operations which occurred immediately prior to the crash can be reapplied and completed as recorded in the journal, which is how data stored in XFS file systems remain consistent. Recovery is performed automatically the first time the file system is mounted after the crash. The speed of recovery is independent of the size of the file system, instead depending on the amount of file system operations to be reapplied. Allocation groups XFS file systems are internally partitioned into allocation groups, which are equally sized linear regions within the file system. Files and directories can span allocation groups. Each allocation group manages its own inodes and free space separately, providing scalability and parallelism so multiple threads and processes can perform I/O operations on the same file system simultaneously. This architecture helps to optimize parallel I/O performance on systems with multiple processors and/or cores, as metadata updates can also be parallelized. The internal partitioning provided by allocation groups can be especially beneficial when the file system spans multiple physical devices, allowing optimal usage of throughput of the underlying storage components. Striped allocation If an XFS file system is to be created on a striped RAID array, a stripe unit can be specified when the file system is created. This maximizes throughput by ensuring that data allocations, inode allocations and the internal log (the journal) are aligned with the stripe unit. Extent-based allocation Blocks used in files stored on XFS file systems are managed with variable length extents where one extent describes one or more contiguous blocks. This can shorten the list of blocks considerably, compared to file systems that list all blocks used by a file individually. Block-oriented file systems manage space allocation with one or more block-oriented bitmaps; in XFS, these structures are replaced with an extent oriented structure consisting of a pair of B+ trees for each file system allocation group. One of the B+ trees is indexed by the length of the free extents, while the other is indexed by the starting block of the free extents. This dual indexing scheme allows for the highly efficient allocation of free extents for file system operations. Variable block sizes The file system block size represents the minimum allocation unit. XFS allows file systems to be created with block sizes ranging between 512 bytes and 64 KB, allowing the file system to be tuned for the expected degree of usage. When many