.\" the Free Software Foundation; either version 2 of the License, or
.\" (at your option) any later version.
.\" See file COPYING in distribution for details.
+.if n .pl 1000v
.TH MD 4
.SH NAME
md \- Multiple Device driver aka Linux Software RAID
and FAULTY (a layer over a single device into which errors can be injected).
.SS MD METADATA
-Each device in an array may have some
+Each device in an array may have some
.I metadata
stored in the device. This metadata is sometimes called a
.BR superblock .
.TP
LEVEL
The manner in which the devices are arranged into the array
-(linear, raid0, raid1, raid4, raid5, raid10, multipath).
+(LINEAR, RAID0, RAID1, RAID4, RAID5, RAID10, MULTIPATH).
.TP
UUID
a 128 bit Universally Unique Identifier that identifies the array that
LEGACY ARRAYS
Early versions of the
.B md
-driver only supported Linear and Raid0 configurations and did not use
+driver only supported LINEAR and RAID0 configurations and did not use
a superblock (which is less critical with these configurations).
While such arrays should be rebuilt with superblocks if possible,
.B md
a MULTIPATH array with no superblock makes sense.
.TP
RAID1
-In some configurations it might be desired to create a raid1
+In some configurations it might be desired to create a RAID1
configuration that does not use a superblock, and to maintain the state of
-the array elsewhere. While not encouraged for general us, it does
+the array elsewhere. While not encouraged for general use, it does
have special-purpose uses and is supported.
.SS ARRAYS WITH EXTERNAL METADATA
From release 2.6.28, the
.I md
driver supports arrays with externally managed metadata. That is,
-the metadata is not managed by the kernel by rather by a user-space
+the metadata is not managed by the kernel but rather by a user-space
program which is external to the kernel. This allows support for a
variety of metadata formats without cluttering the kernel with lots of
details.
.I md
is able to communicate with the user-space program through various
sysfs attributes so that it can make appropriate changes to the
-metadata \- for example to make a device as faulty. When necessary,
+metadata \- for example to mark a device as faulty. When necessary,
.I md
will wait for the program to acknowledge the event by writing to a
sysfs attribute.
.SS LINEAR
-A linear array simply catenates the available space on each
+A LINEAR array simply catenates the available space on each
drive to form one large virtual drive.
One advantage of this arrangement over the more common RAID0
A RAID0 array (which has zero redundancy) is also known as a
striped array.
A RAID0 array is configured at creation with a
-.B "Chunk Size"
+.B "Chunk Size"
which must be a power of two (prior to Linux 2.6.31), and at least 4
kibibytes.
collecting chunks into smaller stripes that only span the drives which
still have remaining space.
+A bug was introduced in linux 3.14 which changed the layout of blocks in
+a RAID0 beyond the region that is striped over all devices. This bug
+does not affect an array with all devices the same size, but can affect
+other RAID0 arrays.
+
+Linux 5.4 (and some stable kernels to which the change was backported)
+will not normally assemble such an array as it cannot know which layout
+to use. There is a module parameter "raid0.default_layout" which can be
+set to "1" to force the kernel to use the pre-3.14 layout or to "2" to
+force it to use the 3.14-and-later layout. when creating a new RAID0
+array,
+.I mdadm
+will record the chosen layout in the metadata in a way that allows newer
+kernels to assemble the array without needing a module parameter.
+
+To assemble an old array on a new kernel without using the module parameter,
+use either the
+.B "--update=layout-original"
+option or the
+.B "--update=layout-alternate"
+option.
.SS RAID1
threads to read from all disks.
Individual devices in a RAID1 can be marked as "write-mostly".
-This drives are excluded from the normal read balancing and will only
+These drives are excluded from the normal read balancing and will only
be read from when there is no other option. This can be useful for
devices connected over a slow link.
drives.
When configuring a RAID10 array, it is necessary to specify the number
-of replicas of each data block that are required (this will normally
-be 2) and whether the replicas should be 'near', 'offset' or 'far'.
