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.
To use this file system, you need to have an object store to run it on. You
may download a target from:
See Documentation/scsi/osd.txt for how to setup a working osd environment.
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
-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
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]
* 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
* 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
* 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
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: 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.
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.