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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>
This commit is contained in:
Guoqing Jiang 2019-05-31 10:10:00 +08:00 committed by Jes Sorensen
parent 2c2d9c48d2
commit e53cb96869
1 changed files with 69 additions and 0 deletions
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@ -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.