-(Note that the 'offset' layout is only available from 2.6.18).
-
-When 'near' replicas are chosen, the multiple copies of a given chunk
-are laid out consecutively across the stripes of the array, so the two
-copies of a datablock will likely be at the same offset on two
-adjacent devices.
-
-When 'far' replicas are chosen, the multiple copies of a given chunk
-are laid out quite distant from each other. The first copy of all
-data blocks will be striped across the early part of all drives in
-RAID0 fashion, and then the next copy of all blocks will be striped
-across a later section of all drives, always ensuring that all copies
-of any given block are on different drives.
-
-The 'far' arrangement can give sequential read performance equal to
-that of a RAID0 array, but at the cost of reduced write performance.
-
-When 'offset' replicas are chosen, the multiple copies of a given
-chunk are laid out on consecutive drives and at consecutive offsets.
-Effectively each stripe is duplicated and the copies are offset by one
-device. This should give similar read characteristics to 'far' if a
-suitably large chunk size is used, but without as much seeking for
-writes.
+of replicas of each data block that are required (this will usually
+be\ 2) and whether their layout should be "near", "far" or "offset"
+(with "offset" being available since Linux\ 2.6.18).
+
+.B About the RAID10 Layout Examples:
+.br
+The examples below visualise the chunk distribution on the underlying
+devices for the respective layout.
+
+For simplicity it is assumed that the size of the chunks equals the
+size of the blocks of the underlying devices as well as those of the
+RAID10 device exported by the kernel (for example \fB/dev/md/\fPname).
+.br
+Therefore the chunks\ /\ chunk numbers map directly to the blocks\ /\
+block addresses of the exported RAID10 device.
+
+Decimal numbers (0,\ 1, 2,\ ...) are the chunks of the RAID10 and due
+to the above assumption also the blocks and block addresses of the
+exported RAID10 device.
+.br
+Repeated numbers mean copies of a chunk\ /\ block (obviously on
+different underlying devices).
+.br
+Hexadecimal numbers (0x00,\ 0x01, 0x02,\ ...) are the block addresses
+of the underlying devices.
+
+.TP
+\fB "near" Layout\fP
+When "near" replicas are chosen, the multiple copies of a given chunk are laid
+out consecutively ("as close to each other as possible") across the stripes of
+the array.
+
+With an even number of devices, they will likely (unless some misalignment is
+present) lay at the very same offset on the different devices.
+.br
+This is as the "classic" RAID1+0; that is two groups of mirrored devices (in the
+example below the groups Device\ #1\ /\ #2 and Device\ #3\ /\ #4 are each a
+RAID1) both in turn forming a striped RAID0.
+
+.ne 10
+.B Example with 2\ copies per chunk and an even number\ (4) of devices:
+.TS
+tab(;);
+ C - - - -
+ C | C | C | C | C |
+| - | - | - | - | - |
+| C | C | C | C | C |
+| C | C | C | C | C |
+| C | C | C | C | C |
+| C | C | C | C | C |
+| C | C | C | C | C |
+| C | C | C | C | C |
+| - | - | - | - | - |
+ C C S C S
+ C C S C S
+ C C S S S
+ C C S S S.
+;
+;Device #1;Device #2;Device #3;Device #4
+0x00;0;0;1;1
+0x01;2;2;3;3
+\.\.\.;\.\.\.;\.\.\.;\.\.\.;\.\.\.
+:;:;:;:;:
+\.\.\.;\.\.\.;\.\.\.;\.\.\.;\.\.\.
+0x80;254;254;255;255
+;\\---------v---------/;\\---------v---------/
+;RAID1;RAID1
+;\\---------------------v---------------------/
+;RAID0
+.TE
+
+.ne 10
+.B Example with 2\ copies per chunk and an odd number\ (5) of devices:
+.TS
+tab(;);
+ C - - - - -
+ C | C | C | C | C | C |
+| - | - | - | - | - | - |
+| C | C | C | C | C | C |
+| C | C | C | C | C | C |
+| C | C | C | C | C | C |
+| C | C | C | C | C | C |
+| C | C | C | C | C | C |
+| C | C | C | C | C | C |
+| - | - | - | - | - | - |
+C.
