1RAID arrays
2===========
3
4Boot time assembly of RAID arrays
5---------------------------------
6
7Tools that manage md devices can be found at
8   https://www.kernel.org/pub/linux/utils/raid/
9
10
11You can boot with your md device with the following kernel command
12lines:
13
14for old raid arrays without persistent superblocks::
15
16  md=<md device no.>,<raid level>,<chunk size factor>,<fault level>,dev0,dev1,...,devn
17
18for raid arrays with persistent superblocks::
19
20  md=<md device no.>,dev0,dev1,...,devn
21
22or, to assemble a partitionable array::
23
24  md=d<md device no.>,dev0,dev1,...,devn
25
26``md device no.``
27+++++++++++++++++
28
29The number of the md device
30
31================= =========
32``md device no.`` device
33================= =========
34              0		md0
35	      1		md1
36	      2		md2
37	      3		md3
38	      4		md4
39================= =========
40
41``raid level``
42++++++++++++++
43
44level of the RAID array
45
46=============== =============
47``raid level``  level
48=============== =============
49-1		linear mode
500		striped mode
51=============== =============
52
53other modes are only supported with persistent super blocks
54
55``chunk size factor``
56+++++++++++++++++++++
57
58(raid-0 and raid-1 only)
59
60Set  the chunk size as 4k << n.
61
62``fault level``
63+++++++++++++++
64
65Totally ignored
66
67``dev0`` to ``devn``
68++++++++++++++++++++
69
70e.g. ``/dev/hda1``, ``/dev/hdc1``, ``/dev/sda1``, ``/dev/sdb1``
71
72A possible loadlin line (Harald Hoyer <HarryH@Royal.Net>)  looks like this::
73
74	e:\loadlin\loadlin e:\zimage root=/dev/md0 md=0,0,4,0,/dev/hdb2,/dev/hdc3 ro
75
76
77Boot time autodetection of RAID arrays
78--------------------------------------
79
80When md is compiled into the kernel (not as module), partitions of
81type 0xfd are scanned and automatically assembled into RAID arrays.
82This autodetection may be suppressed with the kernel parameter
83``raid=noautodetect``.  As of kernel 2.6.9, only drives with a type 0
84superblock can be autodetected and run at boot time.
85
86The kernel parameter ``raid=partitionable`` (or ``raid=part``) means
87that all auto-detected arrays are assembled as partitionable.
88
89Boot time assembly of degraded/dirty arrays
90-------------------------------------------
91
92If a raid5 or raid6 array is both dirty and degraded, it could have
93undetectable data corruption.  This is because the fact that it is
94``dirty`` means that the parity cannot be trusted, and the fact that it
95is degraded means that some datablocks are missing and cannot reliably
96be reconstructed (due to no parity).
97
98For this reason, md will normally refuse to start such an array.  This
99requires the sysadmin to take action to explicitly start the array
100despite possible corruption.  This is normally done with::
101
102   mdadm --assemble --force ....
103
104This option is not really available if the array has the root
105filesystem on it.  In order to support this booting from such an
106array, md supports a module parameter ``start_dirty_degraded`` which,
107when set to 1, bypassed the checks and will allows dirty degraded
108arrays to be started.
109
110So, to boot with a root filesystem of a dirty degraded raid 5 or 6, use::
111
112   md-mod.start_dirty_degraded=1
113
114
115Superblock formats
116------------------
117
118The md driver can support a variety of different superblock formats.
119Currently, it supports superblock formats ``0.90.0`` and the ``md-1`` format
120introduced in the 2.5 development series.
121
122The kernel will autodetect which format superblock is being used.
123
124Superblock format ``0`` is treated differently to others for legacy
125reasons - it is the original superblock format.
126
127
128General Rules - apply for all superblock formats
129------------------------------------------------
130
131An array is ``created`` by writing appropriate superblocks to all
132devices.
133
134It is ``assembled`` by associating each of these devices with an
135particular md virtual device.  Once it is completely assembled, it can
136be accessed.
137
138An array should be created by a user-space tool.  This will write
139superblocks to all devices.  It will usually mark the array as
140``unclean``, or with some devices missing so that the kernel md driver
141can create appropriate redundancy (copying in raid 1, parity
142calculation in raid 4/5).
