File system fragmentation: Difference between revisions

Revision as of 06:03, 16 March 2016

Revision as of 11:06, 27 November 2016

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[[BitTorrent (protocol)|BitTorrent]] and other [[peer-to-peer]] [[filesharing]] applications limit fragmentation by preallocating the full space needed for a file when initiating [[download]]s.<ref>{{cite journal |date=29 March 2009 |first=Jeffrey |last=Layton |title=From ext3 to ext4: An Interview with Theodore Ts'o |work=[[Linux Magazine]] |publisher=[[QuinStreet]] |url=http://www.linux-mag.com/id/7272/ }}</ref>

A relatively recent technique is [[delayed allocation]] in [[XFS]], [[HFS+]]<ref>{{cite web |first=Amit |last=Singh |date=May 2004 |title=Fragmentation in HFS Plus Volumes |work=Mac OS X Internals |url=http://osxbook.com/software/hfsdebug/fragmentation.html }}</ref> and [[ZFS]]; the same technique is also called allocate-on-flush in [[reiser4]] and [[ext4]]. When the file system is being written to, file system blocks are reserved, but the locations of specific files are not laid down yet. Later, when the file system is forced to flush changes as a result of memory pressure or a transaction commit, the allocator will have much better knowledge of the files' characteristics. Most file systems with this approach try to flush files in a single directory contiguously. Assuming that multiple reads from a single directory are common, locality of reference is improved.<ref name=xfs-scalability>{{cite conference |first=Adam |last=Sweeney |first2=Doug |last2=Doucette |first3=Wei |last3=Hu |first4=Curtis |last4=Anderson |first5=Mike |last5=Nishimoto |first6=Geoff |last6=Peck |date=January 1996 |title=Scalability in the XFS File System |publisher=[[Silicon Graphics]] |booktitle=Proceedings of the USENIX 1996 Annual Technical Conference |___location=[[San Diego, California]] |url=http://www.soe.ucsc.edu/~sbrandt/290S/xfs.pdf |format=[[PDF]] |accessdate=2006-12-14 }}</ref> Reiser4 also orders the layout of files according to the directory [[hash table]], so that when files are being accessed in the natural file system order (as dictated by [[readdir]]), they are always read sequentially.<ref name=reiser4-google>{{cite web |first=Hans |last=Reiser |date=2006-02-06 |title=The Reiser4 Filesystem |url=http://video.google.com/videoplay?docid=6866770590245111825&q=reiser4 |work=[[Google TechTalks]] |accessdate=2006-12-14 |archiveurl=httphttps://www.youtube.com/watch?v=HytWhfo7-zI |archivedate=22 August 2012}}</ref>

<!-- TODO: Cylinder groups and locality of reference; XFS allocation groups (are they actually relevant?) -->