+;
+;Dev #1;Dev #2;Dev #3;Dev #4;Dev #5
+0x00;0;0;1;1;2
+0x01;2;3;3;4;4
+\.\.\.;\.\.\.;\.\.\.;\.\.\.;\.\.\.;\.\.\.
+:;:;:;:;:;:
+\.\.\.;\.\.\.;\.\.\.;\.\.\.;\.\.\.;\.\.\.
+0x80;317;318;318;319;319
+;
+.TE
+
+.TP
+\fB "far" Layout\fP
+When "far" replicas are chosen, the multiple copies of a given chunk
+are laid out quite distant ("as far as reasonably possible") from each
+other.
+
+First a complete sequence of all data blocks (that is all the data one
+sees on the exported RAID10 block device) is striped over the
+devices. Then another (though "shifted") complete sequence of all data
+blocks; and so on (in the case of more than 2\ copies per chunk).
+
+The "shift" needed to prevent placing copies of the same chunks on the
+same devices is actually a cyclic permutation with offset\ 1 of each
+of the stripes within a complete sequence of chunks.
+.br
+The offset\ 1 is relative to the previous complete sequence of chunks,
+so in case of more than 2\ copies per chunk one gets the following
+offsets:
+.br
+1.\ complete sequence of chunks: offset\ =\ \ 0
+.br
+2.\ complete sequence of chunks: offset\ =\ \ 1
+.br
+3.\ complete sequence of chunks: offset\ =\ \ 2
+.br
+ :
+.br
+n.\ complete sequence of chunks: offset\ =\ n-1
+
+.ne 10
+.B Example with 2\ copies per chunk and an even number\ (4) of devices:
+.TS
+tab(;);
+ C - - - -
+ C | C | C | C | C |
+| - | - | - | - | - |
+| C | C | C | C | C | L
+| C | C | C | C | C | L
+| C | C | C | C | C | L
+| C | C | C | C | C | L
+| C | C | C | C | C | L
+| C | C | C | C | C | L
+| C | C | C | C | C | L
+| C | C | C | C | C | L
+| C | C | C | C | C | L
+| C | C | C | C | C | L
+| C | C | C | C | C | L
+| C | C | C | C | C | L
+| - | - | - | - | - |
+C.
+;
+;Device #1;Device #2;Device #3;Device #4
+;
+0x00;0;1;2;3;\\
+0x01;4;5;6;7;> [#]
+\.\.\.;\.\.\.;\.\.\.;\.\.\.;\.\.\.;:
+:;:;:;:;:;:
+\.\.\.;\.\.\.;\.\.\.;\.\.\.;\.\.\.;:
+0x40;252;253;254;255;/
+0x41;3;0;1;2;\\
+0x42;7;4;5;6;> [#]~
+\.\.\.;\.\.\.;\.\.\.;\.\.\.;\.\.\.;:
+:;:;:;:;:;:
+\.\.\.;\.\.\.;\.\.\.;\.\.\.;\.\.\.;:
+0x80;255;252;253;254;/
+;
+.TE
+
+.ne 10
+.B Example with 2\ copies per chunk and an odd number\ (5) of devices:
+.TS
+tab(;);
+ C - - - - -
+ C | C | C | C | C | C |
+| - | - | - | - | - | - |
+| C | C | C | C | C | C | L
+| C | C | C | C | C | C | L
+| C | C | C | C | C | C | L
+| C | C | C | C | C | C | L
+| C | C | C | C | C | C | L
+| C | C | C | C | C | C | L
+| C | C | C | C | C | C | L
+| C | C | C | C | C | C | L
+| C | C | C | C | C | C | L
+| C | C | C | C | C | C | L
+| C | C | C | C | C | C | L
+| C | C | C | C | C | C | L
+| - | - | - | - | - | - |
+C.
