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-===============================================================================
-WHAT IS EXOFS?
-===============================================================================
-
-exofs is a file system that uses an OSD and exports the API of a normal Linux
-file system. Users access exofs like any other local file system, and exofs
-will in turn issue commands to the local OSD initiator.
-
-OSD is a new T10 command set that views storage devices not as a large/flat
-array of sectors but as a container of objects, each having a length, quota,
-time attributes and more. Each object is addressed by a 64bit ID, and is
-contained in a 64bit ID partition. Each object has associated attributes
-attached to it, which are integral part of the object and provide metadata about
-the object. The standard defines some common obligatory attributes, but user
-attributes can be added as needed.
-
-===============================================================================
-ENVIRONMENT
-===============================================================================
-
-To use this file system, you need to have an object store to run it on. You
-may download a target from:
-http://open-osd.org
-
-See Documentation/scsi/osd.txt for how to setup a working osd environment.
-
-===============================================================================
-USAGE
-===============================================================================
-
-1. Download and compile exofs and open-osd initiator:
- You need an external Kernel source tree or kernel headers from your
- distribution. (anything based on 2.6.26 or later).
-
- a. download open-osd including exofs source using:
- [parent-directory]$ git clone git://git.open-osd.org/open-osd.git
-
- b. Build the library module like this:
- [parent-directory]$ make -C KSRC=$(KER_DIR) open-osd
-
- This will build both the open-osd initiator as well as the exofs kernel
- module. Use whatever parameters you compiled your Kernel with and
- $(KER_DIR) above pointing to the Kernel you compile against. See the file
- open-osd/top-level-Makefile for an example.
-
-2. Get the OSD initiator and target set up properly, and login to the target.
- See Documentation/scsi/osd.txt for farther instructions. Also see ./do-osd
- for example script that does all these steps.
-
-3. Insmod the exofs.ko module:
- [exofs]$ insmod exofs.ko
-
-4. Make sure the directory where you want to mount exists. If not, create it.
- (For example, mkdir /mnt/exofs)
-
-5. At first run you will need to invoke the mkfs.exofs application
-
- As an example, this will create the file system on:
- /dev/osd0 partition ID 65536
-
- mkfs.exofs --pid=65536 --format /dev/osd0
-
- The --format is optional. If not specified, no OSD_FORMAT will be
- performed and a clean file system will be created in the specified pid,
- in the available space of the target. (Use --format=size_in_meg to limit
- the total LUN space available)
-
- If pid already exists, it will be deleted and a new one will be created in
- its place. Be careful.
-
- An exofs lives inside a single OSD partition. You can create multiple exofs
- filesystems on the same device using multiple pids.
-
- (run mkfs.exofs without any parameters for usage help message)
-
-6. Mount the file system.
-
- For example, to mount /dev/osd0, partition ID 0x10000 on /mnt/exofs:
-
- mount -t exofs -o pid=65536 /dev/osd0 /mnt/exofs/
-
-7. For reference (See do-exofs example script):
- do-exofs start - an example of how to perform the above steps.
- do-exofs stop - an example of how to unmount the file system.
- do-exofs format - an example of how to format and mkfs a new exofs.
-
-8. Extra compilation flags (uncomment in fs/exofs/Kbuild):
- CONFIG_EXOFS_DEBUG - for debug messages and extra checks.
-
-===============================================================================
-exofs mount options
-===============================================================================
-Similar to any mount command:
- mount -t exofs -o exofs_options /dev/osdX mount_exofs_directory
-
-Where:
- -t exofs: specifies the exofs file system
-
- /dev/osdX: X is a decimal number. /dev/osdX was created after a successful
- login into an OSD target.
-
- mount_exofs_directory: The directory to mount the file system on
-
- exofs specific options: Options are separated by commas (,)
- pid=<integer> - The partition number to mount/create as
- container of the filesystem.
- This option is mandatory. integer can be
- Hex by pre-pending an 0x to the number.
- osdname=<id> - Mount by a device's osdname.
- osdname is usually a 36 character uuid of the
- form "d2683732-c906-4ee1-9dbd-c10c27bb40df".
- It is one of the device's uuid specified in the
- mkfs.exofs format command.
- If this option is specified then the /dev/osdX
- above can be empty and is ignored.
- to=<integer> - Timeout in ticks for a single command.
- default is (60 * HZ) [for debugging only]
-
-===============================================================================
-DESIGN
-===============================================================================
-
-* The file system control block (AKA on-disk superblock) resides in an object
- with a special ID (defined in common.h).
- Information included in the file system control block is used to fill the
- in-memory superblock structure at mount time. This object is created before
- the file system is used by mkexofs.c. It contains information such as:
- - The file system's magic number
- - The next inode number to be allocated
-
-* Each file resides in its own object and contains the data (and it will be
- possible to extend the file over multiple objects, though this has not been
- implemented yet).
-
-* A directory is treated as a file, and essentially contains a list of <file
- name, inode #> pairs for files that are found in that directory. The object
- IDs correspond to the files' inode numbers and will be allocated according to
- a bitmap (stored in a separate object). Now they are allocated using a
- counter.
-
-* Each file's control block (AKA on-disk inode) is stored in its object's
- attributes. This applies to both regular files and other types (directories,
- device files, symlinks, etc.).
-
-* Credentials are generated per object (inode and superblock) when they are
- created in memory (read from disk or created). The credential works for all
- operations and is used as long as the object remains in memory.
-
-* Async OSD operations are used whenever possible, but the target may execute
- them out of order. The operations that concern us are create, delete,
- readpage, writepage, update_inode, and truncate. The following pairs of
- operations should execute in the order written, and we need to prevent them
- from executing in reverse order:
- - The following are handled with the OBJ_CREATED and OBJ_2BCREATED
- flags. OBJ_CREATED is set when we know the object exists on the OSD -
- in create's callback function, and when we successfully do a
- read_inode.
- OBJ_2BCREATED is set in the beginning of the create function, so we
- know that we should wait.
- - create/delete: delete should wait until the object is created
- on the OSD.
- - create/readpage: readpage should be able to return a page
- full of zeroes in this case. If there was a write already
- en-route (i.e. create, writepage, readpage) then the page
- would be locked, and so it would really be the same as
- create/writepage.
- - create/writepage: if writepage is called for a sync write, it
- should wait until the object is created on the OSD.
- Otherwise, it should just return.
- - create/truncate: truncate should wait until the object is
- created on the OSD.
- - create/update_inode: update_inode should wait until the
- object is created on the OSD.
- - Handled by VFS locks:
- - readpage/delete: shouldn't happen because of page lock.
- - writepage/delete: shouldn't happen because of page lock.
- - readpage/writepage: shouldn't happen because of page lock.
-
-===============================================================================
-LICENSE/COPYRIGHT
-===============================================================================
-The exofs file system is based on ext2 v0.5b (distributed with the Linux kernel
-version 2.6.10). All files include the original copyrights, and the license
-is GPL version 2 (only version 2, as is true for the Linux kernel). The
-Linux kernel can be downloaded from www.kernel.org.