md: Support write-intent bitmaps with externally managed metadata.
In this case, the metadata needs to not be in the same
sector as the bitmap.
md will not read/write any bitmap metadata. Config must be
done via sysfs and when a recovery makes the array non-degraded
again, writing 'true' to 'bitmap/can_clear' will allow bits in
the bitmap to be cleared again.
Signed-off-by: NeilBrown <neilb@suse.de>
diff --git a/Documentation/md.txt b/Documentation/md.txt
index 18fad68..21d26fb 100644
--- a/Documentation/md.txt
+++ b/Documentation/md.txt
@@ -322,6 +322,22 @@
'backlog' sets a limit on the number of concurrent background
writes. If there are more than this, new writes will by
synchronous.
+ bitmap/metadata
+ This can be either 'internal' or 'external'.
+ 'internal' is the default and means the metadata for the bitmap
+ is stored in the first 256 bytes of the allocated space and is
+ managed by the md module.
+ 'external' means that bitmap metadata is managed externally to
+ the kernel (i.e. by some userspace program)
+ bitmap/can_clear
+ This is either 'true' or 'false'. If 'true', then bits in the
+ bitmap will be cleared when the corresponding blocks are thought
+ to be in-sync. If 'false', bits will never be cleared.
+ This is automatically set to 'false' if a write happens on a
+ degraded array, or if the array becomes degraded during a write.
+ When metadata is managed externally, it should be set to true
+ once the array becomes non-degraded, and this fact has been
+ recorded in the metadata.
