XFS: A high-performance journaling filesystem

XFS combines advanced journaling technology with full 64-bit addressing andscalable structures and algorithms. This combination delivers the mostscalable high-performance filesystem ever conceived.

Current XFS community activity: XFS.org Wiki

Questions and Problems

If you have any questions or problems with the installation oradministration of XFS, you can send email toxfs@oss.sgi.com.Note that this address is a public mailing list; please search thelist archiveto see if your question has been answered previously.

To report any bugs you encounter in XFS, use theSGI Bugzilla database.

Features

The XFS filesystem provides the following major features:

• Quick Recovery

  The XFS journaling technology allows it to restart very quickly after an unexpected interruption, regardless of the number of files it is managing. Traditional filesystems must do special filesystem checks after an interruption, which can take many hours to complete. The XFS journaling avoids these lengthy filesystem checks.

• Fast Transactions

  The XFS filesystem provides the advantages of journaling while minimizing the performance impact of journaling on read and write data transactions. Its journaling structures and algorithms are tuned to log the transactions rapidly.

  XFS uses efficient tree structures for fast searches and rapid space allocation. XFS continues to deliver rapid response times, even for directories with tens of thousands of entries.

• Massive Scalability

  XFS is a full 64-bit filesystem, and thus is capable of handling filesystems as large as a million terabytes.

  263  = 9 x 1018 = 9 exabytes 

   

  A million terabytes is thousands of times larger than most large filesystems in use today. This may seem to be an extremely large address space, but it is needed to plan for the exponential disk density improvements observed in the storage industry in recent years. As disk capacity grows, not only does the address space need to be sufficiently large, but the structures and algorithms need to scale. XFS is ready today with the technologies needed for this scalability.

  XFS also continues to evolve to match the capabilities of the hardware it is being deployed on. Efficiency when dealing with large amounts (terabytes) of main memory and hence large numbers of active files and large amounts of cached file data are areas demanding continual improvements. Extending XFS to improve performance on large NUMA machines is also an area of active research and development.

• Efficient Allocations

  XFS implements extremely sophisticated space management techniques. Efficiency in space management has been achieved through the use of variable sized extents, rather than the simple single-block-at-a-time mechanism of many other filesystems. XFS was the first filesystem to implement delayed space allocation for buffered writes, supports direct I/O, provides an optional realtime allocator, and is able to align allocations based on the geometry of the underlying storage device. The XFS allocator performs admirably in the presence of multiple parallel writers, and is renowned for its resistance to space fragmentation under such conditions.

• Excellent Bandwidth

  XFS is capable of delivering very close to the raw I/O performance that the underlying hardware can provide. XFS has proven scalability on SGI Altix systems of multiple gigabytes-per-second on multiple terabyte filesystems.

Technical Specifications

Technology

Journaled 64-bit filesystem with guaranteedfilesystem consistency.

Availability

XFS is available for Linux 2.4 and later Linux kernels.

Online Administration

XFS supports filesystem growth for mounted volumes, allowsfilesystem "freeze" and "thaw" operations to supportvolume level snapshots, and provides an online file defragmentationutility.

Quotas

XFS supports user and group quotas.XFS considers quota information as filesystem metadata and usesjournaling to avoid the need for lengthy quota consistency checksafter a crash.Project quota are now also supported, and these can be used toprovide a form of directory tree quota.

Extended Attributes

XFS implements fully journaled extended attributes. An extended attributeis a name/value pair associated with a file. Attributes can be attached toall types of inodes: regular files, directories, symbolic links,device nodes, and so forth. Attribute values can contain up to 64KB ofarbitrary binary data. XFS implements three attribute namespaces: a usernamespace available to all users, protected by the normal filepermissions; a system namespace, accessible only to privilegedusers; and a security namespace, used by security modules (SELinux).The system namespace can be used for protected filesystem meta-datasuch as access control lists (ACLs) and hierarchical storage manager(HSM) file migration status.

POSIX Access Control Lists (ACLs)

XFS supports the ACL semantics and interfaces described in the draftPOSIX 1003.1e standard.

Maximum File Size

For Linux 2.4, the maximum accessible file offset is 16TB on 4K page sizeand 64TB on 16K page size. For Linux 2.6, when using 64 bit addressing inthe block devices layer (CONFIG_LBD), file size limit increases to 9million terabytes (or the device limits).

Maximum Filesystem Size

For Linux 2.4, 2 TB. For Linux 2.6 and beyond, when using 64 bit addressingin the block devices layer (CONFIG_LBD) and a 64 bit platform, filesystemsize limit increases to 9 million terabytes (or the device limits).For these later kernels on 32 bit platforms, 16TB is the current limiteven with 64 bit addressing enabled in the block layer.

Filesystem Block Size

The minimum filesystem block size is 512 bytes. The maximumfilesystem block size is the page size of the kernel, which is4K on x86 architecture and is set as a kernel compile optionon the IA64 architecture (up to 64 kilobyte pages). So, XFSsupports filesystem block sizes up to 64 kilobytes (from 512bytes, in powers of 2), when the kernel page size allows it.

Filesystemextents (contiguous data) are configurable at file creation time usingxfsctl(3) and are multiples of the filesystem block size.Individual extents can be up to 4 GB in size.

Physical Disk Sector Sizes Supported

512 bytes through to 32 kilobytes (in powers of 2), with the caveat thatthe sector size must be less than or equal to the filesystem blocksize.

NFS Compatibility

With NFS version 3, 64-bit filesystems can be exported to other systems thatsupport the NFS V3 protocol. Systems that use NFS V2 protocol may access XFSfilesystems within the 32-bit limit imposed by the protocol.

Windows Compatibility

SGI uses the Open Source Samba server to export XFS filesystems to MicrosoftWindows systems. Samba speaks the SMB (Server Message Block) and CIFS(Common Internet File System) protocols.

Backup/Restore

xfsdump and xfsrestore can be used for backup and restore of XFS filesystems to local/remote SCSI tapes or files. It supports dumping of extendedattributes and quota information. As the xfsdump format has been preservedand is now endian neutral, dumps created on one platform can be restoredonto an XFS filesystem on another (different architectures, and evendifferent operating systems - IRIX to Linux, and vice-versa).

Support for Hierarchical Storage

The Data Management API (DMAPI/XDSM) allows implementation of hierarchicalstorage management software with no kernel modifications as well ashigh-performance dump programs without requiring "raw" access tothe disk and knowledge of filesystem structures.

Optional Realtime Allocator

XFS supports the notion of a "realtime subvolume" - a separatearea of disk space where only file data is stored.Space on this subvolume is managed using the realtime allocator(as opposed to the default, B+ tree space allocator).The realtime subvolume is designed to provide very deterministicdata rates suitable for media streaming applications.