+;
+;Dev #1;Dev #2;Dev #3;Dev #4;Dev #5
+;
+0x00;0;1;2;3;4;\\
+0x01;5;6;7;8;9;> [#]
+\.\.\.;\.\.\.;\.\.\.;\.\.\.;\.\.\.;\.\.\.;:
+:;:;:;:;:;:;:
+\.\.\.;\.\.\.;\.\.\.;\.\.\.;\.\.\.;\.\.\.;:
+0x40;315;316;317;318;319;/
+0x41;4;0;1;2;3;\\
+0x42;9;5;6;7;8;> [#]~
+\.\.\.;\.\.\.;\.\.\.;\.\.\.;\.\.\.;\.\.\.;:
+:;:;:;:;:;:;:
+\.\.\.;\.\.\.;\.\.\.;\.\.\.;\.\.\.;\.\.\.;:
+0x80;319;315;316;317;318;/
+;
+.TE
+
+With [#]\ being the complete sequence of chunks and [#]~\ the cyclic permutation
+with offset\ 1 thereof (in the case of more than 2 copies per chunk there would
+be ([#]~)~,\ (([#]~)~)~,\ ...).
+
+The advantage of this layout is that MD can easily spread sequential reads over
+the devices, making them similar to RAID0 in terms of speed.
+.br
+The cost is more seeking for writes, making them substantially slower.
+
+.TP
+\fB"offset" Layout\fP
+When "offset" replicas are chosen, all the copies of a given chunk are
+striped consecutively ("offset by the stripe length after each other")
+over the devices.
+
+Explained in detail, <number of devices> consecutive chunks are
+striped over the devices, immediately followed by a "shifted" copy of
+these chunks (and by further such "shifted" copies in the case of more
+than 2\ copies per chunk).
+.br
+This pattern repeats for all further consecutive chunks of the
+exported RAID10 device (in other words: all further data blocks).
+
+The "shift" needed to prevent placing copies of the same chunks on the
+same devices is actually a cyclic permutation with offset\ 1 of each
+of the striped copies of <number of devices> consecutive chunks.
+.br
+The offset\ 1 is relative to the previous striped copy of <number of
+devices> consecutive chunks, so in case of more than 2\ copies per
+chunk one gets the following offsets:
+.br
+1.\ <number of devices> consecutive chunks: offset\ =\ \ 0
+.br
+2.\ <number of devices> consecutive chunks: offset\ =\ \ 1
+.br
+3.\ <number of devices> consecutive chunks: offset\ =\ \ 2
+.br
+ :
+.br
+n.\ <number of devices> consecutive chunks: offset\ =\ n-1
+
+.ne 10
+.B Example with 2\ copies per chunk and an even number\ (4) of devices:
+.TS
+tab(;);
+ C - - - -
+ C | C | C | C | C |
+| - | - | - | - | - |
+| C | C | C | C | C | L
+| C | C | C | C | C | L
+| C | C | C | C | C | L
+| C | C | C | C | C | L
+| C | C | C | C | C | L
+| C | C | C | C | C | L
+| C | C | C | C | C | L
+| C | C | C | C | C | L
+| C | C | C | C | C | L
+| - | - | - | - | - |
+C.
+;
+;Device #1;Device #2;Device #3;Device #4
+;
+0x00;0;1;2;3;) AA
+0x01;3;0;1;2;) AA~
+0x02;4;5;6;7;) AB
+0x03;7;4;5;6;) AB~
+\.\.\.;\.\.\.;\.\.\.;\.\.\.;\.\.\.;) \.\.\.
+:;:;:;:;:; :
+\.\.\.;\.\.\.;\.\.\.;\.\.\.;\.\.\.;) \.\.\.
