+Default object size:
+
$ rbd create --size 20M img
$ DEV=$(sudo rbd map img)
$ sudo rbd unmap $DEV
$ rbd rm --no-progress img
+
+Custom object size:
+
+ $ rbd create --size 20M --object-size 1M img
+
+ $ DEV=$(sudo rbd map img)
+ $ blockdev --getiomin $DEV
+ 65536
+ $ blockdev --getioopt $DEV
+ 1048576
+ $ cat /sys/block/${DEV#/dev/}/queue/discard_granularity
+ 65536
+ $ sudo rbd unmap $DEV
+
+ $ DEV=$(sudo rbd map -o alloc_size=512 img)
+ $ blockdev --getiomin $DEV
+ 512
+ $ blockdev --getioopt $DEV
+ 1048576
+ $ cat /sys/block/${DEV#/dev/}/queue/discard_granularity
+ 512
+ $ sudo rbd unmap $DEV
+
+ $ DEV=$(sudo rbd map -o alloc_size=1048576 img)
+ $ blockdev --getiomin $DEV
+ 1048576
+ $ blockdev --getioopt $DEV
+ 1048576
+ $ cat /sys/block/${DEV#/dev/}/queue/discard_granularity
+ 1048576
+ $ sudo rbd unmap $DEV
+
+ $ DEV=$(sudo rbd map -o alloc_size=2097152 img)
+ $ blockdev --getiomin $DEV
+ 1048576
+ $ blockdev --getioopt $DEV
+ 1048576
+ $ cat /sys/block/${DEV#/dev/}/queue/discard_granularity
+ 1048576
+ $ sudo rbd unmap $DEV
+
+ $ rbd rm --no-progress img