Create: default to bitmap=internal for large arrays.

Here, "large" means components are 100G or more.  It is
usually beneficial to have write-intent bitmaps on such arrays.
They can be suppressed with --bitmap=none

Signed-off-by: NeilBrown <neilb@suse.de>

1 files changed, 15 insertions, 0 deletions

diff --git a/mdadm.8.in b/mdadm.8.in
index 1c06533c..39034704 100644
--- a/mdadm.8.in
+++ b/mdadm.8.in
@@ -686,6 +686,12 @@ slash ('/') if it is a real file (not 'internal' or 'none').
 Note: external bitmaps are only known to work on ext2 and ext3.
 Storing bitmap files on other filesystems may result in serious problems.
 
+When creating an array on devices which are 100G or larger,
+.I mdadm
+automatically adds an internal bitmap as it will usually be
+beneficial.  This can be suppressed with
+.B "\-\-bitmap=none".
+
 .TP
 .BR \-\-bitmap\-chunk=
 Set the chunksize of the bitmap.  Each bit corresponds to that many
@@ -1932,6 +1938,15 @@ setting.
 .\".B \-\-size
 .\"is given, the apparent size of the smallest drive given is used.
 
+If the array type supports a write-intent bitmap, and if the devices
+in the array exceed 100G is size, an internal write-intent bitmap
+will automatically be added unless some other option is explicitly
+requested with the
+.B \-\-bitmap
+option.  In any case space for a bitmap will be reserved so that one
+can be added layer with
+.BR "\-\-grow \-\-bitmap=internal" .
+
 If the metadata type supports it (currently only 1.x metadata), space
 will be allocated to store a bad block list.  This allows a modest
 number of bad blocks to be recorded, allowing the drive to remain in