+0x79;251;252;253;254;) EX
+0x80;254;251;252;253;) EX~
+;
+.TE
+
+.ne 10
+.B Example with 2\ copies per chunk and an odd number\ (5) of devices:
+.TS
+tab(;);
+ C - - - - -
+ C | C | C | C | C | C |
+| - | - | - | - | - | - |
+| C | C | C | C | C | C | L
+| C | C | C | C | C | C | L
+| C | C | C | C | C | C | L
+| C | C | C | C | C | C | L
+| C | C | C | C | C | C | L
+| C | C | C | C | C | C | L
+| C | C | C | C | C | C | L
+| C | C | C | C | C | C | L
+| C | C | C | C | C | C | L
+| - | - | - | - | - | - |
+C.
+;
+;Dev #1;Dev #2;Dev #3;Dev #4;Dev #5
+;
+0x00;0;1;2;3;4;) AA
+0x01;4;0;1;2;3;) AA~
+0x02;5;6;7;8;9;) AB
+0x03;9;5;6;7;8;) AB~
+\.\.\.;\.\.\.;\.\.\.;\.\.\.;\.\.\.;\.\.\.;) \.\.\.
+:;:;:;:;:;:; :
+\.\.\.;\.\.\.;\.\.\.;\.\.\.;\.\.\.;\.\.\.;) \.\.\.
+0x79;314;315;316;317;318;) EX
+0x80;318;314;315;316;317;) EX~
+;
+.TE
+
+With AA,\ AB,\ ..., AZ,\ BA,\ ... being the sets of <number of devices> consecutive
+chunks and AA~,\ AB~,\ ..., AZ~,\ BA~,\ ... the cyclic permutations with offset\ 1
+thereof (in the case of more than 2 copies per chunk there would be (AA~)~,\ ...
+as well as ((AA~)~)~,\ ... and so on).
+
+This should give similar read characteristics to "far" if a suitably large chunk
+size is used, but without as much seeking for writes.
+.PP
+
It should be noted that the number of devices in a RAID10 array need
not be a multiple of the number of replica of each data block; however,
then space equivalent to 2.5 of the devices will be available, and
every block will be stored on two different devices.
-Finally, it is possible to have an array with both 'near' and 'far'
+Finally, it is possible to have an array with both "near" and "far"
copies. If an array is configured with 2 near copies and 2 far
copies, then there will be a total of 4 copies of each block, each on
a different drive. This is an artifact of the implementation and is
A MULTIPATH array is composed of a number of logically different
devices, often fibre channel interfaces, that all refer the the same
real device. If one of these interfaces fails (e.g. due to cable
-problems), the multipath driver will attempt to redirect requests to
+problems), the MULTIPATH driver will attempt to redirect requests to
another interface.
The MULTIPATH drive is not receiving any ongoing development and
multipath drivers should be preferred for new installations.
.SS FAULTY
-The FAULTY md module is provided for testing purposes. A faulty array
+The FAULTY md module is provided for testing purposes. A FAULTY array
has exactly one component device and is normally assembled without a
superblock, so the md array created provides direct access to all of
the data in the component device.
.B speed_limit_max
control files mentioned below.
+.SS SCRUBBING AND MISMATCHES
+
+As storage devices can develop bad blocks at any time it is valuable
+to regularly read all blocks on all devices in an array so as to catch
+such bad blocks early. This process is called
+.IR scrubbing .
+
+md arrays can be scrubbed by writing either
+.I check
+or
+.I repair
+to the file
+.I md/sync_action
+in the
+.I sysfs
+directory for the device.
+
+Requesting a scrub will cause
+.I md
+to read every block on every device in the array, and check that the
+data is consistent. For RAID1 and RAID10, this means checking that the copies
+are identical. For RAID4, RAID5, RAID6 this means checking that the
+parity block is (or blocks are) correct.
+
+If a read error is detected during this process, the normal read-error
+handling causes correct data to be found from other devices and to be
+written back to the faulty device. In many case this will
+effectively
+.I fix
+the bad block.
+
+If all blocks read successfully but are found to not be consistent,
+then this is regarded as a
+.IR mismatch .
+
+If
+.I check
+was used, then no action is taken to handle the mismatch, it is simply
+recorded.
