Editing Andrew File System (section)

== Features ==

AFS<ref>{{cite journal|author =Howard, J.H.|author2 =Kazar, M.L.|author3 =Nichols, S.G.|author4 =Nichols, D.A.|author5 =Satyanarayanan, M.|author6 =Sidebotham, R.N.|author7 =West, M.J.|name-list-style =amp|title=Scale and Performance in a Distributed File System|journal=ACM Transactions on Computer Systems|volume=6|issue=1|date=February 1988|pages=51–81|doi=10.1145/35037.35059|citeseerx =10.1.1.71.5072|s2cid =52848606}}</ref> has several benefits over traditional networked [[file system]]s, particularly in the areas of security and scalability.  One enterprise AFS deployment at [[Morgan Stanley]] exceeds 25,000 clients.<ref>{{cite web|url=http://www-conf.slac.stanford.edu/AFSBestPractices/Slides/MorganStanley.pdf|title=When Your Business Depends On It — The Evolution of a Global File System for a Global Enterprise|first=Phillip|last=Moore|date=2004|access-date=2009-06-18|archive-date=2017-07-09|archive-url=https://web.archive.org/web/20170709042700/http://www-conf.slac.stanford.edu/AFSBestPractices/Slides/MorganStanley.pdf|url-status=dead}}</ref>  AFS uses [[Kerberos protocol|Kerberos]] for authentication, and implements [[access control list]]s on directories for users and groups.  Each client caches files on the local filesystem for increased speed on subsequent requests for the same file.  This also allows limited filesystem access in the event of a [[crash (computing)|server crash]] or a [[network outage]].

AFS uses the [[weak consistency]] model.<ref>{{Citation
|publisher = Amazon
|author = Yaniv Pessach
|title = Distributed Storage
|edition = Distributed Storage: Concepts, Algorithms, and Implementations
|date = 2013
|ol = 25423189M
}}</ref> Read and write operations on an open file are directed only to the locally cached copy. When a modified file is closed, the changed portions are copied back to the file server. Cache consistency is maintained by [[Callback (computer science)|callback]] mechanism. When a file is cached, the server makes a note of this and promises to inform the client if the file is updated by someone else.  Callbacks are discarded and must be re-established after any client, server, or network failure, including a timeout.  Re-establishing a callback involves a status check and does not require re-reading the file itself.

A consequence of the [[file locking]] strategy is that AFS does not support large shared databases or record updating within files shared between client systems. This was a deliberate design decision based on the perceived needs of the university computing environment. For example, in the original email system for the Andrew Project, the Andrew Message System, a single file per message is used, like [[maildir]], rather than a single file per mailbox, like [[mbox]]. See [[file locking#Buffered I/O problems|AFS and buffered I/O Problems]] for handling shared databases.

A significant feature of AFS is the [[Volume (computing)|volume]], a tree of files, sub-directories and AFS [[mount (computing)|mountpoints]] (links to other AFS volumes). Volumes are created by administrators and linked at a specific named path in an AFS cell. Once created, users of the filesystem may create directories and
files as usual without concern for the physical location of the volume.  A volume may have a [[Disk quota|quota]] assigned to it in order to limit the amount of space consumed. As needed, AFS administrators can move that volume to another server and disk location without the need to notify users; the operation can even occur while files in that volume are being used.

AFS volumes can be replicated to read-only cloned copies. When accessing files in a read-only volume, a client system will retrieve data from a particular read-only copy. If at some point, that copy becomes unavailable, clients will look for any of the remaining copies. Again, users of that data are unaware of the location of the read-only copy; administrators can create and relocate such copies as needed. The AFS command suite guarantees that all read-only volumes contain exact copies of the original read-write volume at the time the read-only copy was created.

The file name space on an Andrew workstation is partitioned into a ''shared'' and ''local'' name space. The shared name space (usually mounted as /afs on the Unix filesystem) is identical on all workstations. The local name space is unique to each workstation. It only contains temporary files needed for workstation initialization and symbolic links to files in the shared name space.

The Andrew File System heavily influenced Version 4 of [[Sun Microsystems]]' popular [[Network File System (protocol)|Network File System]] (NFS). Additionally, a variant of AFS, the [[DCE Distributed File System]] (DFS) was adopted by the [[Open Software Foundation]] in 1989 as part of their [[Distributed Computing Environment]]. Finally AFS (version two) was the predecessor of the [[Coda (file system)|Coda]] file system.