Calculating RADOS objects for RBD images

Ceph’s RBD (RADOS Block Device) is just a thin wrapper on top of RADOS, the object store of Ceph.

It stripes (by default) over 4MB objects in RADOS. It’s very simple to calculate which RADOS object corresponds with which sector on your RBD image/block device.

First you have to find out the block device’s object prefix name and the stripe size:

ceph@daisy:~$ sudo rbd info test
rbd image 'test':
	size 128 MB in 32 objects
	order 22 (4096 KB objects)
	block_name_prefix: rb.0.1066.2ae8944a
	format: 1
ceph@daisy:~$

In this case the stripe size is 4MB (order 2^22) and the object name prefix is rb.0.1066.2ae8944a

With one line of Perl we can calculate the object name in RADOS:

perl -e 'printf "BLOCK_NAME_PREFIX.%012x\n", ((SECTOR_OFFSET * 512) / (4 * 1024 * 1024))'

Let’s say that we want the object for sector 1 of our block device:

perl -e 'printf "rb.0.1066.2ae8944a.%012x\n", ((0 * 512) / (4 * 1024 * 1024))'

This tells us that we need to fetch object rb.0.1066.2ae8944a.000000000000 from RADOS. This can be done using the ‘rados’ command:

sudo rados -p rbd get rb.0.1066.2ae8944a.000000000000 rb.0.1066.2ae8944a.000000000000

Voila, you just fetched 4MB of your drive. Might be useful if you want to do some data recovery or such.

Published by

Wido den Hollander

I am the CTO at PCextreme B.V. and i will keep this blog just to post some interesting information about my daily work on Linux (and other) systems.