dm thin: add data block size limits to Documentation

Inform users that the data block size can't be any arbitrary number,
i.e. its value must be between 64KB and 1GB.  Also, it should be a
multiple of 64KB.

Signed-off-by: Carlos Maiolino <cmaiolino@redhat.com>
Signed-off-by: Mike Snitzer <snitzer@redhat.com>
Signed-off-by: Alasdair G Kergon <agk@redhat.com>
Acked-by: Joe Thornber <ejt@redhat.com>

Documentation/device-mapper/thin-provisioning.txt

@@ -99,13 +99,14 @@ Using an existing pool device
 		 $data_block_size $low_water_mark"
 
 $data_block_size gives the smallest unit of disk space that can be
-allocated at a time expressed in units of 512-byte sectors.  People
+allocated at a time expressed in units of 512-byte sectors.
-primarily interested in thin provisioning may want to use a value such
+$data_block_size must be between 128 (64KB) and 2097152 (1GB) and a
-as 1024 (512KB).  People doing lots of snapshotting may want a smaller value
+multiple of 128 (64KB).  $data_block_size cannot be changed after the
-such as 128 (64KB).  If you are not zeroing newly-allocated data,
+thin-pool is created.  People primarily interested in thin provisioning
-a larger $data_block_size in the region of 256000 (128MB) is suggested.
+may want to use a value such as 1024 (512KB).  People doing lots of
-$data_block_size must be the same for the lifetime of the
+snapshotting may want a smaller value such as 128 (64KB).  If you are
-metadata device.
+not zeroing newly-allocated data, a larger $data_block_size in the
+region of 256000 (128MB) is suggested.
 
 $low_water_mark is expressed in blocks of size $data_block_size.  If
 free space on the data device drops below this level then a dm event