Introduction to High-Availability Clusters On Linux And Other Systems

In order for an active-active cluster to work, it must use a shared file-system (or an equivalent application-support for working with raw devices).
In a shared file-system, the same file-system is mounted on several nodes at the same time, and applications from all nodes may access files at the same time (if they use proper locking).
A shared file-system must provide proper distributed locking support, to avoid data corruption.
To reduce lock contention, a shared file-system such as GFS avoids sharing meta-data about files (e.g. i-nodes) in the same file-system block. Instead, they store the file data (for very small files) in the same block.
Further, a shared file-system should have "on-line recovery" (e.g. in case one node crashes in the middle of a transaction).
In GFS, this is done by having a separate journal per node - and having another node re-play the journal in case the first node crashes.

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