dm-raid.txt 4.03 KB
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dm-raid
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The device-mapper RAID (dm-raid) target provides a bridge from DM to MD.
It allows the MD RAID drivers to be accessed using a device-mapper
interface.
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The target is named "raid" and it accepts the following parameters:

  <raid_type> <#raid_params> <raid_params> \
    <#raid_devs> <metadata_dev0> <dev0> [.. <metadata_devN> <devN>]

<raid_type>:
  raid4		RAID4 dedicated parity disk
  raid5_la	RAID5 left asymmetric
		- rotating parity 0 with data continuation
  raid5_ra	RAID5 right asymmetric
		- rotating parity N with data continuation
  raid5_ls	RAID5 left symmetric
		- rotating parity 0 with data restart
  raid5_rs 	RAID5 right symmetric
		- rotating parity N with data restart
  raid6_zr	RAID6 zero restart
		- rotating parity zero (left-to-right) with data restart
  raid6_nr	RAID6 N restart
		- rotating parity N (right-to-left) with data restart
  raid6_nc	RAID6 N continue
		- rotating parity N (right-to-left) with data continuation

  Refererence: Chapter 4 of
  http://www.snia.org/sites/default/files/SNIA_DDF_Technical_Position_v2.0.pdf

<#raid_params>: The number of parameters that follow.

<raid_params> consists of
    Mandatory parameters:
        <chunk_size>: Chunk size in sectors.  This parameter is often known as
		      "stripe size".  It is the only mandatory parameter and
		      is placed first.

    followed by optional parameters (in any order):
	[sync|nosync]   Force or prevent RAID initialization.

	[rebuild <idx>]	Rebuild drive number idx (first drive is 0).

	[daemon_sleep <ms>]
		Interval between runs of the bitmap daemon that
		clear bits.  A longer interval means less bitmap I/O but
		resyncing after a failure is likely to take longer.

	[min_recovery_rate <kB/sec/disk>]  Throttle RAID initialization
	[max_recovery_rate <kB/sec/disk>]  Throttle RAID initialization
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	[write_mostly <idx>]		   Drive index is write-mostly
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	[max_write_behind <sectors>]       See '-write-behind=' (man mdadm)
	[stripe_cache <sectors>]           Stripe cache size (higher RAIDs only)
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	[region_size <sectors>]
		The region_size multiplied by the number of regions is the
		logical size of the array.  The bitmap records the device
		synchronisation state for each region.
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<#raid_devs>: The number of devices composing the array.
	Each device consists of two entries.  The first is the device
	containing the metadata (if any); the second is the one containing the
	data.  Currently, separate metadata devices are not supported and '-'
	is required in place of the metadata device.

	If a drive has failed or is missing at creation time, a '-' can be
	given for both the metadata and data drives for a given position.


Example tables
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# RAID4 - 4 data drives, 1 parity
# No metadata devices specified to hold superblock/bitmap info
# Chunk size of 1MiB
# (Lines separated for easy reading)
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0 1960893648 raid \
        raid4 1 2048 \
        5 - 8:17 - 8:33 - 8:49 - 8:65 - 8:81

# RAID4 - 4 data drives, 1 parity (no metadata devices)
# Chunk size of 1MiB, force RAID initialization,
#       min recovery rate at 20 kiB/sec/disk
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0 1960893648 raid \
        raid4 4 2048 min_recovery_rate 20 sync\
        5 - 8:17 - 8:33 - 8:49 - 8:65 - 8:81

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'dmsetup table' displays the table used to construct the mapping.
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The optional parameters are always printed in the order listed
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above with "sync" or "nosync" always output ahead of the other
arguments, regardless of the order used when originally loading the table.
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Arguments that can be repeated are ordered by value.
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'dmsetup status' yields information on the state and health of the
array.
The output is as follows:
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1: <s> <l> raid \
2:      <raid_type> <#devices> <1 health char for each dev> <resync_ratio>

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Line 1 is the standard output produced by device-mapper.
Line 2 is produced by the raid target, and best explained by example:
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        0 1960893648 raid raid4 5 AAAAA 2/490221568
Here we can see the RAID type is raid4, there are 5 devices - all of
which are 'A'live, and the array is 2/490221568 complete with recovery.
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Faulty or missing devices are marked 'D'.  Devices that are out-of-sync
are marked 'a'.