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md: allow a reshape operation to be reversed.

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Currently a reshape operation always progresses from the start
of the array to the end unless the number of devices is being
reduced, in which case it progressed in the opposite direction.

To reverse a partial reshape which changes the number of devices
you can stop the array and re-assemble with the raid-disks numbers
reversed and it will undo.

However for a reshape that does not change the number of devices
it is not possible to reverse the reshape in the middle - you have to
wait until it completes.

So add a 'reshape_direction' attribute with is either 'forwards' or
'backwards' and can be explicitly set when delta_disks is zero.

This will become more important when we allow the data_offset to
change in a reshape.  Then the explicit statement of what direction is
being used will be more useful.

This can be enabled in raid5 trivially as it already supports
reverse reshape and just needs to use a different trigger to request it.

Signed-off-by: NeilBrown <neilb@suse.de>
This commit is contained in:
NeilBrown 2012-05-21 09:27:00 +10:00
parent b5e1b8cee7
commit 2c810cddc4
4 changed files with 84 additions and 14 deletions
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7
include/linux/raid/md_p.h
View file

@ -281,10 +281,15 @@ struct mdp_superblock_1 {
* active device with same 'role'.
* 'recovery_offset' is also set.
*/
#define MD_FEATURE_RESHAPE_BACKWARDS 32 /* Reshape doesn't change number
* of devices, but is going
* backwards anyway.
*/
#define MD_FEATURE_ALL (MD_FEATURE_BITMAP_OFFSET \
|MD_FEATURE_RECOVERY_OFFSET \
|MD_FEATURE_RESHAPE_ACTIVE \
|MD_FEATURE_BAD_BLOCKS \
|MD_FEATURE_REPLACEMENT)
|MD_FEATURE_REPLACEMENT \
|MD_FEATURE_RESHAPE_BACKWARDS)
#endif