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Subject[PATCH 7/8] exofs: Documentation
Added some documentation in exofs.txt, as well as a BUGS file.

For further reading, operation instructions, example scripts
and up to date infomation and code please see:

Signed-off-by: Boaz Harrosh <>
Documentation/filesystems/exofs.txt | 176 +++++++++++++++++++++++++++++++++++
fs/exofs/BUGS | 3 +
2 files changed, 179 insertions(+), 0 deletions(-)
create mode 100644 Documentation/filesystems/exofs.txt
create mode 100644 fs/exofs/BUGS

diff --git a/Documentation/filesystems/exofs.txt b/Documentation/filesystems/exofs.txt
new file mode 100644
index 0000000..0ced74c
--- /dev/null
+++ b/Documentation/filesystems/exofs.txt
@@ -0,0 +1,176 @@
+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://
+ 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
+ preformed 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 exist it will be deleted and a new one will be created in it's
+ 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
+ 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
+ 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 is
+ created in memory (read off 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.
+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
diff --git a/fs/exofs/BUGS b/fs/exofs/BUGS
new file mode 100644
index 0000000..1b2d4c6
--- /dev/null
+++ b/fs/exofs/BUGS
@@ -0,0 +1,3 @@
+- Out-of-space may cause a severe problem if the object (and directory entry)
+ were written, but the inode attributes failed. Then if the filesystem was
+ unmounted and mounted the kernel can get into an endless loop doing a readdir.

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