143
144When an array is assembled, it is first initialized with the
145SET_ARRAY_INFO ioctl.  This contains, in particular, a major and minor
146version number.  The major version number selects which superblock
147format is to be used.  The minor number might be used to tune handling
148of the format, such as suggesting where on each device to look for the
149superblock.
150
151Then each device is added using the ADD_NEW_DISK ioctl.  This
152provides, in particular, a major and minor number identifying the
153device to add.
154
155The array is started with the RUN_ARRAY ioctl.
156
157Once started, new devices can be added.  They should have an
158appropriate superblock written to them, and then be passed in with
159ADD_NEW_DISK.
160
161Devices that have failed or are not yet active can be detached from an
162array using HOT_REMOVE_DISK.
163
164
165Specific Rules that apply to format-0 super block arrays, and arrays with no superblock (non-persistent)
166--------------------------------------------------------------------------------------------------------
167
168An array can be ``created`` by describing the array (level, chunksize
169etc) in a SET_ARRAY_INFO ioctl.  This must have ``major_version==0`` and
170``raid_disks != 0``.
171
172Then uninitialized devices can be added with ADD_NEW_DISK.  The
173structure passed to ADD_NEW_DISK must specify the state of the device
174and its role in the array.
175
176Once started with RUN_ARRAY, uninitialized spares can be added with
177HOT_ADD_DISK.
178
179
180MD devices in sysfs
181-------------------
182
183md devices appear in sysfs (``/sys``) as regular block devices,
184e.g.::
185
186   /sys/block/md0
187
188Each ``md`` device will contain a subdirectory called ``md`` which
189contains further md-specific information about the device.
190
191All md devices contain:
192
193  level
194     a text file indicating the ``raid level``. e.g. raid0, raid1,
195     raid5, linear, multipath, faulty.
196     If no raid level has been set yet (array is still being
197     assembled), the value will reflect whatever has been written
198     to it, which may be a name like the above, or may be a number
199     such as ``0``, ``5``, etc.
200
201  raid_disks
202     a text file with a simple number indicating the number of devices
203     in a fully functional array.  If this is not yet known, the file
204     will be empty.  If an array is being resized this will contain
205     the new number of devices.
206     Some raid levels allow this value to be set while the array is
207     active.  This will reconfigure the array.   Otherwise it can only
208     be set while assembling an array.
209     A change to this attribute will not be permitted if it would
210     reduce the size of the array.  To reduce the number of drives
211     in an e.g. raid5, the array size must first be reduced by
212     setting the ``array_size`` attribute.
213
214  chunk_size
215     This is the size in bytes for ``chunks`` and is only relevant to
216     raid levels that involve striping (0,4,5,6,10). The address space
217     of the array is conceptually divided into chunks and consecutive
218     chunks are striped onto neighbouring devices.
219     The size should be at least PAGE_SIZE (4k) and should be a power
220     of 2.  This can only be set while assembling an array
221
222  layout
223     The ``layout`` for the array for the particular level.  This is
224     simply a number that is interpreted differently by different
225     levels.  It can be written while assembling an array.
226
227  array_size
228     This can be used to artificially constrain the available space in
229     the array to be less than is actually available on the combined
230     devices.  Writing a number (in Kilobytes) which is less than
231     the available size will set the size.  Any reconfiguration of the
232     array (e.g. adding devices) will not cause the size to change.
233     Writing the word ``default`` will cause the effective size of the
234     array to be whatever size is actually available based on
235     ``level``, ``chunk_size`` and ``component_size``.
236
237     This can be used to reduce the size of the array before reducing
238     the number of devices in a raid4/5/6, or to support external
239     metadata formats which mandate such clipping.
240
241  reshape_position
242     This is either ``none`` or a sector number within the devices of
243     the array where ``reshape`` is up to.  If this is set, the three
244     attributes mentioned above (raid_disks, chunk_size, layout) can
245     potentially have 2 values, an old and a new value.  If these
246     values differ, reading the attribute returns::
247
248        new (old)
249
250     and writing will effect the ``new`` value, leaving the ``old``
251     unchanged.
252
253  component_size
254     For arrays with data redundancy (i.e. not raid0, linear, faulty,
255     multipath), all components must be the same size - or at least
256     there must a size that they all provide space for.  This is a key
257     part or the geometry of the array.  It is measured in sectors
258     and can be read from here.  Writing to this value may resize
259     the array if the personality supports it (raid1, raid5, raid6),
260     and if the component drives are large enough.
