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For reads, only the object extent needs to be reverse mapped:
$ rbd create --size 5M img
$ DEV=$(sudo rbd map img)
$ xfs_io -c 'pwrite 0 5M' $DEV >/dev/null
$ sudo rbd unmap $DEV
$ rbd snap create --no-progress img@snap
$ rbd snap protect img@snap
$ rbd clone img@snap cloneimg
$ rbd resize --no-progress --size 12M cloneimg
$ DEV=$(sudo rbd map cloneimg)
$ hexdump $DEV
0000000 cdcd cdcd cdcd cdcd cdcd cdcd cdcd cdcd
*
0500000 0000 0000 0000 0000 0000 0000 0000 0000
*
0c00000
$ dd if=$DEV iflag=direct bs=4M status=none | hexdump
0000000 cdcd cdcd cdcd cdcd cdcd cdcd cdcd cdcd
*
0500000 0000 0000 0000 0000 0000 0000 0000 0000
*
0c00000
$ sudo rbd unmap $DEV
$ rbd rm --no-progress cloneimg
$ rbd snap unprotect img@snap
$ rbd snap rm --no-progress img@snap
$ rbd rm --no-progress img
For writes, the entire object needs to be reverse mapped:
$ rbd create --size 2M img
$ DEV=$(sudo rbd map img)
$ xfs_io -c 'pwrite 0 1M' $DEV >/dev/null
$ sudo rbd unmap $DEV
$ rbd snap create --no-progress img@snap
$ rbd snap protect img@snap
$ rbd clone img@snap cloneimg
$ rbd resize --no-progress --size 8M cloneimg
$ DEV=$(sudo rbd map cloneimg)
$ xfs_io -c 'pwrite -S 0xef 3M 1M' $DEV >/dev/null
$ hexdump $DEV
0000000 cdcd cdcd cdcd cdcd cdcd cdcd cdcd cdcd
*
0100000 0000 0000 0000 0000 0000 0000 0000 0000
*
0300000 efef efef efef efef efef efef efef efef
*
0400000 0000 0000 0000 0000 0000 0000 0000 0000
*
0800000
$ sudo rbd unmap $DEV
$ DEV=$(sudo rbd map cloneimg)
$ hexdump $DEV
0000000 cdcd cdcd cdcd cdcd cdcd cdcd cdcd cdcd
*
0100000 0000 0000 0000 0000 0000 0000 0000 0000
*
0300000 efef efef efef efef efef efef efef efef
*
0400000 0000 0000 0000 0000 0000 0000 0000 0000
*
0800000
$ sudo rbd unmap $DEV
$ rbd rm --no-progress cloneimg
$ rbd snap unprotect img@snap
$ rbd snap rm --no-progress img@snap
$ rbd rm --no-progress img
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