Adding upstream version 5.10.209.upstream/5.10.209upstream

Signed-off-by: Daniel Baumann <daniel.baumann@progress-linux.org>

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+RAID arrays
+===========
+
+Boot time assembly of RAID arrays
+---------------------------------
+
+Tools that manage md devices can be found at
+   https://www.kernel.org/pub/linux/utils/raid/
+
+
+You can boot with your md device with the following kernel command
+lines:
+
+for old raid arrays without persistent superblocks::
+
+  md=<md device no.>,<raid level>,<chunk size factor>,<fault level>,dev0,dev1,...,devn
+
+for raid arrays with persistent superblocks::
+
+  md=<md device no.>,dev0,dev1,...,devn
+
+or, to assemble a partitionable array::
+
+  md=d<md device no.>,dev0,dev1,...,devn
+
+``md device no.``
++++++++++++++++++
+
+The number of the md device
+
+================= =========
+``md device no.`` device
+================= =========
+              0		md0
+	      1		md1
+	      2		md2
+	      3		md3
+	      4		md4
+================= =========
+
+``raid level``
+++++++++++++++
+
+level of the RAID array
+
+=============== =============
+``raid level``  level
+=============== =============
+-1		linear mode
+0		striped mode
+=============== =============
+
+other modes are only supported with persistent super blocks
+
+``chunk size factor``
++++++++++++++++++++++
+
+(raid-0 and raid-1 only)
+
+Set  the chunk size as 4k << n.
+
+``fault level``
++++++++++++++++
+
+Totally ignored
+
+``dev0`` to ``devn``
+++++++++++++++++++++
+
+e.g. ``/dev/hda1``, ``/dev/hdc1``, ``/dev/sda1``, ``/dev/sdb1``
+
+A possible loadlin line (Harald Hoyer <HarryH@Royal.Net>)  looks like this::
+
+	e:\loadlin\loadlin e:\zimage root=/dev/md0 md=0,0,4,0,/dev/hdb2,/dev/hdc3 ro
+
+
+Boot time autodetection of RAID arrays
+--------------------------------------
+
+When md is compiled into the kernel (not as module), partitions of
+type 0xfd are scanned and automatically assembled into RAID arrays.
+This autodetection may be suppressed with the kernel parameter
+``raid=noautodetect``.  As of kernel 2.6.9, only drives with a type 0
+superblock can be autodetected and run at boot time.
+
+The kernel parameter ``raid=partitionable`` (or ``raid=part``) means
+that all auto-detected arrays are assembled as partitionable.
+
+Boot time assembly of degraded/dirty arrays
+-------------------------------------------
+
+If a raid5 or raid6 array is both dirty and degraded, it could have
+undetectable data corruption.  This is because the fact that it is
+``dirty`` means that the parity cannot be trusted, and the fact that it
+is degraded means that some datablocks are missing and cannot reliably
+be reconstructed (due to no parity).
+
+For this reason, md will normally refuse to start such an array.  This
+requires the sysadmin to take action to explicitly start the array
+despite possible corruption.  This is normally done with::
+
+   mdadm --assemble --force ....
+
+This option is not really available if the array has the root
+filesystem on it.  In order to support this booting from such an
+array, md supports a module parameter ``start_dirty_degraded`` which,
+when set to 1, bypassed the checks and will allows dirty degraded
+arrays to be started.
+
+So, to boot with a root filesystem of a dirty degraded raid 5 or 6, use::
+
+   md-mod.start_dirty_degraded=1
+
+
+Superblock formats
+------------------
+
+The md driver can support a variety of different superblock formats.
+Currently, it supports superblock formats ``0.90.0`` and the ``md-1`` format
+introduced in the 2.5 development series.
+
+The kernel will autodetect which format superblock is being used.
+
+Superblock format ``0`` is treated differently to others for legacy
+reasons - it is the original superblock format.
+
+
+General Rules - apply for all superblock formats
+------------------------------------------------
+
+An array is ``created`` by writing appropriate superblocks to all
+devices.
+
+It is ``assembled`` by associating each of these devices with an
+particular md virtual device.  Once it is completely assembled, it can
+be accessed.
+
+An array should be created by a user-space tool.  This will write
+superblocks to all devices.  It will usually mark the array as
+``unclean``, or with some devices missing so that the kernel md driver
+can create appropriate redundancy (copying in raid 1, parity
+calculation in raid 4/5).
