2.3.2. Have Each Node Allocate its Own Files, If Possible

Due to the way the distributed lock manager (DLM) works, there will be more lock contention if all files are allocated by one node and other nodes need to add blocks to those files.

In GFS (version 1), all locks were managed by a central lock manager whose job was to control locking throughout the cluster. This grand unified lock manager (GULM) was problematic because it was a single point of failure. GFS2’s replacement locking scheme, DLM, spreads the locks throughout the cluster. If any node in the cluster goes down, its locks are recovered by the other nodes.

With DLM, the first node to lock a resource (like a file) becomes the “lock master” for that lock. Other nodes may lock that resource, but they have to ask permission from the lock master first. Each node knows which locks for which it is the lock master, and each node knows which node it has lent a lock to. Locking a lock on the master node is much faster than locking one on another node that has to stop and ask permission from the lock’s master.

As in many file systems, the GFS2 allocator tries to keep blocks in the same file close to one another to reduce the movement of disk heads and boost performance. A node that allocates blocks to a file will likely need to use and lock the same resource groups for the new blocks (unless all the blocks in that resource group are in use). The file system will run faster if the lock master for the resource group containing the file allocates its data blocks (that is, it is faster to have the node that first opened the file do all the writing of new blocks).