Re: RAID10-shrink does not work

Dear Neil,

quoted

Unfortunately shrinking the raid10-array back to 13 devices
does not work:

# mdadm --grow /dev/md5 --array-size 12696988928
# mdadm --grow /dev/md5 --raid-devices=13
mdadm: Cannot set array shape for /dev/md5

Where did you get that array-size from?  It isn't a multiple
of 512K.  Hence the chunksize setting reports an error.

mdadm told me to use this :-)

Here's what I did:

# mdadm --detail /dev/md5
..
     Array Size : 15627063296 (14903.13 GiB 16002.11 GB)
  Used Dev Size : 1953382912 (1862.89 GiB 2000.26 GB)
   Raid Devices : 16
..

Note: 15627063296 = 1953382912 * 16 / 2

Now if I reduce the number of devices to 13 mdadm complains
about the array-size and suggests a value to use:

# mdadm --grow /dev/md5 --raid-devices=13
mdadm: this change will reduce the size of the array.
       use --grow --array-size first to truncate array.
       e.g. mdadm --grow /dev/md5 --array-size 12696988928

mdadm calculates the new array-size from the componentsize
by multiplying it with 13 and dividing it by 2:
newarraysize = componentsize * 13 / copies

While the old array-size is a multiple of 512 the new is not.
So I truncated the array to the size of the underlying
filesystem (which is 12696988672K, a multiple of 512K)
and that worked.

Thanks again for your support

Peter