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Documentation / filesystems / dlmfs.txt

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Based on kernel version 4.16.1. Page generated on 2018-04-09 11:53 EST.

1	dlmfs
2	==================
3	A minimal DLM userspace interface implemented via a virtual file
4	system.
6	dlmfs is built with OCFS2 as it requires most of its infrastructure.
8	Project web page:    http://ocfs2.wiki.kernel.org
9	Tools web page:      https://github.com/markfasheh/ocfs2-tools
10	OCFS2 mailing lists: http://oss.oracle.com/projects/ocfs2/mailman/
12	All code copyright 2005 Oracle except when otherwise noted.
15	=======
17	Some code taken from ramfs which is Copyright (C) 2000 Linus Torvalds
18	and Transmeta Corp.
20	Mark Fasheh <mark.fasheh@oracle.com>
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.
27	Mount options
28	=============
29	None
31	Usage
32	=====
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.
39	Setup
40	=====
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.
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 its heartbeat area, though it will eventually support heartbeat
51	against raw disks.
53	Please see the ocfs2_hb_ctl and mkfs.ocfs2 manual pages distributed
54	with ocfs2-tools.
56	Once you're heartbeating, DLM lock 'domains' can be easily created /
57	destroyed and locks within them accessed.
59	Locking
60	=======
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.
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.
71	Two levels of locks are supported - Shared Read, and Exclusive.
72	Also supported is a Trylock operation.
74	For information on the libo2dlm interface, please see o2dlm.h,
75	distributed with ocfs2-tools.
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.
83	mkdir(2) signals dlmfs to join a domain (which will have the same name
84	as the resulting directory)
86	rmdir(2) signals dlmfs to leave the domain
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.
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.
96	open(2) with O_CREAT to ensure the resource inode is created - dlmfs does
97	not automatically create inodes for existing lock resources.
99	Open Flag     Lock Request Type
100	---------     -----------------
101	O_RDONLY      Shared Read
102	O_RDWR        Exclusive
104	Open Flag     Resulting Locking Behavior
105	---------     --------------------------
106	O_NONBLOCK    Trylock operation
108	You must provide exactly one of O_RDONLY or O_RDWR.
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.
113	close(2) drops the lock associated with your fd.
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.
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.
123	Once written, an LVB will be visible to other nodes who obtain Read
124	Only or higher level locks on the resource.
126	See Also
127	========
128	http://opendlm.sourceforge.net/cvsmirror/opendlm/docs/dlmbook_final.pdf
130	For more information on the VMS distributed locking API.
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