mdadm/md.4: add the descriptions for bitmap sysfs nodes

The sysfs nodes under bitmap are not recorded in md.4,
add them based on md.rst and kernel source code.

Cc: NeilBrown <neilb@suse.com>
Signed-off-by: Guoqing Jiang <gqjiang@suse.com>
Signed-off-by: Jes Sorensen <jsorensen@fb.com>

diff --git a/md.4 b/md.4
index 3a1d6777e5b75e172ef4c060aeeb5f80bada55a1..e86707a2cbb37f7a09fe5fa4776762a4b058afdc 100644 (file)
--- a/md.4
+++ b/md.4
@@ -1101,6 +1101,75 @@ stripe that requires some "prereading".  For fairness this defaults to
 maximizes sequential-write throughput at the cost of fairness to threads
 doing small or random writes.
 
+.TP
+.B md/bitmap/backlog
+The value stored in the file only has any effect on RAID1 when write-mostly
+devices are active, and write requests to those devices are proceed in the
+background.
+
+This variable sets a limit on the number of concurrent background writes,
+the valid values are 0 to 16383, 0 means that write-behind is not allowed,
+while any other number means it can happen.  If there are more write requests
+than the number, new writes will by synchronous.
+
+.TP
+.B md/bitmap/can_clear
+This is for externally managed bitmaps, where the kernel writes the bitmap
+itself, but metadata describing the bitmap is managed by mdmon or similar.
+
+When the array is degraded, bits mustn't be cleared. When the array becomes
+optimal again, bit can be cleared, but first the metadata needs to record
+the current event count. So md sets this to 'false' and notifies mdmon,
+then mdmon updates the metadata and writes 'true'.
+
+There is no code in mdmon to actually do this, so maybe it doesn't even
+work.
+
+.TP
+.B md/bitmap/chunksize
+The bitmap chunksize can only be changed when no bitmap is active, and
+the value should be power of 2 and at least 512.
+
+.TP
+.B md/bitmap/location
+This indicates where the write-intent bitmap for the array is stored.
+It can be "none" or "file" or a signed offset from the array metadata
+- measured in sectors. You cannot set a file by writing here - that can
+only be done with the SET_BITMAP_FILE ioctl.
+
+Write 'none' to 'bitmap/location' will clear bitmap, and the previous
+location value must be write to it to restore bitmap.
+
+.TP
+.B md/bitmap/max_backlog_used
+This keeps track of the maximum number of concurrent write-behind requests
+for an md array, writing any value to this file will clear it.
+
+.TP
+.B md/bitmap/metadata
+This can be 'internal' or 'clustered' or 'external'. 'internal' is set
+by default, which means the metadata for bitmap is stored in the first 256
+bytes of the bitmap space. 'clustered' means separate bitmap metadata are
+used for each cluster node. 'external' means that bitmap metadata is managed
+externally to the kernel.
+
+.TP
+.B md/bitmap/space
+This shows the space (in sectors) which is available at md/bitmap/location,
+and allows the kernel to know when it is safe to resize the bitmap to match
+a resized array. It should big enough to contain the total bytes in the bitmap.
+
+For 1.0 metadata, assume we can use up to the superblock if before, else
+to 4K beyond superblock. For other metadata versions, assume no change is
+possible.
+
+.TP
+.B md/bitmap/time_base
+This shows the time (in seconds) between disk flushes, and is used to looking
+for bits in the bitmap to be cleared.
+
+The default value is 5 seconds, and it should be an unsigned long value.
+
 .SS KERNEL PARAMETERS
 
 The md driver recognised several different kernel parameters.