+
+When an array is assembled, it is first initialized with the
+SET_ARRAY_INFO ioctl.  This contains, in particular, a major and minor
+version number.  The major version number selects which superblock
+format is to be used.  The minor number might be used to tune handling
+of the format, such as suggesting where on each device to look for the
+superblock.
+
+Then each device is added using the ADD_NEW_DISK ioctl.  This
+provides, in particular, a major and minor number identifying the
+device to add.
+
+The array is started with the RUN_ARRAY ioctl.
+
+Once started, new devices can be added.  They should have an
+appropriate superblock written to them, and then be passed in with
+ADD_NEW_DISK.
+
+Devices that have failed or are not yet active can be detached from an
+array using HOT_REMOVE_DISK.
+
+
+Specific Rules that apply to format-0 super block arrays, and arrays with no superblock (non-persistent)
+--------------------------------------------------------------------------------------------------------
+
+An array can be ``created`` by describing the array (level, chunksize
+etc) in a SET_ARRAY_INFO ioctl.  This must have ``major_version==0`` and
+``raid_disks != 0``.
+
+Then uninitialized devices can be added with ADD_NEW_DISK.  The
+structure passed to ADD_NEW_DISK must specify the state of the device
+and its role in the array.
+
+Once started with RUN_ARRAY, uninitialized spares can be added with
+HOT_ADD_DISK.
+
+
+MD devices in sysfs
+-------------------
+
+md devices appear in sysfs (``/sys``) as regular block devices,
+e.g.::
+
+   /sys/block/md0
+
+Each ``md`` device will contain a subdirectory called ``md`` which
+contains further md-specific information about the device.
+
+All md devices contain:
+
+  level
+     a text file indicating the ``raid level``. e.g. raid0, raid1,
+     raid5, linear, multipath, faulty.
+     If no raid level has been set yet (array is still being
+     assembled), the value will reflect whatever has been written
+     to it, which may be a name like the above, or may be a number
+     such as ``0``, ``5``, etc.
+
+  raid_disks
+     a text file with a simple number indicating the number of devices
+     in a fully functional array.  If this is not yet known, the file
+     will be empty.  If an array is being resized this will contain
+     the new number of devices.
+     Some raid levels allow this value to be set while the array is
+     active.  This will reconfigure the array.   Otherwise it can only
+     be set while assembling an array.
+     A change to this attribute will not be permitted if it would
+     reduce the size of the array.  To reduce the number of drives
+     in an e.g. raid5, the array size must first be reduced by
+     setting the ``array_size`` attribute.
+
+  chunk_size
+     This is the size in bytes for ``chunks`` and is only relevant to
+     raid levels that involve striping (0,4,5,6,10). The address space
+     of the array is conceptually divided into chunks and consecutive
+     chunks are striped onto neighbouring devices.
+     The size should be at least PAGE_SIZE (4k) and should be a power
+     of 2.  This can only be set while assembling an array
+
+  layout
+     The ``layout`` for the array for the particular level.  This is
+     simply a number that is interpretted differently by different
+     levels.  It can be written while assembling an array.
+
+  array_size
+     This can be used to artificially constrain the available space in
+     the array to be less than is actually available on the combined
+     devices.  Writing a number (in Kilobytes) which is less than
+     the available size will set the size.  Any reconfiguration of the
+     array (e.g. adding devices) will not cause the size to change.
+     Writing the word ``default`` will cause the effective size of the
+     array to be whatever size is actually available based on
+     ``level``, ``chunk_size`` and ``component_size``.
+
+     This can be used to reduce the size of the array before reducing
+     the number of devices in a raid4/5/6, or to support external
+     metadata formats which mandate such clipping.
+
+  reshape_position
+     This is either ``none`` or a sector number within the devices of
+     the array where ``reshape`` is up to.  If this is set, the three
+     attributes mentioned above (raid_disks, chunk_size, layout) can
+     potentially have 2 values, an old and a new value.  If these
+     values differ, reading the attribute returns::
+
+        new (old)
+
+     and writing will effect the ``new`` value, leaving the ``old``
+     unchanged.