+If
+.I repair
+was used, then a mismatch will be repaired in the same way that
+.I resync
+repairs arrays. For RAID5/RAID6 new parity blocks are written. For RAID1/RAID10,
+all but one block are overwritten with the content of that one block.
+
+A count of mismatches is recorded in the
+.I sysfs
+file
+.IR md/mismatch_cnt .
+This is set to zero when a
+scrub starts and is incremented whenever a sector is
+found that is a mismatch.
+.I md
+normally works in units much larger than a single sector and when it
+finds a mismatch, it does not determine exactly how many actual sectors were
+affected but simply adds the number of sectors in the IO unit that was
+used. So a value of 128 could simply mean that a single 64KB check
+found an error (128 x 512bytes = 64KB).
+
+If an array is created by
+.I mdadm
+with
+.I \-\-assume\-clean
+then a subsequent check could be expected to find some mismatches.
+
+On a truly clean RAID5 or RAID6 array, any mismatches should indicate
+a hardware problem at some level - software issues should never cause
+such a mismatch.
+
+However on RAID1 and RAID10 it is possible for software issues to
+cause a mismatch to be reported. This does not necessarily mean that
+the data on the array is corrupted. It could simply be that the
+system does not care what is stored on that part of the array - it is
+unused space.
+
+The most likely cause for an unexpected mismatch on RAID1 or RAID10
+occurs if a swap partition or swap file is stored on the array.
+
+When the swap subsystem wants to write a page of memory out, it flags
+the page as 'clean' in the memory manager and requests the swap device
+to write it out. It is quite possible that the memory will be
+changed while the write-out is happening. In that case the 'clean'
+flag will be found to be clear when the write completes and so the
+swap subsystem will simply forget that the swapout had been attempted,
+and will possibly choose a different page to write out.
+
+If the swap device was on RAID1 (or RAID10), then the data is sent
+from memory to a device twice (or more depending on the number of
+devices in the array). Thus it is possible that the memory gets changed
+between the times it is sent, so different data can be written to
+the different devices in the array. This will be detected by
+.I check
+as a mismatch. However it does not reflect any corruption as the
+block where this mismatch occurs is being treated by the swap system as
+being empty, and the data will never be read from that block.
+
+It is conceivable for a similar situation to occur on non-swap files,
+though it is less likely.
+
+Thus the
+.I mismatch_cnt
+value can not be interpreted very reliably on RAID1 or RAID10,
+especially when the device is used for swap.
+
+
.SS BITMAP WRITE-INTENT LOGGING
From Linux 2.6.13,
In 2.6.13, intent bitmaps are only supported with RAID1. Other levels
with redundancy are supported from 2.6.15.
+.SS BAD BLOCK LIST
+
+From Linux 3.5 each device in an
+.I md
+array can store a list of known-bad-blocks. This list is 4K in size
+and usually positioned at the end of the space between the superblock
+and the data.
+
+When a block cannot be read and cannot be repaired by writing data
+recovered from other devices, the address of the block is stored in
+the bad block list. Similarly if an attempt to write a block fails,
+the address will be recorded as a bad block. If attempting to record
+the bad block fails, the whole device will be marked faulty.
+
+Attempting to read from a known bad block will cause a read error.
+Attempting to write to a known bad block will be ignored if any write
+errors have been reported by the device. If there have been no write
+errors then the data will be written to the known bad block and if
+that succeeds, the address will be removed from the list.
+
+This allows an array to fail more gracefully - a few blocks on
+different devices can be faulty without taking the whole array out of
+action.
+
+The list is particularly useful when recovering to a spare. If a few blocks
+cannot be read from the other devices, the bulk of the recovery can
+complete and those few bad blocks will be recorded in the bad block list.
+
+.SS RAID456 WRITE JOURNAL
+
+Due to non-atomicity nature of RAID write operations, interruption of
+write operations (system crash, etc.) to RAID456 array can lead to
+inconsistent parity and data loss (so called RAID-5 write hole).
