diff --git a/Documentation/md.txt b/Documentation/md.txt
index 866a1a887547..7b3d471bb9f0 100644
--- a/Documentation/md.txt
+++ b/Documentation/md.txt
@@ -250,6 +250,13 @@ Each directory contains:
         the device before this offset us not touched, unless it is
         used for storing metadata (Formats 1.1 and 1.2).
 
+      size
+        The amount of the device, after the offset, that can be used
+        for storage of data.  This will normally be the same as the
+	component_size.  This can be written while assembling an
+        array.  If a value less than the current component_size is
+        written, component_size will be reduced to this value.
+
 
 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 27db100d8b12..40ac7fbab61f 100644
--- a/drivers/md/md.c
+++ b/drivers/md/md.c
@@ -1686,12 +1686,37 @@ offset_store(mdk_rdev_t *rdev, const char *buf, size_t len)
 static struct rdev_sysfs_entry rdev_offset =
 __ATTR(offset, 0644, offset_show, offset_store);
 
+static ssize_t
+rdev_size_show(mdk_rdev_t *rdev, char *page)
+{
+	return sprintf(page, "%llu\n", (unsigned long long)rdev->size);
+}
+
+static ssize_t
+rdev_size_store(mdk_rdev_t *rdev, const char *buf, size_t len)
+{
+	char *e;
+	unsigned long long size = simple_strtoull(buf, &e, 10);
+	if (e==buf || (*e && *e != '\n'))
+		return -EINVAL;
+	if (rdev->mddev->pers)
+		return -EBUSY;
+	rdev->size = size;
+	if (size < rdev->mddev->size || rdev->mddev->size == 0)
+		rdev->mddev->size = size;
+	return len;
+}
+
+static struct rdev_sysfs_entry rdev_size =
+__ATTR(size, 0644, rdev_size_show, rdev_size_store);
+
 static struct attribute *rdev_default_attrs[] = {
 	&rdev_state.attr,
 	&rdev_super.attr,
 	&rdev_errors.attr,
 	&rdev_slot.attr,
 	&rdev_offset.attr,
+	&rdev_size.attr,
 	NULL,
 };
 static ssize_t