+
+  component_size
+     For arrays with data redundancy (i.e. not raid0, linear, faulty,
+     multipath), all components must be the same size - or at least
+     there must a size that they all provide space for.  This is a key
+     part or the geometry of the array.  It is measured in sectors
+     and can be read from here.  Writing to this value may resize
+     the array if the personality supports it (raid1, raid5, raid6),
+     and if the component drives are large enough.
+
+  metadata_version
+     This indicates the format that is being used to record metadata
+     about the array.  It can be 0.90 (traditional format), 1.0, 1.1,
+     1.2 (newer format in varying locations) or ``none`` indicating that
+     the kernel isn't managing metadata at all.
+     Alternately it can be ``external:`` followed by a string which
+     is set by user-space.  This indicates that metadata is managed
+     by a user-space program.  Any device failure or other event that
+     requires a metadata update will cause array activity to be
+     suspended until the event is acknowledged.
+
+  resync_start
+     The point at which resync should start.  If no resync is needed,
+     this will be a very large number (or ``none`` since 2.6.30-rc1).  At
+     array creation it will default to 0, though starting the array as
+     ``clean`` will set it much larger.
+
+  new_dev
+     This file can be written but not read.  The value written should
+     be a block device number as major:minor.  e.g. 8:0
+     This will cause that device to be attached to the array, if it is
+     available.  It will then appear at md/dev-XXX (depending on the
+     name of the device) and further configuration is then possible.
+
+  safe_mode_delay
+     When an md array has seen no write requests for a certain period
+     of time, it will be marked as ``clean``.  When another write
+     request arrives, the array is marked as ``dirty`` before the write
+     commences.  This is known as ``safe_mode``.
+     The ``certain period`` is controlled by this file which stores the
+     period as a number of seconds.  The default is 200msec (0.200).
+     Writing a value of 0 disables safemode.
+
+  array_state
+     This file contains a single word which describes the current
+     state of the array.  In many cases, the state can be set by
+     writing the word for the desired state, however some states
+     cannot be explicitly set, and some transitions are not allowed.
+
+     Select/poll works on this file.  All changes except between
+     Active_idle and active (which can be frequent and are not
+     very interesting) are notified.  active->active_idle is
+     reported if the metadata is externally managed.
+
+     clear
+         No devices, no size, no level
+
+         Writing is equivalent to STOP_ARRAY ioctl
+
+     inactive
+         May have some settings, but array is not active
+         all IO results in error
+
+         When written, doesn't tear down array, but just stops it
+
+     suspended (not supported yet)
+         All IO requests will block. The array can be reconfigured.
+
+         Writing this, if accepted, will block until array is quiessent
+
+     readonly
+         no resync can happen.  no superblocks get written.
+
+         Write requests fail
+
+     read-auto
+         like readonly, but behaves like ``clean`` on a write request.
+
+     clean
+         no pending writes, but otherwise active.
+
+         When written to inactive array, starts without resync
+
+         If a write request arrives then
+         if metadata is known, mark ``dirty`` and switch to ``active``.
+         if not known, block and switch to write-pending
+
+         If written to an active array that has pending writes, then fails.
+     active
+         fully active: IO and resync can be happening.
+         When written to inactive array, starts with resync
+
+     write-pending
+         clean, but writes are blocked waiting for ``active`` to be written.
+
+     active-idle
+         like active, but no writes have been seen for a while (safe_mode_delay).
+
+  bitmap/location
+     This indicates where the write-intent bitmap for the array is
+     stored.
+
+     It can be one of ``none``, ``file`` or ``[+-]N``.
+     ``file`` may later be extended to ``file:/file/name``
+     ``[+-]N`` means that many sectors from the start of the metadata.
+
+     This is replicated on all devices.  For arrays with externally
+     managed metadata, the offset is from the beginning of the
+     device.
+
+  bitmap/chunksize
+     The size, in bytes, of the chunk which will be represented by a
+     single bit.  For RAID456, it is a portion of an individual
+     device. For RAID10, it is a portion of the array.  For RAID1, it
+     is both (they come to the same thing).
+
+  bitmap/time_base
+     The time, in seconds, between looking for bits in the bitmap to
+     be cleared. In the current implementation, a bit will be cleared
+     between 2 and 3 times ``time_base`` after all the covered blocks
+     are known to be in-sync.
+
+  bitmap/backlog
+     When write-mostly devices are active in a RAID1, write requests
+     to those devices proceed in the background - the filesystem (or
+     other user of the device) does not have to wait for them.