+
+To plug the write hole, from Linux 4.4 (to be confirmed),
+.I md
+supports write ahead journal for RAID456. When the array is created,
+an additional journal device can be added to the array through
+.IR write-journal
+option. The RAID write journal works similar to file system journals.
+Before writing to the data disks, md persists data AND parity of the
+stripe to the journal device. After crashes, md searches the journal
+device for incomplete write operations, and replay them to the data
+disks.
+
+When the journal device fails, the RAID array is forced to run in
+read-only mode.
+
.SS WRITE-BEHIND
From Linux 2.6.14,
operations, but the remote system will still have a reasonably
up-to-date copy of all data.
+.SS FAILFAST
+
+From Linux 4.10,
+.I
+md
+supports FAILFAST for RAID1 and RAID10 arrays. This is a flag that
+can be set on individual drives, though it is usually set on all
+drives, or no drives.
+
+When
+.I md
+sends an I/O request to a drive that is marked as FAILFAST, and when
+the array could survive the loss of that drive without losing data,
+.I md
+will request that the underlying device does not perform any retries.
+This means that a failure will be reported to
+.I md
+promptly, and it can mark the device as faulty and continue using the
+other device(s).
+.I md
+cannot control the timeout that the underlying devices use to
+determine failure. Any changes desired to that timeout must be set
+explictly on the underlying device, separately from using
+.IR mdadm .
+
+If a FAILFAST request does fail, and if it is still safe to mark the
+device as faulty without data loss, that will be done and the array
+will continue functioning on a reduced number of devices. If it is not
+possible to safely mark the device as faulty,
+.I md
+will retry the request without disabling retries in the underlying
+device. In any case,
+.I md
+will not attempt to repair read errors on a device marked as FAILFAST
+by writing out the correct. It will just mark the device as faulty.
+
+FAILFAST is appropriate for storage arrays that have a low probability
+of true failure, but will sometimes introduce unacceptable delays to
+I/O requests while performing internal maintenance. The value of
+setting FAILFAST involves a trade-off. The gain is that the chance of
+unacceptable delays is substantially reduced. The cost is that the
+unlikely event of data-loss on one device is slightly more likely to
+result in data-loss for the array.
+
+When a device in an array using FAILFAST is marked as faulty, it will
+usually become usable again in a short while.
+.I mdadm
+makes no attempt to detect that possibility. Some separate
+mechanism, tuned to the specific details of the expected failure modes,
+needs to be created to monitor devices to see when they return to full
+functionality, and to then re-add them to the array. In order of
+this "re-add" functionality to be effective, an array using FAILFAST
+should always have a write-intent bitmap.
+
.SS RESTRIPING
.IR Restriping ,
new layout. This might involve changing the number of devices in the
array (so the stripes are wider), changing the chunk size (so stripes
are deeper or shallower), or changing the arrangement of data and
-parity (possibly changing the raid level, e.g. 1 to 5 or 5 to 6).
+parity (possibly changing the RAID level, e.g. 1 to 5 or 5 to 6).
-As of Linux 2.6.17, md can reshape a raid5 array to have more
-devices. Other possibilities may follow in future kernels.
+As of Linux 2.6.35, md can reshape a RAID4, RAID5, or RAID6 array to
+have a different number of devices (more or fewer) and to have a
+different layout or chunk size. It can also convert between these
+different RAID levels. It can also convert between RAID0 and RAID10,
+and between RAID0 and RAID4 or RAID5.
+Other possibilities may follow in future kernels.
During any stripe process there is a 'critical section' during which
live data is being overwritten on disk. For the operation of
-increasing the number of drives in a raid5, this critical section
+increasing the number of drives in a RAID5, this critical section
covers the first few stripes (the number being the product of the old
and new number of devices). After this critical section is passed,
data is only written to areas of the array which no longer hold live
data \(em the live data has already been located away.
+For a reshape which reduces the number of devices, the 'critical
+section' is at the end of the reshape process.
