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