====== Shrinking a RAID0 volume to add a smaller device to it ======

**FIXME** Must add the **resize2fs** operation!

Suppose we have:

  * **/dev/md3** is the RAID0 device, running in degraded mode.
  * **/dev/sda7** is the surviving component of md0, **921GB** in size.
  * **/dev/sdb7** is the new component to be added to md0, **451GB** in size.

=== Get the size of the new component sdb7 ===

Get the exact size (in kb) of the **/dev/sdb7** partition. Using **parted** and display sizes in sector units:

<code>
parted /dev/sdb
(parted) unit s
(parted) print
...
Sector size (logical/physical): 512B/4096B
...
 7   95703040s  976773119s  881070080s    home    raid
</code>

so the size of /dev/sdb7 is 881070080 / 2 = **440535040** kb.

=== Get the size of the running component sda7 and RAID device md3 ===

Get the size (in kb) of the **/dev/sda7** partition:

<code>
parted /dev/sda
(parted) unit s
(parted) print
...
Sector size (logical/physical): 512B/4096B
...
 7   95703040s  1894920191s  1799217152s    home    raid
</code>

The size is 1799217152 / 2 = **899608576** kb.

Get the size of the **/dev/md0** RAID volume:

<code>
mdadm -D /dev/md3
...
    Array Size : 899476480 (857.81 GiB 921.06 GB)
...
</code>

The overhead of the RAID volume is 899608576 - 899476480 = **132096** kb. For safety we allow an extra space of **128** kb for the RAID superblock.

=== Calcluate the new array size ===

So we can calculate the new array size:

<code>
new_array_size = new_component_size - raid_overhead - raid_superblock_size
</code>

<code>
440535040 - 132096 - 128 = 440402816
</code>

It is now possibile to shrink the existing array **/dev/md3** to the new smaller size:

<code>
mdadm --grow /dev/md3 -z 440402816
</code>

Finally we can add the new component to the shrinked array:

<code>
mdadm /dev/md3 --add /dev/sdb7
</code>