Add --data-offset flag for Create and Grow

[thirdparty/mdadm.git] / mdadm.8.in

diff --git a/mdadm.8.in b/mdadm.8.in
index 845c00cdb4732d3be497b4265058818ef5c22fe8..255b6505009f8143f6a71ec8cb6d7fce012fd6f2 100644 (file)
--- a/mdadm.8.in
+++ b/mdadm.8.in
@@ -767,6 +767,34 @@ or layout.  See the GROW MODE section below on RAID\-DEVICES CHANGES.
 The file must be stored on a separate device, not on the RAID array
 being reshaped.
 
+.TP
+.B \-\-data\-offset=
+Arrays with 1.x metadata can leave a gap between the start of the
+device and the start of array data.  This gap can be used for various
+metadata.  The start of data is known as the
+.IR data\-offset .
+Normally an appropriate data offset is computed automatically.
+However it can be useful to set it explicitly such as when re-creating
+an array which was originally created using a different version of
+.I mdadm
+which computed a different offset.
+
+Setting the offset explicitly over-rides the default.  The value given
+is in Kilobytes unless an 'M' or 'G' suffix is given.
+
+Since Linux 3.4,
+.B \-\-data\-offset
+can also be used with
+.B --grow
+for some RAID levels (initially on RAID10).  This allows the
+data-offset to be changed as part of the reshape process.  When the
+data offset is changed, no backup file is required as the difference
+in offsets is used to provide the same functionality.
+
+When the new offset is earlier than the old offset, the number of
+devices in the array cannot shrink.  When it is after the old offset,
+the number of devices in the array cannot increase.
+
 .TP
 .BR \-\-continue
 This option is complementary to the