261
262  metadata_version
263     This indicates the format that is being used to record metadata
264     about the array.  It can be 0.90 (traditional format), 1.0, 1.1,
265     1.2 (newer format in varying locations) or ``none`` indicating that
266     the kernel isn't managing metadata at all.
267     Alternately it can be ``external:`` followed by a string which
268     is set by user-space.  This indicates that metadata is managed
269     by a user-space program.  Any device failure or other event that
270     requires a metadata update will cause array activity to be
271     suspended until the event is acknowledged.
272
273  resync_start
274     The point at which resync should start.  If no resync is needed,
275     this will be a very large number (or ``none`` since 2.6.30-rc1).  At
276     array creation it will default to 0, though starting the array as
277     ``clean`` will set it much larger.
278
279  new_dev
280     This file can be written but not read.  The value written should
281     be a block device number as major:minor.  e.g. 8:0
282     This will cause that device to be attached to the array, if it is
283     available.  It will then appear at md/dev-XXX (depending on the
284     name of the device) and further configuration is then possible.
285
286  safe_mode_delay
287     When an md array has seen no write requests for a certain period
288     of time, it will be marked as ``clean``.  When another write
289     request arrives, the array is marked as ``dirty`` before the write
290     commences.  This is known as ``safe_mode``.
291     The ``certain period`` is controlled by this file which stores the
292     period as a number of seconds.  The default is 200msec (0.200).
293     Writing a value of 0 disables safemode.
294
295  array_state
296     This file contains a single word which describes the current
297     state of the array.  In many cases, the state can be set by
298     writing the word for the desired state, however some states
299     cannot be explicitly set, and some transitions are not allowed.
300
301     Select/poll works on this file.  All changes except between
302     Active_idle and active (which can be frequent and are not
303     very interesting) are notified.  active->active_idle is
304     reported if the metadata is externally managed.
305
306     clear
307         No devices, no size, no level
308
309         Writing is equivalent to STOP_ARRAY ioctl
310
311     inactive
312         May have some settings, but array is not active
313         all IO results in error
314
315         When written, doesn't tear down array, but just stops it
316
317     suspended (not supported yet)
318         All IO requests will block. The array can be reconfigured.
319
320         Writing this, if accepted, will block until array is quiessent
321
322     readonly
323         no resync can happen.  no superblocks get written.
324
325         Write requests fail
326
327     read-auto
328         like readonly, but behaves like ``clean`` on a write request.
329
330     clean
331         no pending writes, but otherwise active.
332
333         When written to inactive array, starts without resync
334
335         If a write request arrives then
336         if metadata is known, mark ``dirty`` and switch to ``active``.
337         if not known, block and switch to write-pending
338
339         If written to an active array that has pending writes, then fails.
340     active
341         fully active: IO and resync can be happening.
342         When written to inactive array, starts with resync
343
344     write-pending
345         clean, but writes are blocked waiting for ``active`` to be written.
346
347     active-idle
348         like active, but no writes have been seen for a while (safe_mode_delay).
349
350  bitmap/location
351     This indicates where the write-intent bitmap for the array is
352     stored.
353
354     It can be one of ``none``, ``file`` or ``[+-]N``.
355     ``file`` may later be extended to ``file:/file/name``
356     ``[+-]N`` means that many sectors from the start of the metadata.
357
358     This is replicated on all devices.  For arrays with externally
359     managed metadata, the offset is from the beginning of the
360     device.
361
362  bitmap/chunksize
363     The size, in bytes, of the chunk which will be represented by a
364     single bit.  For RAID456, it is a portion of an individual
365     device. For RAID10, it is a portion of the array.  For RAID1, it
366     is both (they come to the same thing).
367
368  bitmap/time_base
369     The time, in seconds, between looking for bits in the bitmap to
370     be cleared. In the current implementation, a bit will be cleared
371     between 2 and 3 times ``time_base`` after all the covered blocks
372     are known to be in-sync.
373
374  bitmap/backlog
375     When write-mostly devices are active in a RAID1, write requests
376     to those devices proceed in the background - the filesystem (or
377     other user of the device) does not have to wait for them.
378     ``backlog`` sets a limit on the number of concurrent background
379     writes.  If there are more than this, new writes will by
380     synchronous.
