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7.3.2.2.3. Optimising for concurrency

Unlike other file systems, XFS can perform many types of allocation and deallocation operations concurrently provided that the operations are occurring on non-shared objects. Allocation or deallocation of extents can occur concurrently provided that the concurrent operations occur in different allocation groups. Similarly, allocation or deallocation of inodes can occur concurrently provided that the concurrent operations affect different allocation groups.
The number of allocation groups becomes important when using machines with a high CPU count and multi-threaded applications that attempt to perform operations concurrently. If only four allocation groups exist, then sustained, parallel metadata operations will only scale as far as those four CPUs (the concurrency limit provided by the system). For small file systems, ensure that the number of allocation groups is supported by the concurrency provided by the system. For large file systems (tens of terabytes and larger) the default formatting options generally create sufficient allocation groups to avoid limiting concurrency.
Applications must be aware of single points of contention in order to use the parallelism inherent in the structure of the XFS file system. It is not possible to modify a directory concurrently, so applications that create and remove large numbers of files should avoid storing all files in a single directory. Each directory created is placed in a different allocation group, so techniques such as hashing files over multiple sub-directories provide a more scalable storage pattern compared to using a single large directory.