The RADOS object store and Ceph filesystem: Part 2

Magical Storage

Alternative: The RADOS Block Device

In the first part of this workshop http://1, I took an in-depth look at the Ceph filesystem, which is a front end for RADOS. Ceph is not, however, the only way to access the data deposited in a RADOS store – the RADOS block driver (RBD) is an alternative. With rbd, objects in RADOS can be addressed as if they were on a hard disk.

This functionality is especially helpful in the context of virtualization because presenting a block device to KVM as a hard disk saves the virtualization a detour over container formats like qcow2. Additionally, using rbd is very easy – an rbd pool is already included that admins can take advantage of. For example, to create an rbd drive with a size of 1GB, you only need to use the command

rbd create test --size 1024

Subsequently, the block device can be used on any host with an rbd kernel module, which is now part of the mainline kernel and is found on practically every system. Because ceph.conf from the first part of this workshop has already specified that users must authenticate themselves to gain access to the RADOS services, that is also true for rbd. Note that login credentials can be called with:

ceph-authtool -l /etc/ceph/admin.keyring

Then, on a machine that has loaded rbd, you can type

echo "IP addresses of the MONs, separated by commas name=admin,secret=Authkeyrbd test" > /sys/bus/rbd/add

to activate the RBD drive.

Conclusions

RADOS lets administrators replace two-node storage with seamlessly scalable RADOS-based storage. Currently, the project developers plan to turn version 0.48 into version 1.0, which will then receive the official "Ready for Enterprise" stamp. Because the previous version 0.47 has already been released, the enterprise version may be expected soon.

Infos

"RADOS and Ceph" by Martin Loschwitz, ADMIN , Issue 09, pg. 28, http://hpc.admin-magazine.com/Articles/The-RADOS-Object-Store-and-Ceph-Filesystem/(language)/eng-US
Data placement in RADOS: http://www.ssrc.ucsc.edu/Papers/weil-osdi06.pdf
Description of the Crush algorithm: http://www.ssrc.ucsc.edu/Papers/weil-sc06.pdf

The Author

Martin Gerhard Loschwitz is Principal Consultant at hastexo, where he is intensively involved with high-availability solutions. In his spare time, he maintains the Linux cluster stack for Debian GNU/Linux.

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