18.17 About the XFS File System

18.17.1 About External XFS Journals
18.17.2 About XFS Write Barriers
18.17.3 About Lazy Counters
Note

You must have an Oracle Linux Premier Support account to obtain technical support for XFS with Oracle Linux.

The XFS file system is supported for the Unbreakable Enterprise Kernel Release 2 (2.6.39) and the Unbreakable Enterprise Kernel Release 3 (3.8.13) on the x86_64 architecture only.

XFS is a high-performance journaling file system that was initially created by Silicon Graphics, Inc. for the IRIX operating system and later ported to Linux. The parallel I/O performance of XFS provides high scalability for I/O threads, file system bandwidth, file and file system size, even when the file system spans many storage devices.

A typical use case for XFS is to implement a several-hundred terabyte file system across multiple storage servers, each server consisting of multiple FC-connected disk arrays.

XFS is not supported for use with the root (/) or boot file systems on Oracle Linux.

XFS has a large number of features that make it suitable for deployment in an enterprise-level computing environment that requires the implementation of very large file systems:

You can find more information about XFS at http://xfs.org/index.php/XFS_Papers_and_Documentation.

18.17.1 About External XFS Journals

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.

18.17.2 About XFS Write Barriers

A write barrier assures file system consistency on storage hardware that supports flushing of in-memory data to the underlying device. This ability is particularly important for write operations to an XFS journal that is held on a device with a volatile write-back cache.

By default, an XFS file system is mounted with a write barrier. If you create an XFS file system on a LUN that has a battery-backed, non-volatile cache, using a write barrier degrades I/O performance by requiring data to be flushed more often than necessary. In such cases, you can remove the write barrier by mounting the file system with the -o nobarrier option to the mount command.

18.17.3 About Lazy Counters

With lazy-counters enabled on an XFS file system, the free-space and inode counters are maintained in parts of the file system other than the superblock. This arrangement can significantly improve I/O performance for application workloads that are metadata intensive.

Lazy counters are enabled by default, but if required, you can disable them by specifying the -l lazy-count=0 option to the mkfs.xfs command.