The default location for an XFS journal is on the same block device as the data. As synchronous metadata writes to the journal must complete successfully before any associated data writes can start, such a layout can lead to disk contention for the typical workload pattern on a database server. To overcome this problem, you can place the journal on a separate physical device with a low-latency I/O path. As the journal typically requires very little storage space, such an arrangement can significantly improve the file system's I/O throughput. A suitable host device for the journal is a solid-state drive (SSD) device or a RAID device with a battery-backed write-back cache.

To reserve an external journal with a specified size when you create an XFS file system, specify the -l logdev=device,size=size option to the mkfs.xfs command. If you omit the size parameter, mkfs.xfs selects a journal size based on the size of the file system. To mount the XFS file system so that it uses the external journal, specify the -o logdev=device option to the mount command.