+     ``backlog`` sets a limit on the number of concurrent background
+     writes.  If there are more than this, new writes will by
+     synchronous.
+
+  bitmap/metadata
+     This can be either ``internal`` or ``external``.
+
+     ``internal``
+       is the default and means the metadata for the bitmap
+       is stored in the first 256 bytes of the allocated space and is
+       managed by the md module.
+
+     ``external``
+       means that bitmap metadata is managed externally to
+       the kernel (i.e. by some userspace program)
+
+  bitmap/can_clear
+     This is either ``true`` or ``false``.  If ``true``, then bits in the
+     bitmap will be cleared when the corresponding blocks are thought
+     to be in-sync.  If ``false``, bits will never be cleared.
+     This is automatically set to ``false`` if a write happens on a
+     degraded array, or if the array becomes degraded during a write.
+     When metadata is managed externally, it should be set to true
+     once the array becomes non-degraded, and this fact has been
+     recorded in the metadata.
+
+  consistency_policy
+     This indicates how the array maintains consistency in case of unexpected
+     shutdown. It can be:
+
+     none
+       Array has no redundancy information, e.g. raid0, linear.
+
+     resync
+       Full resync is performed and all redundancy is regenerated when the
+       array is started after unclean shutdown.
+
+     bitmap
+       Resync assisted by a write-intent bitmap.
+
+     journal
+       For raid4/5/6, journal device is used to log transactions and replay
+       after unclean shutdown.
+
+     ppl
+       For raid5 only, Partial Parity Log is used to close the write hole and
+       eliminate resync.
+
+     The accepted values when writing to this file are ``ppl`` and ``resync``,
+     used to enable and disable PPL.
+
+  uuid
+     This indicates the UUID of the array in the following format:
+     xxxxxxxx-xxxx-xxxx-xxxx-xxxxxxxxxxxx
+
+
+As component devices are added to an md array, they appear in the ``md``
+directory as new directories named::
+
+      dev-XXX
+
+where ``XXX`` is a name that the kernel knows for the device, e.g. hdb1.
+Each directory contains:
+
+      block
+        a symlink to the block device in /sys/block, e.g.::
+
+	     /sys/block/md0/md/dev-hdb1/block -> ../../../../block/hdb/hdb1
+
+      super
+        A file containing an image of the superblock read from, or
+        written to, that device.
+
+      state
+	A file recording the current state of the device in the array
+	which can be a comma separated list of:
+
+	      faulty
+			device has been kicked from active use due to
+			a detected fault, or it has unacknowledged bad
+			blocks
+
+	      in_sync
+			device is a fully in-sync member of the array
+
+	      writemostly
+			device will only be subject to read
+			requests if there are no other options.
+
+			This applies only to raid1 arrays.
+
+	      blocked
+			device has failed, and the failure hasn't been
+			acknowledged yet by the metadata handler.
+
+			Writes that would write to this device if
+			it were not faulty are blocked.
+
+	      spare
+			device is working, but not a full member.
+
+			This includes spares that are in the process
+			of being recovered to
+
+	      write_error
+			device has ever seen a write error.
+
+	      want_replacement
+			device is (mostly) working but probably
+			should be replaced, either due to errors or
+			due to user request.
+
+	      replacement
+			device is a replacement for another active
+			device with same raid_disk.
+
+
+	This list may grow in future.
+
+	This can be written to.
+
+	Writing ``faulty``  simulates a failure on the device.
+
+	Writing ``remove`` removes the device from the array.
+
+	Writing ``writemostly`` sets the writemostly flag.
+
+	Writing ``-writemostly`` clears the writemostly flag.
+
+	Writing ``blocked`` sets the ``blocked`` flag.
+
+	Writing ``-blocked`` clears the ``blocked`` flags and allows writes
+	to complete and possibly simulates an error.
+
+	Writing ``in_sync`` sets the in_sync flag.
+
+	Writing ``write_error`` sets writeerrorseen flag.
+
+	Writing ``-write_error`` clears writeerrorseen flag.
+
+	Writing ``want_replacement`` is allowed at any time except to a
+	replacement device or a spare.  It sets the flag.
+
+	Writing ``-want_replacement`` is allowed at any time.  It clears
+	the flag.
+
+	Writing ``replacement`` or ``-replacement`` is only allowed before
+	starting the array.  It sets or clears the flag.
