Shrinking a RAID0 volume to add a smaller device to it

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:

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

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:

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

The size is 1799217152 / 2 = 899608576 kb.

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

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

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:

new_array_size = new_component_size - raid_overhead - raid_superblock_size

440535040 - 132096 - 128 = 440402816

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

mdadm --grow /dev/md3 -z 440402816

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

mdadm /dev/md3 --add /dev/sdb7