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Based on kernel version 2.6.33. Page generated on 2010-02-24 15:36 EST.

1	===============================================================================
2	WHAT IS EXOFS?
3	===============================================================================
4	
5	exofs is a file system that uses an OSD and exports the API of a normal Linux
6	file system. Users access exofs like any other local file system, and exofs
7	will in turn issue commands to the local OSD initiator.
8	
9	OSD is a new T10 command set that views storage devices not as a large/flat
10	array of sectors but as a container of objects, each having a length, quota,
11	time attributes and more. Each object is addressed by a 64bit ID, and is
12	contained in a 64bit ID partition. Each object has associated attributes
13	attached to it, which are integral part of the object and provide metadata about
14	the object. The standard defines some common obligatory attributes, but user
15	attributes can be added as needed.
16	
17	===============================================================================
18	ENVIRONMENT
19	===============================================================================
20	
21	To use this file system, you need to have an object store to run it on.  You
22	may download a target from:
23	http://open-osd.org
24	
25	See Documentation/scsi/osd.txt for how to setup a working osd environment.
26	
27	===============================================================================
28	USAGE
29	===============================================================================
30	
31	1. Download and compile exofs and open-osd initiator:
32	  You need an external Kernel source tree or kernel headers from your
33	  distribution. (anything based on 2.6.26 or later).
34	
35	  a. download open-osd including exofs source using:
36	     [parent-directory]$ git clone git://git.open-osd.org/open-osd.git
37	
38	  b. Build the library module like this:
39	     [parent-directory]$ make -C KSRC=$(KER_DIR) open-osd
40	
41	     This will build both the open-osd initiator as well as the exofs kernel
42	     module. Use whatever parameters you compiled your Kernel with and
43	     $(KER_DIR) above pointing to the Kernel you compile against. See the file
44	     open-osd/top-level-Makefile for an example.
45	
46	2. Get the OSD initiator and target set up properly, and login to the target.
47	  See Documentation/scsi/osd.txt for farther instructions. Also see ./do-osd
48	  for example script that does all these steps.
49	
50	3. Insmod the exofs.ko module:
51	   [exofs]$ insmod exofs.ko
52	
53	4. Make sure the directory where you want to mount exists. If not, create it.
54	   (For example, mkdir /mnt/exofs)
55	
56	5. At first run you will need to invoke the mkfs.exofs application
57	
58	   As an example, this will create the file system on:
59	   /dev/osd0 partition ID 65536
60	
61	   mkfs.exofs --pid=65536 --format /dev/osd0
62	
63	   The --format is optional. If not specified, no OSD_FORMAT will be
64	   performed and a clean file system will be created in the specified pid,
65	   in the available space of the target. (Use --format=size_in_meg to limit
66	   the total LUN space available)
67	
68	   If pid already exists, it will be deleted and a new one will be created in
69	   its place. Be careful.
70	
71	   An exofs lives inside a single OSD partition. You can create multiple exofs
72	   filesystems on the same device using multiple pids.
73	
74	   (run mkfs.exofs without any parameters for usage help message)
75	
76	6. Mount the file system.
77	
78	   For example, to mount /dev/osd0, partition ID 0x10000 on /mnt/exofs:
79	
80		mount -t exofs -o pid=65536 /dev/osd0 /mnt/exofs/
81	
82	7. For reference (See do-exofs example script):
83		do-exofs start - an example of how to perform the above steps.
84		do-exofs stop - an example of how to unmount the file system.
85		do-exofs format - an example of how to format and mkfs a new exofs.
86	
87	8. Extra compilation flags (uncomment in fs/exofs/Kbuild):
88		CONFIG_EXOFS_DEBUG - for debug messages and extra checks.
89	
90	===============================================================================
91	exofs mount options
92	===============================================================================
93	Similar to any mount command:
94		mount -t exofs -o exofs_options /dev/osdX mount_exofs_directory
95	
96	Where:
97	    -t exofs: specifies the exofs file system
98	
99	    /dev/osdX: X is a decimal number. /dev/osdX was created after a successful
100	               login into an OSD target.
101	
102	    mount_exofs_directory: The directory to mount the file system on
103	
104	    exofs specific options: Options are separated by commas (,)
105			pid=<integer> - The partition number to mount/create as
106	                                container of the filesystem.
107	                                This option is mandatory.
108	                to=<integer>  - Timeout in ticks for a single command.
109	                                default is (60 * HZ) [for debugging only]
110	
111	===============================================================================
112	DESIGN
113	===============================================================================
114	
115	* The file system control block (AKA on-disk superblock) resides in an object
116	  with a special ID (defined in common.h).
117	  Information included in the file system control block is used to fill the
118	  in-memory superblock structure at mount time. This object is created before
119	  the file system is used by mkexofs.c. It contains information such as:
120		- The file system's magic number
121		- The next inode number to be allocated
122	
123	* Each file resides in its own object and contains the data (and it will be
124	  possible to extend the file over multiple objects, though this has not been
125	  implemented yet).
126	
127	* A directory is treated as a file, and essentially contains a list of <file
128	  name, inode #> pairs for files that are found in that directory. The object
129	  IDs correspond to the files' inode numbers and will be allocated according to
130	  a bitmap (stored in a separate object). Now they are allocated using a
131	  counter.
132	
133	* Each file's control block (AKA on-disk inode) is stored in its object's
134	  attributes. This applies to both regular files and other types (directories,
135	  device files, symlinks, etc.).
136	
137	* Credentials are generated per object (inode and superblock) when they are
138	  created in memory (read from disk or created). The credential works for all
139	  operations and is used as long as the object remains in memory.
140	
141	* Async OSD operations are used whenever possible, but the target may execute
142	  them out of order. The operations that concern us are create, delete,
143	  readpage, writepage, update_inode, and truncate. The following pairs of
144	  operations should execute in the order written, and we need to prevent them
145	  from executing in reverse order:
146		- The following are handled with the OBJ_CREATED and OBJ_2BCREATED
147		  flags. OBJ_CREATED is set when we know the object exists on the OSD -
148		  in create's callback function, and when we successfully do a
149		  read_inode.
150		  OBJ_2BCREATED is set in the beginning of the create function, so we
151		  know that we should wait.
152			- create/delete: delete should wait until the object is created
153			  on the OSD.
154			- create/readpage: readpage should be able to return a page
155			  full of zeroes in this case. If there was a write already
156			  en-route (i.e. create, writepage, readpage) then the page
157			  would be locked, and so it would really be the same as
158			  create/writepage.
159			- create/writepage: if writepage is called for a sync write, it
160			  should wait until the object is created on the OSD.
161			  Otherwise, it should just return.
162			- create/truncate: truncate should wait until the object is
163			  created on the OSD.
164			- create/update_inode: update_inode should wait until the
165			  object is created on the OSD.
166		- Handled by VFS locks:
167			- readpage/delete: shouldn't happen because of page lock.
168			- writepage/delete: shouldn't happen because of page lock.
169			- readpage/writepage: shouldn't happen because of page lock.
170	
171	===============================================================================
172	LICENSE/COPYRIGHT
173	===============================================================================
174	The exofs file system is based on ext2 v0.5b (distributed with the Linux kernel
175	version 2.6.10).  All files include the original copyrights, and the license
176	is GPL version 2 (only version 2, as is true for the Linux kernel).  The
177	Linux kernel can be downloaded from www.kernel.org.
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