon's reason for existence to provide functionality
-which is otherwise handled by the kernel.
-
-
-Design overview
----------------
-
-mdmon is structured as two threads with a common address space and
-common data structures. These threads are know as the 'monitor' and
-the 'manager'.
-
-The 'monitor' has the primary role of monitoring the array for
-important state changes and updating the metadata accordingly. As
-writes to the array can be blocked until 'monitor' completes and
-acknowledges the update, it much be very careful not to block itself.
-In particular it must not block waiting for any write to complete else
-it could deadlock. This means that it must not allocate memory as
-doing this can require dirty memory to be written out and if the
-system choose to write to the array that mdmon is monitoring, the
-memory allocation could deadlock.
-
-So 'monitor' must never allocate memory and must limit the number of
-other system call it performs. It may:
- - use select (or poll) to wait for activity on a file descriptor
- - read from a sysfs file descriptor
- - write to a sysfs file descriptor
- - write the metadata out to the block devices using O_DIRECT
- - send a signal (kill) to the manager thread
-
-It must not e.g. open files or do anything similar that might allocate
-resources.
-
-The 'manager' thread does everything else that is needed. If any
-files are to be opened (e.g. because a device has been added to the
-array), the manager does that. If any memory needs to be allocated
-(e.g. to hold data about a new array as can happen when one set of
-metadata describes several arrays), the manager performs that
-allocation.
-
-The 'manager' is also responsible for communicating with mdadm and
-assigning spares to replace failed devices.
-
-
-Handling metadata updates
--------------------------
-
-There are a number of cases in which mdadm needs to update the
-metdata which mdmon is managing. These include:
- - creating a new array in an active container
- - adding a device to a container
- - reconfiguring an array
-etc.
-
-To complete these updates, mdadm must send a message to mdmon which
-will merge the update into the metadata as it is at that moment.
-
-To achieve this, mdmon creates a Unix Domain Socket which the manager
-thread listens on. mdadm sends a message over this socket. The
-manager thread examines the message to see if it will require
-allocating any memory and allocates it. This is done in the
-'prepare_update' metadata method.
-
-The update message is then queued for handling by the monitor thread
-which it will do when convenient. The monitor thread calls
-->process_update which should atomically make the required changes to
-the metadata, making use of the pre-allocate memory as required. Any
-memory the is no-longer needed can be placed back in the request and
-the manager thread will free it.
-
-The exact format of a metadata update is up to the implementer of the
-metadata handlers. It will simply describe a change that needs to be
-made. It will sometimes contain fragments of the metadata to be
-copied in to place. However the ->process_update routine must make
-sure not to over-write any field that the monitor thread might have
-updated, such as a 'device failed' or 'array is dirty' state.
-
-When the monitor thread has completed the update and written it to the
-devices, an acknowledgement message is sent back over the socket so
-that mdadm knows it is complete.
--
2.40.1
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