..
Use the original 0.90 format superblock. This format limits arrays to
28 component devices and limits component devices of levels 1 and
-greater to 2 terabytes.
+greater to 2 terabytes. It is also possible for there to be confusion
+about whether the superblock applies to a whole device or just the
+last partition, if that partition starts on a 64K boundary.
.ie '{DEFAULT_METADATA}'0.90'
.IP "1, 1.0, 1.1, 1.2"
.el
.IP "1, 1.0, 1.1, 1.2 default"
..
-Use the new version-1 format superblock. This has few restrictions.
-The different sub-versions store the superblock at different locations
-on the device, either at the end (for 1.0), at the start (for 1.1) or
-4K from the start (for 1.2). "1" is equivalent to "1.0".
+Use the new version-1 format superblock. This has fewer restrictions.
+It can easily be moved between hosts with different endian-ness, and a
+recovery operation can be checkpointed and restarted. The different
+sub-versions store the superblock at different locations on the
+device, either at the end (for 1.0), at the start (for 1.1) or 4K from
+the start (for 1.2). "1" is equivalent to "1.0".
'if '{DEFAULT_METADATA}'1.2' "default" is equivalent to "1.2".
.IP ddf
Use the "Industry Standard" DDF (Disk Data Format) format defined by
The default is
.BR left\-symmetric .
-It is also possibly to cause RAID5 to use a RAID4-like layout by
+It is also possible to cause RAID5 to use a RAID4-like layout by
choosing
.BR parity\-first ,
or
.BR \-\-backup\-file=
This is needed when
.B \-\-grow
-is used to increase the number of
-raid-devices in a RAID5 if there are no spare devices available.
-See the GROW MODE section below on RAID\-DEVICES CHANGES. The file
-should be stored on a separate device, not on the RAID array being
-reshaped.
+is used to increase the number of raid-devices in a RAID5 or RAID6 if
+there are no spare devices available, or to shrink, change RAID level
+or layout. See the GROW MODE section below on RAID\-DEVICES CHANGES.
+The file must be stored on a separate device, not on the RAID array
+being reshaped.
.TP
.BR \-N ", " \-\-name=
.BR \-\-backup\-file=
If
.B \-\-backup\-file
-was used to grow the number of raid-devices in a RAID5, and the system
-crashed during the critical section, then the same
+was used while reshaping an array (e.g. changing number of devices or
+chunk size) and the system crashed during the critical section, then the same
.B \-\-backup\-file
must be presented to
.B \-\-assemble
-to allow possibly corrupted data to be restored.
+to allow possibly corrupted data to be restored, and the reshape
+to be completed.
+
+.TP
+.BR \-\-invalid\-backup
+If the file needed for the above option is not available for any
+reason an empty file can be given together with this option to
+indicate that the backup file is invalid. In this case the data that
+was being rearranged at the time of the crash could be irrecoverably
+lost, but the rest of the array may still be recoverable. This option
+should only be used as a last resort if there is no way to recover the
+backup file.
+
.TP
.BR \-U ", " \-\-update=
.BR \-a ", " \-\-add
hot-add listed devices.
If a device appears to have recently been part of the array
-(possibly it failed or was removed) the device is re-added as describe
+(possibly it failed or was removed) the device is re\-added as describe
in the next point.
If that fails or the device was never part of the array, the device is
added as a hot-spare.
it will be assumed that bitmap-based recovery is enough to make the
device fully consistent with the array.
+When
+.B \-\-re\-add
+can be accompanied by
+.BR \-\-update=devicesize .
+See the description of this option when used in Assemble mode for an
+explanation of its use.
+
If the device name given is
.B missing
then mdadm will try to find any device that looks like it should be
inaccessible. The integrity of any data can then be checked before
the non-reversible reduction in the number of devices is request.
-When relocating the first few stripes on a RAID5, it is not possible
-to keep the data on disk completely consistent and crash-proof. To
-provide the required safety, mdadm disables writes to the array while
-this "critical section" is reshaped, and takes a backup of the data
-that is in that section. This backup is normally stored in any spare
-devices that the array has, however it can also be stored in a
-separate file specified with the
+When relocating the first few stripes on a RAID5 or RAID6, it is not
+possible to keep the data on disk completely consistent and
+crash-proof. To provide the required safety, mdadm disables writes to
+the array while this "critical section" is reshaped, and takes a
+backup of the data that is in that section. For grows, this backup may be
+stored in any spare devices that the array has, however it can also be
+stored in a separate file specified with the
.B \-\-backup\-file
-option. If this option is used, and the system does crash during the
-critical period, the same file must be passed to
+option, and is required to be specified for shrinks, RAID level
+changes and layout changes. If this option is used, and the system
+does crash during the critical period, the same file must be passed to
.B \-\-assemble
-to restore the backup and reassemble the array.
+to restore the backup and reassemble the array. When shrinking rather
+than growing the array, the reshape is done from the end towards the
+beginning, so the "critical section" is at the end of the reshape.
.SS LEVEL CHANGES
Changing the RAID level of any array happens instantaneously. However
-in the RAID to RAID6 case this requires a non-standard layout of the
+in the RAID5 to RAID6 case this requires a non-standard layout of the
RAID6 data, and in the RAID6 to RAID5 case that non-standard layout is
-required before the change can be accomplish. So while the level
+required before the change can be accomplished. So while the level
change is instant, the accompanying layout change can take quite a
-long time.
+long time. A
+.B \-\-backup\-file
+is required. If the array is not simultaneously being grown or
+shrunk, so that the array size will remain the same - for example,
+reshaping a 3-drive RAID5 into a 4-drive RAID6 - the backup file will
+be used not just for a "cricital section" but throughout the reshape
+operation, as described below under LAYOUT CHANGES.
.SS CHUNK-SIZE AND LAYOUT CHANGES
To ensure against data loss in the case of a crash, a
.B --backup-file
must be provided for these changes. Small sections of the array will
-be copied to the backup file while they are being rearranged.
+be copied to the backup file while they are being rearranged. This
+means that all the data is copied twice, once to the backup and once
+to the new layout on the array, so this type of reshape will go very
+slowly.
If the reshape is interrupted for any reason, this backup file must be
-make available to
+made available to
.B "mdadm --assemble"
so the array can be reassembled. Consequently the file cannot be
stored on the device being reshaped.
projects / thirdparty / mdadm.git / blobdiff