Changes between Version 2 and Version 3 of AFS


Ignore:
Timestamp:
10/17/12 21:00:56 (9 years ago)
Author:
lfaraone
Comment:

typofix

Legend:

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  • AFS

    v2 v3  
    33Athena workstations use a filesystem called AFS and the current implementation is [http://www.openafs.org OpenAFS].  Running AFS allows workstations to access files under the /afs hierarchy.  Of particular interest are the MIT parts of this hierarchy: /afs/athena.mit.edu, /afs/sipb.mit.edu, /afs/net.mit.edu, /afs/ops.mit.edu, and /afs/zone.mit.edu.  (/afs/dev.mit.edu was decommissioned in Februrary 2012). 
    44 
    5 Unlike NFS, AFS includes two layers of indirection which shield a client from having to know what hostname a file resides on in order to access it.  The first layer of indirection is "cells", such as athena.mit.edu.  Each workstation has a directory of cells in /usr/vice/etc/CellServDB, which it can use to look up the database servers for a cell name.  If a cell's database servers change, each client's CellServDB has to be updated, but the canonical paths to files in that cell do not change.  A canonical CellServDB file is maintained at grand.central.org (an OpenAFS community resource).  IS&T Server Operations maintains a local canonica CellServDB at /afs/athena.mit.edu/service/CellServDB.  A cron job on zulu compares this file to our most recent copy and alerts debathena-root if the files differ.  Debathena developers must take action to incorporate the changes into a new version of debathena-afs-config and push it out to the APT repo.  Some AFS clients can check for DNS SRV records for cells which are not listed in CellServDB. 
     5Unlike NFS, AFS includes two layers of indirection which shield a client from having to know what hostname a file resides on in order to access it.  The first layer of indirection is "cells", such as athena.mit.edu.  Each workstation has a directory of cells in /usr/vice/etc/CellServDB, which it can use to look up the database servers for a cell name.  If a cell's database servers change, each client's CellServDB has to be updated, but the canonical paths to files in that cell do not change.  A canonical CellServDB file is maintained at grand.central.org (an OpenAFS community resource).  IS&T Server Operations maintains a local canonical CellServDB at /afs/athena.mit.edu/service/CellServDB.  A cron job on zulu compares this file to our most recent copy and alerts debathena-root if the files differ.  Debathena developers must take action to incorporate the changes into a new version of debathena-afs-config and push it out to the APT repo.  Some AFS clients can check for DNS SRV records for cells which are not listed in CellServDB. 
    66 
    77The second layer of indirection is the volume location database, or VLDB.  Each AFS cell's contents are divided into named volumes of files which are stored together; volumes refer to other volumes using mountpoints within their directory structure.  When a client wishes to access a file in a volume, it uses the VLDB servers to find out which file server the volume lives on.  Volumes can move around from one file server to another and clients will track them without the user noticing anything other than a slight slowdown.