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://subspace.kernel.org/lists.linux.dev.html 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