Commit 7799eaec authored by Heinz Mauelshagen's avatar Heinz Mauelshagen Committed by Mikulas Patocka
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dm raid: add documentation for takeover/reshape raid1 -> raid5 table line examples 



Also enhance possible takeover/reshape information and do some reformatting.

Signed-off-by: default avatarHeinz Mauelshagen <heinzm@redhat.com>
Signed-off-by: default avatarMikulas Patocka <mpatocka@redhat.com>
parent ab08f9c8

Documentation/admin-guide/device-mapper/dm-raid.rst

+73 −7
Original line number Diff line number Diff line
@@ -20,10 +20,10 @@ The target is named "raid" and it accepts the following parameters:: 
  raid0		RAID0 striping (no resilience)

  raid1		RAID1 mirroring

  raid4		RAID4 with dedicated last parity disk

  raid5_n 	RAID5 with dedicated last parity disk supporting takeover

  raid5_n 	RAID5 with dedicated last parity disk supporting takeover from/to raid1

		Same as raid4



		- Transitory layout

		- Transitory layout for takeover from/to raid1

  raid5_la	RAID5 left asymmetric



		- rotating parity 0 with data continuation
@@ -48,8 +48,8 @@ The target is named "raid" and it accepts the following parameters:: 
  raid6_n_6	RAID6 with dedicate parity disks



		- parity and Q-syndrome on the last 2 disks;

		  layout for takeover from/to raid4/raid5_n

  raid6_la_6	Same as "raid_la" plus dedicated last Q-syndrome disk

		  layout for takeover from/to raid0/raid4/raid5_n

  raid6_la_6	Same as "raid_la" plus dedicated last Q-syndrome disk supporting takeover from/to raid5



		- layout for takeover from raid5_la from/to raid6

  raid6_ra_6	Same as "raid5_ra" dedicated last Q-syndrome disk
@@ -173,9 +173,9 @@ The target is named "raid" and it accepts the following parameters:: 
		The delta_disks option value (-251 < N < +251) triggers

		device removal (negative value) or device addition (positive

		value) to any reshape supporting raid levels 4/5/6 and 10.

		RAID levels 4/5/6 allow for addition of devices (metadata

		and data device tuple), raid10_near and raid10_offset only

		allow for device addition. raid10_far does not support any

		RAID levels 4/5/6 allow for addition and removal of devices

                (metadata and data device tuple), raid10_near and raid10_offset

                only allow for device addition. raid10_far does not support any

		reshaping at all.

		A minimum of devices have to be kept to enforce resilience,

		which is 3 devices for raid4/5 and 4 devices for raid6.
@@ -372,6 +372,72 @@ to safely enable discard support for RAID 4/5/6: 
    'devices_handle_discards_safely'





Takeover/Reshape Support

------------------------

The target natively supports these two types of MDRAID conversions:



o Takeover: Converts an array from one RAID level to another



o Reshape: Changes the internal layout while maintaining the current RAID level



Each operation is only valid under specific constraints imposed by the existing array's layout and configuration.





Takeover:

linear -> raid1 with N >= 2 mirrors

raid0 -> raid4 (add dedicated parity device)

raid0 -> raid5 (add dedicated parity device)

raid0 -> raid10 with near layout and N >= 2 mirror groups (raid0 stripes have to become first member within mirror groups)

raid1 -> linear

raid1 -> raid5 with 2 mirrors

raid4 -> raid5 w/ rotating parity

raid5 with dedicated parity device -> raid4

raid5 -> raid6 (with dedicated Q-syndrome)

raid6 (with dedicated Q-syndrome) -> raid5

raid10 with near layout and even number of disks -> raid0 (select any in-sync device from each mirror group)



Reshape:

linear: not possible

raid0:  not possible

raid1:  change number of mirrors

raid4:  add and remove stripes (minimum 3), change stripesize

raid5:  add and remove stripes (minimum 3, special case 2 for raid1 takeover), change rotating parity algorithms, change stripesize

raid6:  add and remove stripes (minimum 4), change rotating syndrome algorithms, change stripesize

raid10 near:   add stripes (minimum 4), change stripesize, no stripe removal possible, change to offset layout

raid10 offset: add stripes, change stripesize, no stripe removal possible, change to near layout

raid10 far:    not possible



Table line examples:



### raid1 -> raid5

#

# 2 devices limitation in raid1.

# raid5 personality is able to just map 2 like raid1.

# Reshape after takeover to change to full raid5 layout



  0 1960886272 raid raid1 3 0 region_size 2048 2 /dev/dm-0 /dev/dm-1 /dev/dm-2 /dev/dm-3



# dm-0 and dm-2 are e.g. 4MiB large metadata devices, dm-1 and dm-3 have to be at least 1960886272 big.

#

# Table line to takeover to raid5



  0 1960886272 raid raid5 3 0 region_size 2048 2 /dev/dm-0 /dev/dm-1 /dev/dm-2 /dev/dm-3



# Add required out-of-place reshape space to the beginniong of the given 2 data devices,

# allocate another metadata/data device tuple with the same sizes for the parity space

# and zero the first 4K of the metadata device.

#

# Example table of the out-of-place reshape space addition for one data device, e.g. dm-1



  0 8192 linear 8:0 0 1960903888 #  <- must be free space segment

  8192 1960886272 linear 8:0 0 2048 # previous data segment



# Mapping table for e.g. raid5_rs reshape causing the size of the raid device to double-fold once the reshape finishes.

# Check the status output (e.g. "dmsetup status $RaidDev") for progess.



  0 $((2 * 1960886272)) raid raid5 7 0 region_size 2048 data_offset 8192 delta_disk 1 2 /dev/dm-0 /dev/dm-1 /dev/dm-2 /dev/dm-3





Version History

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