A comparison between BookKeeper and GlusterFS.

GlusterFS also implements striping and replication (GlusterFS). However, GlusterFS has been designed to be a general purpose file system and it does not provide the same guarantees that BookKeeper for journalling/WAL as we discuss next.We first discuss our understanding of the GlusterFS architecture and point to the differences we see.

Overview of GlusterFS

General setup

In a distributed GlusterFS setup, there is a daemon, glusterd running on all the machines. The administrator uses gluster to create a /trusted server pool/ from these machines. All machines in the pool then share the configuration. The pool is configured by the commandline tool.

Servers in the pool expose bricks1, which represent the lowest level of storage in the system, which is basically a filesystem partition. The administrator can then compose and layer /translators/ on top of this to build a volume. Translators are conceptual objects which expose a filesystem interface. For example, the AFR translator2 provides replication by writing all changes to a number of child translators. At the lowest level there is a Posix translator which sites directly on top of a brick.

AFR replicates by write all changes to all child translators, much like RAID1. If a child is down, write will go to the other children. If all children go offline, the volume itself goes offline.

Consistency issues.

Imagine a system with a AFR translator replicating to two Posix translators (A & B). If one replica(A) goes offline, a client will continue to write to the other (B). Now, B goes offline. The client will stop writing to any replica. A is restarted so the client can continue writing. The client writes updates to A. Now A and B are inconsistent.

Differences to BookKeeper

1 http://www.gluster.com/community/documentation/index.php/GlusterFS_Concepts
2 http://www.gluster.com/community/documentation/index.php/Understanding_AFR_Translator