linux/Documentation/filesystems/dlmfs.rst
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   1.. SPDX-License-Identifier: GPL-2.0
   2.. include:: <isonum.txt>
   3
   4=====
   5DLMFS
   6=====
   7
   8A minimal DLM userspace interface implemented via a virtual file
   9system.
  10
  11dlmfs is built with OCFS2 as it requires most of its infrastructure.
  12
  13:Project web page:    http://ocfs2.wiki.kernel.org
  14:Tools web page:      https://github.com/markfasheh/ocfs2-tools
  15:OCFS2 mailing lists: https://oss.oracle.com/projects/ocfs2/mailman/
  16
  17All code copyright 2005 Oracle except when otherwise noted.
  18
  19Credits
  20=======
  21
  22Some code taken from ramfs which is Copyright |copy| 2000 Linus Torvalds
  23and Transmeta Corp.
  24
  25Mark Fasheh <mark.fasheh@oracle.com>
  26
  27Caveats
  28=======
  29- Right now it only works with the OCFS2 DLM, though support for other
  30  DLM implementations should not be a major issue.
  31
  32Mount options
  33=============
  34None
  35
  36Usage
  37=====
  38
  39If you're just interested in OCFS2, then please see ocfs2.txt. The
  40rest of this document will be geared towards those who want to use
  41dlmfs for easy to setup and easy to use clustered locking in
  42userspace.
  43
  44Setup
  45=====
  46
  47dlmfs requires that the OCFS2 cluster infrastructure be in
  48place. Please download ocfs2-tools from the above url and configure a
  49cluster.
  50
  51You'll want to start heartbeating on a volume which all the nodes in
  52your lockspace can access. The easiest way to do this is via
  53ocfs2_hb_ctl (distributed with ocfs2-tools). Right now it requires
  54that an OCFS2 file system be in place so that it can automatically
  55find its heartbeat area, though it will eventually support heartbeat
  56against raw disks.
  57
  58Please see the ocfs2_hb_ctl and mkfs.ocfs2 manual pages distributed
  59with ocfs2-tools.
  60
  61Once you're heartbeating, DLM lock 'domains' can be easily created /
  62destroyed and locks within them accessed.
  63
  64Locking
  65=======
  66
  67Users may access dlmfs via standard file system calls, or they can use
  68'libo2dlm' (distributed with ocfs2-tools) which abstracts the file
  69system calls and presents a more traditional locking api.
  70
  71dlmfs handles lock caching automatically for the user, so a lock
  72request for an already acquired lock will not generate another DLM
  73call. Userspace programs are assumed to handle their own local
  74locking.
  75
  76Two levels of locks are supported - Shared Read, and Exclusive.
  77Also supported is a Trylock operation.
  78
  79For information on the libo2dlm interface, please see o2dlm.h,
  80distributed with ocfs2-tools.
  81
  82Lock value blocks can be read and written to a resource via read(2)
  83and write(2) against the fd obtained via your open(2) call. The
  84maximum currently supported LVB length is 64 bytes (though that is an
  85OCFS2 DLM limitation). Through this mechanism, users of dlmfs can share
  86small amounts of data amongst their nodes.
  87
  88mkdir(2) signals dlmfs to join a domain (which will have the same name
  89as the resulting directory)
  90
  91rmdir(2) signals dlmfs to leave the domain
  92
  93Locks for a given domain are represented by regular inodes inside the
  94domain directory.  Locking against them is done via the open(2) system
  95call.
  96
  97The open(2) call will not return until your lock has been granted or
  98an error has occurred, unless it has been instructed to do a trylock
  99operation. If the lock succeeds, you'll get an fd.
 100
 101open(2) with O_CREAT to ensure the resource inode is created - dlmfs does
 102not automatically create inodes for existing lock resources.
 103
 104============  ===========================
 105Open Flag     Lock Request Type
 106============  ===========================
 107O_RDONLY      Shared Read
 108O_RDWR        Exclusive
 109============  ===========================
 110
 111
 112============  ===========================
 113Open Flag     Resulting Locking Behavior
 114============  ===========================
 115O_NONBLOCK    Trylock operation
 116============  ===========================
 117
 118You must provide exactly one of O_RDONLY or O_RDWR.
 119
 120If O_NONBLOCK is also provided and the trylock operation was valid but
 121could not lock the resource then open(2) will return ETXTBUSY.
 122
 123close(2) drops the lock associated with your fd.
 124
 125Modes passed to mkdir(2) or open(2) are adhered to locally. Chown is
 126supported locally as well. This means you can use them to restrict
 127access to the resources via dlmfs on your local node only.
 128
 129The resource LVB may be read from the fd in either Shared Read or
 130Exclusive modes via the read(2) system call. It can be written via
 131write(2) only when open in Exclusive mode.
 132
 133Once written, an LVB will be visible to other nodes who obtain Read
 134Only or higher level locks on the resource.
 135
 136See Also
 137========
 138http://opendlm.sourceforge.net/cvsmirror/opendlm/docs/dlmbook_final.pdf
 139
 140For more information on the VMS distributed locking API.
 141