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