+
md is not able to ensure data preservation if there is a crash
(e.g. power failure) during the critical section. If md is asked to
start an array which failed during a critical section of restriping,
increasing chunk size, or converting RAID5 to RAID6 with one extra
device, the entire process is the critical section. In this case, the
restripe will need to progress in stages, as a section is suspended,
-backed up,
-restriped, and released; this is not yet implemented.
+backed up, restriped, and released.
.SS SYSFS INTERFACE
Each block device appears as a directory in
This is the partner of
.B md/sync_speed_min
and overrides
-.B /proc/sys/dev/raid/spool_limit_max
+.B /proc/sys/dev/raid/speed_limit_max
described below.
.TP
stripe that requires some "prereading". For fairness this defaults to
1. Valid values are 0 to stripe_cache_size. Setting this to 0
maximizes sequential-write throughput at the cost of fairness to threads
-doing small or random writes.
+doing small or random writes.
+
+.TP
+.B md/bitmap/backlog
+The value stored in the file only has any effect on RAID1 when write-mostly
+devices are active, and write requests to those devices are proceed in the
+background.
+
+This variable sets a limit on the number of concurrent background writes,
+the valid values are 0 to 16383, 0 means that write-behind is not allowed,
+while any other number means it can happen. If there are more write requests
+than the number, new writes will by synchronous.
+
+.TP
+.B md/bitmap/can_clear
+This is for externally managed bitmaps, where the kernel writes the bitmap
+itself, but metadata describing the bitmap is managed by mdmon or similar.
+
+When the array is degraded, bits mustn't be cleared. When the array becomes
+optimal again, bit can be cleared, but first the metadata needs to record
+the current event count. So md sets this to 'false' and notifies mdmon,
+then mdmon updates the metadata and writes 'true'.
+
+There is no code in mdmon to actually do this, so maybe it doesn't even
+work.
+
+.TP
+.B md/bitmap/chunksize
+The bitmap chunksize can only be changed when no bitmap is active, and
+the value should be power of 2 and at least 512.
+
+.TP
+.B md/bitmap/location
+This indicates where the write-intent bitmap for the array is stored.
+It can be "none" or "file" or a signed offset from the array metadata
+- measured in sectors. You cannot set a file by writing here - that can
+only be done with the SET_BITMAP_FILE ioctl.
+
+Write 'none' to 'bitmap/location' will clear bitmap, and the previous
+location value must be write to it to restore bitmap.
+
+.TP
+.B md/bitmap/max_backlog_used
+This keeps track of the maximum number of concurrent write-behind requests
+for an md array, writing any value to this file will clear it.
+
+.TP
+.B md/bitmap/metadata
+This can be 'internal' or 'clustered' or 'external'. 'internal' is set
+by default, which means the metadata for bitmap is stored in the first 256
+bytes of the bitmap space. 'clustered' means separate bitmap metadata are
+used for each cluster node. 'external' means that bitmap metadata is managed
+externally to the kernel.
+
+.TP
+.B md/bitmap/space
+This shows the space (in sectors) which is available at md/bitmap/location,
+and allows the kernel to know when it is safe to resize the bitmap to match
+a resized array. It should big enough to contain the total bytes in the bitmap.
+
+For 1.0 metadata, assume we can use up to the superblock if before, else
+to 4K beyond superblock. For other metadata versions, assume no change is
+possible.
+
+.TP
+.B md/bitmap/time_base
+This shows the time (in seconds) between disk flushes, and is used to looking
+for bits in the bitmap to be cleared.
+
+The default value is 5 seconds, and it should be an unsigned long value.
.SS KERNEL PARAMETERS
.I n
gives the md device number,
.I l
-gives the level, 0 for RAID0 or -1 for LINEAR,
+gives the level, 0 for RAID0 or \-1 for LINEAR,
.I c
gives the chunk size as a base-2 logarithm offset by twelve, so 0
means 4K, 1 means 8K.
.SH SEE ALSO
.BR mdadm (8),
-.BR mkraid (8).
projects / thirdparty / mdadm.git / blobdiff