2 files changed, 29 insertions, 0 deletions

diff --git a/Documentation/md.txt b/Documentation/md.txt
index d525fffc873b..866a1a887547 100644
--- a/Documentation/md.txt
+++ b/Documentation/md.txt
@@ -244,6 +244,12 @@ Each directory contains:
         it currently fills.  This can only be set while assembling an
         array.  A device for which this is set is assumed to be working.
 
+      offset
+        This gives the location in the device (in sectors from the
+        start) where data from the array will be stored.  Any part of
+        the device before this offset us not touched, unless it is
+        used for storing metadata (Formats 1.1 and 1.2).
+
 
 An active md device will also contain and entry for each active device
 in the array.  These are named
diff --git a/drivers/md/md.c b/drivers/md/md.c
index a8169564209d..742a82a4d10b 100644
--- a/drivers/md/md.c
+++ b/drivers/md/md.c
@@ -1664,11 +1664,34 @@ slot_store(mdk_rdev_t *rdev, const char *buf, size_t len)
 static struct rdev_sysfs_entry rdev_slot =
 __ATTR(slot, 0644, slot_show, slot_store);
 
+static ssize_t
+offset_show(mdk_rdev_t *rdev, char *page)
+{
+        return sprintf(page, "%llu\n", rdev->data_offset);
+}
+
+static ssize_t
+offset_store(mdk_rdev_t *rdev, const char *buf, size_t len)
+{
+        char *e;
+        unsigned long long offset = simple_strtoull(buf, &e, 10);
+        if (e==buf || (*e && *e != '\n'))
+                return -EINVAL;
+        if (rdev->mddev->pers)
+                return -EBUSY;
+        rdev->data_offset = offset;
+        return len;
+}
+
+static struct rdev_sysfs_entry rdev_offset =
+__ATTR(offset, 0644, offset_show, offset_store);
+
 static struct attribute *rdev_default_attrs[] = {
         &rdev_state.attr,
         &rdev_super.attr,
         &rdev_errors.attr,
         &rdev_slot.attr,
+        &rdev_offset.attr,
         NULL,
 };
 static ssize_t