Shrink Multipath Block Device
However, it is possible to shrink an LVM PV formatted device, or an ext4 filesystem. Please see the LVM shrinking page for methods for how to do this. Be aware there are still potential risks, so it's a good idea to have a backup of the data in case of issues.
Long story short - this will only ever happen if your block devices are being transmitted over a SAN. Once you've cleaned up the last blocks on a device as depicted in the above link, then allow the block device to be reduced at the originator. Rescan the devices as per the instructions for growing:
# echo 1 > /sys/class/scsi_device/X\:X\:X\:X/device/rescan
Resize the path in multipathd with:
# multipathd -k"resize multipath san4_Data"
Once that's done, multipath should now report the new size:
# multipath -ll san4_Data
san4_Data (360002ac00000000000000dfa00004ca6) dm-3 3PARdata,VV size=250G features='1 queue_if_no_path' hwhandler='1 alua' wp=rw `-+- policy='round-robin 0' prio=50 status=active |- 0:0:1:2 sdb 8:16 active ready running `- 1:0:1:2 sdg 8:96 active ready running