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.
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.
.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
projects / thirdparty / mdadm.git / blobdiff