.BI /dev/md n
.br
.BI /dev/md/ n
+.br
+.BR /dev/md/ name
.SH DESCRIPTION
The
.B md
MULTIPATH (a set of different interfaces to the same device),
and FAULTY (a layer over a single device into which errors can be injected).
-.SS MD SUPER BLOCK
-Each device in an array may have a
-.I superblock
-which records information about the structure and state of the array.
+.SS MD METADATA
+Each device in an array may have some
+.I metadata
+stored in the device. This metadata is sometimes called a
+.BR superblock .
+The metadata records information about the structure and state of the array.
This allows the array to be reliably re-assembled after a shutdown.
From Linux kernel version 2.6.10,
.B md
-provides support for two different formats of this superblock, and
+provides support for two different formats of metadata, and
other formats can be added. Prior to this release, only one format is
supported.
and 12K from the end of the device, on a 4K boundary, though
variations can be stored at the start of the device (version 1.1) or 4K from
the start of the device (version 1.2).
-This superblock format stores multibyte data in a
+This metadata format stores multibyte data in a
processor-independent format and supports up to hundreds of
component devices (version 0.90 only supports 28).
-The superblock contains, among other things:
+The metadata contains, among other things:
.TP
LEVEL
The manner in which the devices are arranged into the array
a 128 bit Universally Unique Identifier that identifies the array that
contains this device.
+.PP
When a version 0.90 array is being reshaped (e.g. adding extra devices
to a RAID5), the version number is temporarily set to 0.91. This
ensures that if the reshape process is stopped in the middle (e.g. by
would cause data corruption) but will be left untouched until a kernel
that can complete the reshape processes is used.
-.SS ARRAYS WITHOUT SUPERBLOCKS
+.SS ARRAYS WITHOUT METADATA
While it is usually best to create arrays with superblocks so that
they can be assembled reliably, there are some circumstances when an
array without superblocks is preferred. These include:
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 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.
+.PP
+.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 mark a device as faulty. When necessary,
+.I md
+will wait for the program to acknowledge the event by writing to a
+sysfs attribute.
+The manual page for
+.IR mdmon (8)
+contains more detail about this interaction.
+
+.SS CONTAINERS
+Many metadata formats use a single block of metadata to describe a
+number of different arrays which all use the same set of devices.
+In this case it is helpful for the kernel to know about the full set
+of devices as a whole. This set is known to md as a
+.IR container .
+A container is an
+.I md
+array with externally managed metadata and with device offset and size
+so that it just covers the metadata part of the devices. The
+remainder of each device is available to be incorporated into various
+arrays.
+
.SS LINEAR
A linear array simply catenates the available space on each
striped array.
A RAID0 array is configured at creation with a
.B "Chunk Size"
-which must be a power of two, and at least 4 kibibytes.
+which must be a power of two (prior to Linux 2.6.31), and at least 4
+kibibytes.
The RAID0 driver assigns the first chunk of the array to the first
device, the second chunk to the second device, and so on until all
-drives have been assigned one chunk. This collection of chunks forms
-a
+drives have been assigned one chunk. This collection of chunks forms a
.BR stripe .
Further chunks are gathered into stripes in the same way, and are
assigned to the remaining space in the drives.
spindle. In theory, having an N-disk RAID1 will allow N sequential
threads to read from all disks.
+Individual devices in a RAID1 can be marked as "write-mostly".
+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.
+
.SS RAID4
A RAID4 array is like a RAID0 array with an extra device for storing
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 degraded write performance.
+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.
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
-another interface.
+another interface.
+
+The MULTIPATH drive is not receiving any ongoing development and
+should be considered a legacy driver. The device-mapper based
+multipath drivers should be preferred for new installations.
.SS FAULTY
The FAULTY md module is provided for testing purposes. A faulty array
.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,
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).
-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
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
.TP
.B md_mod.start_ro=1
+.TP
+.B /sys/module/md_mod/parameters/start_ro
This tells md to start all arrays in read-only mode. This is a soft
read-only that will automatically switch to read-write on the first
write request. However until that write request, nothing is written
.TP
.B md_mod.start_dirty_degraded=1
+.TP
+.B /sys/module/md_mod/parameters/start_dirty_degraded
As mentioned above, md will not normally start a RAID4, RAID5, or
RAID6 that is both dirty and degraded as this situation can imply
hidden data loss. This can be awkward if the root filesystem is
speed for times when non-rebuild activity is current on an array.
The speed is in Kibibytes per second, and is a per-device rate, not a
per-array rate (which means that an array with more disks will shuffle
-more data for a given speed). The default is 100.
+more data for a given speed). The default is 1000.
.TP
.B /proc/sys/dev/raid/speed_limit_max
A readable and writable file that reflects the current "goal" rebuild
speed for times when no non-rebuild activity is current on an array.
-The default is 100,000.
+The default is 200,000.
.SH SEE ALSO
.BR mdadm (8),
projects / thirdparty / mdadm.git / blobdiff