+
+
+	This file responds to select/poll. Any change to ``faulty``
+	or ``blocked`` causes an event.
+
+      errors
+	An approximate count of read errors that have been detected on
+	this device but have not caused the device to be evicted from
+	the array (either because they were corrected or because they
+	happened while the array was read-only).  When using version-1
+	metadata, this value persists across restarts of the array.
+
+	This value can be written while assembling an array thus
+	providing an ongoing count for arrays with metadata managed by
+	userspace.
+
+      slot
+        This gives the role that the device has in the array.  It will
+	either be ``none`` if the device is not active in the array
+        (i.e. is a spare or has failed) or an integer less than the
+	``raid_disks`` number for the array indicating which position
+	it currently fills.  This can only be set while assembling an
+	array.  A device for which this is set is assumed to be working.
+
+      offset
+        This gives the location in the device (in sectors from the
+        start) where data from the array will be stored.  Any part of
+        the device before this offset is not touched, unless it is
+        used for storing metadata (Formats 1.1 and 1.2).
+
+      size
+        The amount of the device, after the offset, that can be used
+        for storage of data.  This will normally be the same as the
+	component_size.  This can be written while assembling an
+        array.  If a value less than the current component_size is
+        written, it will be rejected.
+
+      recovery_start
+        When the device is not ``in_sync``, this records the number of
+	sectors from the start of the device which are known to be
+	correct.  This is normally zero, but during a recovery
+	operation it will steadily increase, and if the recovery is
+	interrupted, restoring this value can cause recovery to
+	avoid repeating the earlier blocks.  With v1.x metadata, this
+	value is saved and restored automatically.
+
+	This can be set whenever the device is not an active member of
+	the array, either before the array is activated, or before
+	the ``slot`` is set.
+
+	Setting this to ``none`` is equivalent to setting ``in_sync``.
+	Setting to any other value also clears the ``in_sync`` flag.
+
+      bad_blocks
+	This gives the list of all known bad blocks in the form of
+	start address and length (in sectors respectively). If output
+	is too big to fit in a page, it will be truncated. Writing
+	``sector length`` to this file adds new acknowledged (i.e.
+	recorded to disk safely) bad blocks.
+
+      unacknowledged_bad_blocks
+	This gives the list of known-but-not-yet-saved-to-disk bad
+	blocks in the same form of ``bad_blocks``. If output is too big
+	to fit in a page, it will be truncated. Writing to this file
+	adds bad blocks without acknowledging them. This is largely
+	for testing.
+
+      ppl_sector, ppl_size
+        Location and size (in sectors) of the space used for Partial Parity Log
+        on this device.
+
+
+An active md device will also contain an entry for each active device
+in the array.  These are named::
+
+    rdNN
+
+where ``NN`` is the position in the array, starting from 0.
+So for a 3 drive array there will be rd0, rd1, rd2.
+These are symbolic links to the appropriate ``dev-XXX`` entry.
+Thus, for example::
+
+       cat /sys/block/md*/md/rd*/state
+
+will show ``in_sync`` on every line.
+
+
+
+Active md devices for levels that support data redundancy (1,4,5,6,10)
+also have
+
+   sync_action
+     a text file that can be used to monitor and control the rebuild
+     process.  It contains one word which can be one of:
+
+       resync
+		redundancy is being recalculated after unclean
+                shutdown or creation
+
+       recover
+		a hot spare is being built to replace a
+		failed/missing device
+
+       idle
+		nothing is happening
+       check
+		A full check of redundancy was requested and is
+                happening.  This reads all blocks and checks
+                them. A repair may also happen for some raid
+                levels.
+
+       repair
+		A full check and repair is happening.  This is
+		similar to ``resync``, but was requested by the
+                user, and the write-intent bitmap is NOT used to
+		optimise the process.
+
+      This file is writable, and each of the strings that could be
+      read are meaningful for writing.
+
+	``idle`` will stop an active resync/recovery etc.  There is no
+	guarantee that another resync/recovery may not be automatically
+	started again, though some event will be needed to trigger
+	this.
+
+	``resync`` or ``recovery`` can be used to restart the
+        corresponding operation if it was stopped with ``idle``.
+
+	``check`` and ``repair`` will start the appropriate process
+	providing the current state is ``idle``.
