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md: support updating bitmap parameters via sysfs.

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A new attribute directory 'bitmap' in 'md' is created which
contains files for configuring the bitmap.
'location' identifies where the bitmap is, either 'none',
or 'file' or 'sector offset from metadata'.
Writing 'location' can create or remove a bitmap.
Adding a 'file' bitmap this way is not yet supported.
'chunksize' and 'time_base' must be set before 'location'
can be set.

'chunksize' can be set before creating a bitmap, but is
currently always over-ridden by the bitmap superblock.

'time_base' and 'backlog' can be updated at any time.


Signed-off-by: NeilBrown <[email protected]>
Reviewed-by: Andre Noll <[email protected]>
This commit is contained in:
NeilBrown 2009-12-14 12:49:55 +11:00
commit 43a705076e
4 changed files with 240 additions and 2 deletions
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29
Documentation/md.txt
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@ -296,6 +296,35 @@ All md devices contain:
active-idle
like active, but no writes have been seen for a while (safe_mode_delay).
bitmap/location
This indicates where the write-intent bitmap for the array is
stored.
It can be one of "none", "file" or "[+-]N".
"file" may later be extended to "file:/file/name"
"[+-]N" means that many sectors from the start of the metadata.
This is replicated on all devices. For arrays with externally
managed metadata, the offset is from the beginning of the
device.
bitmap/chunksize
The size, in bytes, of the chunk which will be represented by a
single bit. For RAID456, it is a portion of an individual
device. For RAID10, it is a portion of the array. For RAID1, it
is both (they come to the same thing).
bitmap/time_base
The time, in seconds, between looking for bits in the bitmap to
be cleared. In the current implementation, a bit will be cleared
between 2 and 3 times "time_base" after all the covered blocks
are known to be in-sync.
bitmap/backlog
When write-mostly devices are active in a RAID1, write requests
to those devices proceed in the background - the filesystem (or
other user of the device) does not have to wait for them.
'backlog' sets a limit on the number of concurrent background
writes. If there are more than this, new writes will by
synchronous.
As component devices are added to an md array, they appear in the 'md'
directory as new directories named