381
382  bitmap/metadata
383     This can be either ``internal`` or ``external``.
384
385     ``internal``
386       is the default and means the metadata for the bitmap
387       is stored in the first 256 bytes of the allocated space and is
388       managed by the md module.
389
390     ``external``
391       means that bitmap metadata is managed externally to
392       the kernel (i.e. by some userspace program)
393
394  bitmap/can_clear
395     This is either ``true`` or ``false``.  If ``true``, then bits in the
396     bitmap will be cleared when the corresponding blocks are thought
397     to be in-sync.  If ``false``, bits will never be cleared.
398     This is automatically set to ``false`` if a write happens on a
399     degraded array, or if the array becomes degraded during a write.
400     When metadata is managed externally, it should be set to true
401     once the array becomes non-degraded, and this fact has been
402     recorded in the metadata.
403
404  consistency_policy
405     This indicates how the array maintains consistency in case of unexpected
406     shutdown. It can be:
407
408     none
409       Array has no redundancy information, e.g. raid0, linear.
410
411     resync
412       Full resync is performed and all redundancy is regenerated when the
413       array is started after unclean shutdown.
414
415     bitmap
416       Resync assisted by a write-intent bitmap.
417
418     journal
419       For raid4/5/6, journal device is used to log transactions and replay
420       after unclean shutdown.
421
422     ppl
423       For raid5 only, Partial Parity Log is used to close the write hole and
424       eliminate resync.
425
426     The accepted values when writing to this file are ``ppl`` and ``resync``,
427     used to enable and disable PPL.
428
429  uuid
430     This indicates the UUID of the array in the following format:
431     xxxxxxxx-xxxx-xxxx-xxxx-xxxxxxxxxxxx
432
433
434As component devices are added to an md array, they appear in the ``md``
435directory as new directories named::
436
437      dev-XXX
438
439where ``XXX`` is a name that the kernel knows for the device, e.g. hdb1.
440Each directory contains:
441
442      block
443        a symlink to the block device in /sys/block, e.g.::
444
445	     /sys/block/md0/md/dev-hdb1/block -> ../../../../block/hdb/hdb1
446
447      super
448        A file containing an image of the superblock read from, or
449        written to, that device.
450
451      state
452	A file recording the current state of the device in the array
453	which can be a comma separated list of:
454
455	      faulty
456			device has been kicked from active use due to
457			a detected fault, or it has unacknowledged bad
458			blocks
459
460	      in_sync
461			device is a fully in-sync member of the array
462
463	      writemostly
464			device will only be subject to read
465			requests if there are no other options.
466
467			This applies only to raid1 arrays.
468
469	      blocked
470			device has failed, and the failure hasn't been
471			acknowledged yet by the metadata handler.
472
473			Writes that would write to this device if
474			it were not faulty are blocked.
475
476	      spare
477			device is working, but not a full member.
478
479			This includes spares that are in the process
480			of being recovered to
481
482	      write_error
483			device has ever seen a write error.
484
485	      want_replacement
486			device is (mostly) working but probably
487			should be replaced, either due to errors or
488			due to user request.
489
490	      replacement
491			device is a replacement for another active
492			device with same raid_disk.
493
494
495	This list may grow in future.
496
497	This can be written to.
498
499	Writing ``faulty``  simulates a failure on the device.
500
501	Writing ``remove`` removes the device from the array.
502
503	Writing ``writemostly`` sets the writemostly flag.
504
505	Writing ``-writemostly`` clears the writemostly flag.
506
507	Writing ``blocked`` sets the ``blocked`` flag.
508
509	Writing ``-blocked`` clears the ``blocked`` flags and allows writes
510	to complete and possibly simulates an error.
511
512	Writing ``in_sync`` sets the in_sync flag.
513
514	Writing ``write_error`` sets writeerrorseen flag.
515
516	Writing ``-write_error`` clears writeerrorseen flag.
517
518	Writing ``want_replacement`` is allowed at any time except to a
519	replacement device or a spare.  It sets the flag.
520
521	Writing ``-want_replacement`` is allowed at any time.  It clears
522	the flag.
523
524	Writing ``replacement`` or ``-replacement`` is only allowed before
525	starting the array.  It sets or clears the flag.