+
+      This file responds to select/poll.  Any important change in the value
+      triggers a poll event.  Sometimes the value will briefly be
+      ``recover`` if a recovery seems to be needed, but cannot be
+      achieved. In that case, the transition to ``recover`` isn't
+      notified, but the transition away is.
+
+   degraded
+      This contains a count of the number of devices by which the
+      arrays is degraded.  So an optimal array will show ``0``.  A
+      single failed/missing drive will show ``1``, etc.
+
+      This file responds to select/poll, any increase or decrease
+      in the count of missing devices will trigger an event.
+
+   mismatch_count
+      When performing ``check`` and ``repair``, and possibly when
+      performing ``resync``, md will count the number of errors that are
+      found.  The count in ``mismatch_cnt`` is the number of sectors
+      that were re-written, or (for ``check``) would have been
+      re-written.  As most raid levels work in units of pages rather
+      than sectors, this may be larger than the number of actual errors
+      by a factor of the number of sectors in a page.
+
+   bitmap_set_bits
+      If the array has a write-intent bitmap, then writing to this
+      attribute can set bits in the bitmap, indicating that a resync
+      would need to check the corresponding blocks. Either individual
+      numbers or start-end pairs can be written.  Multiple numbers
+      can be separated by a space.
+
+      Note that the numbers are ``bit`` numbers, not ``block`` numbers.
+      They should be scaled by the bitmap_chunksize.
+
+   sync_speed_min, sync_speed_max
+     This are similar to ``/proc/sys/dev/raid/speed_limit_{min,max}``
+     however they only apply to the particular array.
+
+     If no value has been written to these, or if the word ``system``
+     is written, then the system-wide value is used.  If a value,
+     in kibibytes-per-second is written, then it is used.
+
+     When the files are read, they show the currently active value
+     followed by ``(local)`` or ``(system)`` depending on whether it is
+     a locally set or system-wide value.
+
+   sync_completed
+     This shows the number of sectors that have been completed of
+     whatever the current sync_action is, followed by the number of
+     sectors in total that could need to be processed.  The two
+     numbers are separated by a ``/``  thus effectively showing one
+     value, a fraction of the process that is complete.
+
+     A ``select`` on this attribute will return when resync completes,
+     when it reaches the current sync_max (below) and possibly at
+     other times.
+
+   sync_speed
+     This shows the current actual speed, in K/sec, of the current
+     sync_action.  It is averaged over the last 30 seconds.
+
+   suspend_lo, suspend_hi
+     The two values, given as numbers of sectors, indicate a range
+     within the array where IO will be blocked.  This is currently
+     only supported for raid4/5/6.
+
+   sync_min, sync_max
+     The two values, given as numbers of sectors, indicate a range
+     within the array where ``check``/``repair`` will operate. Must be
+     a multiple of chunk_size. When it reaches ``sync_max`` it will
+     pause, rather than complete.
+     You can use ``select`` or ``poll`` on ``sync_completed`` to wait for
+     that number to reach sync_max.  Then you can either increase
+     ``sync_max``, or can write ``idle`` to ``sync_action``.
+
+     The value of ``max`` for ``sync_max`` effectively disables the limit.
+     When a resync is active, the value can only ever be increased,
+     never decreased.
+     The value of ``0`` is the minimum for ``sync_min``.
+
+
+
+Each active md device may also have attributes specific to the
+personality module that manages it.
+These are specific to the implementation of the module and could
+change substantially if the implementation changes.
+
+These currently include:
+
+  stripe_cache_size  (currently raid5 only)
+      number of entries in the stripe cache.  This is writable, but
+      there are upper and lower limits (32768, 17).  Default is 256.
+
+  strip_cache_active (currently raid5 only)
+      number of active entries in the stripe cache
+
+  preread_bypass_threshold (currently raid5 only)
+      number of times a stripe requiring preread will be bypassed by
+      a stripe that does not require preread.  For fairness defaults
+      to 1.  Setting this to 0 disables bypass accounting and
+      requires preread stripes to wait until all full-width stripe-
+      writes are complete.  Valid values are 0 to stripe_cache_size.
+
+  journal_mode (currently raid5 only)
+      The cache mode for raid5. raid5 could include an extra disk for
+      caching. The mode can be "write-throuth" and "write-back". The
+      default is "write-through".
+
+  ppl_write_hint
+      NVMe stream ID to be set for each PPL write request.