79 lines
2.5 KiB
Groff
79 lines
2.5 KiB
Groff
.TH MD 4
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.SH NAME
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md \- Multiple Device driver aka Linux Software Raid
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.SH SYNOPSIS
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.BI /dev/md n
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.br
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.BI /dev/md/ n
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.SH DESCRIPTION
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The
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.B md
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driver provides virtual devices that are created from one or more
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independant underlying devices. This array of devices often contains
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redundancy, and hence the acronym RAID which stands for a Redundant
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Array of Independant Devices.
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.PP
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.B md
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support RAID levels 1 (mirroring) 4 (striped array with parity device) and 5
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(striped array with distributed parity information. If a single underlying
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device fails while using one of these level, they array will continue
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to function.
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.PP
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.B md
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also supports a number of pseudo RAID (non-redundant) configuations
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including RAID0 (striped array), LINEAR (catenated array) and
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MULTIPATH (a set of different interfaces to the same device).
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.SS RAID SUPER BLOCK
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With the exception of Legacy Arrays described below, each device that
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is incorportated into an MD array has a
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.I super block
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written towards the end of the device. This superblock records
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information about the structure and state of the array so that the
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array an be reliably re-assembled after a shutdown.
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The superblock is 4K long and is written into a 64K aligned block that
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start at least 64K and less than 128K from the end of the device
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(i.e. to get the address of the superblock round the size of the
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device down to a multiple of 64K and then subtract 64K).
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The available size of each device is the ammount of space before the
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super block, so between 64K and 128K is lost when a device in
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incorporated into an MD array.
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The superblock contains, among other things:
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.TP
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LEVEL
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The
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.TP
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UUID
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a 128 bit Universally Unique Identifier that identifies the array that
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this device is part of.
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.SS LINEAR
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.SS RAID0
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.SS RAID1
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.SS RAID4
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.SS RAID5
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.SS REBUILD/RESYNC
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.SH FILES
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.TP
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.B /proc/mdstat
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Contains information about the status of currently running array.
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.TP
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.B /proc/sys/dev/raid/speed_limit_min
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A readable and writable file that reflects the current goal rebuild
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speed for times when non-rebuild activity is current on an array.
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The speed is in Kibibytes per second, and is a per-device rate, not a
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per-array rate (which means that an array with more disc will shuffle
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more data for a given speed). The default is 100.
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.TP
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.B /proc/sys/dev/raid/speed_limit_max
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A readable and writable file that reflects the current goal rebuild
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speed for times when no non-rebuild activity is current on an array.
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The default is 100,000.
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.SH SEE ALSO
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.BR mdadm (8),
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.BR mkraid (8).
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