526
527
528	This file responds to select/poll. Any change to ``faulty``
529	or ``blocked`` causes an event.
530
531      errors
532	An approximate count of read errors that have been detected on
533	this device but have not caused the device to be evicted from
534	the array (either because they were corrected or because they
535	happened while the array was read-only).  When using version-1
536	metadata, this value persists across restarts of the array.
537
538	This value can be written while assembling an array thus
539	providing an ongoing count for arrays with metadata managed by
540	userspace.
541
542      slot
543        This gives the role that the device has in the array.  It will
544	either be ``none`` if the device is not active in the array
545        (i.e. is a spare or has failed) or an integer less than the
546	``raid_disks`` number for the array indicating which position
547	it currently fills.  This can only be set while assembling an
548	array.  A device for which this is set is assumed to be working.
549
550      offset
551        This gives the location in the device (in sectors from the
552        start) where data from the array will be stored.  Any part of
553        the device before this offset is not touched, unless it is
554        used for storing metadata (Formats 1.1 and 1.2).
555
556      size
557        The amount of the device, after the offset, that can be used
558        for storage of data.  This will normally be the same as the
559	component_size.  This can be written while assembling an
560        array.  If a value less than the current component_size is
561        written, it will be rejected.
562
563      recovery_start
564        When the device is not ``in_sync``, this records the number of
565	sectors from the start of the device which are known to be
566	correct.  This is normally zero, but during a recovery
567	operation it will steadily increase, and if the recovery is
568	interrupted, restoring this value can cause recovery to
569	avoid repeating the earlier blocks.  With v1.x metadata, this
570	value is saved and restored automatically.
571
572	This can be set whenever the device is not an active member of
573	the array, either before the array is activated, or before
574	the ``slot`` is set.
575
576	Setting this to ``none`` is equivalent to setting ``in_sync``.
577	Setting to any other value also clears the ``in_sync`` flag.
578
579      bad_blocks
580	This gives the list of all known bad blocks in the form of
581	start address and length (in sectors respectively). If output
582	is too big to fit in a page, it will be truncated. Writing
583	``sector length`` to this file adds new acknowledged (i.e.
584	recorded to disk safely) bad blocks.
585
586      unacknowledged_bad_blocks
587	This gives the list of known-but-not-yet-saved-to-disk bad
588	blocks in the same form of ``bad_blocks``. If output is too big
589	to fit in a page, it will be truncated. Writing to this file
590	adds bad blocks without acknowledging them. This is largely
591	for testing.
592
593      ppl_sector, ppl_size
594        Location and size (in sectors) of the space used for Partial Parity Log
595        on this device.
596
597
598An active md device will also contain an entry for each active device
599in the array.  These are named::
600
601    rdNN
602
603where ``NN`` is the position in the array, starting from 0.
604So for a 3 drive array there will be rd0, rd1, rd2.
605These are symbolic links to the appropriate ``dev-XXX`` entry.
606Thus, for example::
607
608       cat /sys/block/md*/md/rd*/state
609
610will show ``in_sync`` on every line.
611
612
613
614Active md devices for levels that support data redundancy (1,4,5,6,10)
615also have
616
617   sync_action
618     a text file that can be used to monitor and control the rebuild
619     process.  It contains one word which can be one of:
620
621       resync
622		redundancy is being recalculated after unclean
623                shutdown or creation
624
625       recover
626		a hot spare is being built to replace a
627		failed/missing device
628
629       idle
630		nothing is happening
631       check
632		A full check of redundancy was requested and is
633                happening.  This reads all blocks and checks
634                them. A repair may also happen for some raid
635                levels.
636
637       repair
638		A full check and repair is happening.  This is
639		similar to ``resync``, but was requested by the
640                user, and the write-intent bitmap is NOT used to
641		optimise the process.
642
643      This file is writable, and each of the strings that could be
644      read are meaningful for writing.
645
646	``idle`` will stop an active resync/recovery etc.  There is no
647	guarantee that another resync/recovery may not be automatically
648	started again, though some event will be needed to trigger
649	this.
650
651	``resync`` or ``recovery`` can be used to restart the
652        corresponding operation if it was stopped with ``idle``.
653
654	``check`` and ``repair`` will start the appropriate process
655	providing the current state is ``idle``.
656
657      This file responds to select/poll.  Any important change in the value
658      triggers a poll event.  Sometimes the value will briefly be
659      ``recover`` if a recovery seems to be needed, but cannot be
660      achieved. In that case, the transition to ``recover`` isn't
661      notified, but the transition away is.
662
663   degraded
664      This contains a count of the number of devices by which the
665      arrays is degraded.  So an optimal array will show ``0``.  A
666      single failed/missing drive will show ``1``, etc.
667
668      This file responds to select/poll, any increase or decrease
669      in the count of missing devices will trigger an event.
670
671   mismatch_count
672      When performing ``check`` and ``repair``, and possibly when
673      performing ``resync``, md will count the number of errors that are
674      found.  The count in ``mismatch_cnt`` is the number of sectors
675      that were re-written, or (for ``check``) would have been
676      re-written.  As most raid levels work in units of pages rather
677      than sectors, this may be larger than the number of actual errors
678      by a factor of the number of sectors in a page.
679
680   bitmap_set_bits
681      If the array has a write-intent bitmap, then writing to this
682      attribute can set bits in the bitmap, indicating that a resync
683      would need to check the corresponding blocks. Either individual
684      numbers or start-end pairs can be written.  Multiple numbers
685      can be separated by a space.
686
687      Note that the numbers are ``bit`` numbers, not ``block`` numbers.
688      They should be scaled by the bitmap_chunksize.
689
690   sync_speed_min, sync_speed_max
691     This are similar to ``/proc/sys/dev/raid/speed_limit_{min,max}``
692     however they only apply to the particular array.
693
694     If no value has been written to these, or if the word ``system``
695     is written, then the system-wide value is used.  If a value,
696     in kibibytes-per-second is written, then it is used.
697
698     When the files are read, they show the currently active value
699     followed by ``(local)`` or ``(system)`` depending on whether it is
700     a locally set or system-wide value.
701
702   sync_completed
703     This shows the number of sectors that have been completed of
704     whatever the current sync_action is, followed by the number of
705     sectors in total that could need to be processed.  The two
706     numbers are separated by a ``/``  thus effectively showing one
707     value, a fraction of the process that is complete.
708
709     A ``select`` on this attribute will return when resync completes,
710     when it reaches the current sync_max (below) and possibly at
711     other times.
712
713   sync_speed
714     This shows the current actual speed, in K/sec, of the current
715     sync_action.  It is averaged over the last 30 seconds.
716
717   suspend_lo, suspend_hi
718     The two values, given as numbers of sectors, indicate a range
719     within the array where IO will be blocked.  This is currently
720     only supported for raid4/5/6.
721
722   sync_min, sync_max
723     The two values, given as numbers of sectors, indicate a range
724     within the array where ``check``/``repair`` will operate. Must be
725     a multiple of chunk_size. When it reaches ``sync_max`` it will
726     pause, rather than complete.
727     You can use ``select`` or ``poll`` on ``sync_completed`` to wait for
728     that number to reach sync_max.  Then you can either increase
729     ``sync_max``, or can write ``idle`` to ``sync_action``.
730
731     The value of ``max`` for ``sync_max`` effectively disables the limit.
732     When a resync is active, the value can only ever be increased,
733     never decreased.
734     The value of ``0`` is the minimum for ``sync_min``.
735
736
737
738Each active md device may also have attributes specific to the
739personality module that manages it.
740These are specific to the implementation of the module and could
741change substantially if the implementation changes.
742
743These currently include:
744
745  stripe_cache_size  (currently raid5 only)
746      number of entries in the stripe cache.  This is writable, but
747      there are upper and lower limits (32768, 17).  Default is 256.
748
749  strip_cache_active (currently raid5 only)
750      number of active entries in the stripe cache
751
752  preread_bypass_threshold (currently raid5 only)
753      number of times a stripe requiring preread will be bypassed by
754      a stripe that does not require preread.  For fairness defaults
755      to 1.  Setting this to 0 disables bypass accounting and
756      requires preread stripes to wait until all full-width stripe-
757      writes are complete.  Valid values are 0 to stripe_cache_size.
758
759  journal_mode (currently raid5 only)
760      The cache mode for raid5. raid5 could include an extra disk for
761      caching. The mode can be "write-throuth" and "write-back". The
762      default is "write-through".
763
764  ppl_write_hint
765      NVMe stream ID to be set for each PPL write request.
766