5exofs is a file system that uses an OSD and exports the API of a normal Linux
   6file system. Users access exofs like any other local file system, and exofs
   7will in turn issue commands to the local OSD initiator.
   9OSD is a new T10 command set that views storage devices not as a large/flat
  10array of sectors but as a container of objects, each having a length, quota,
  11time attributes and more. Each object is addressed by a 64bit ID, and is
  12contained in a 64bit ID partition. Each object has associated attributes
  13attached to it, which are integral part of the object and provide metadata about
  14the object. The standard defines some common obligatory attributes, but user
  15attributes can be added as needed.
  21To use this file system, you need to have an object store to run it on.  You
  22may download a target from:
  25See Documentation/scsi/osd.txt for how to setup a working osd environment.
  311. 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).
  35  a. download open-osd including exofs source using:
  36     [parent-directory]$ git clone git://
  38  b. Build the library module like this:
  39     [parent-directory]$ make -C KSRC=$(KER_DIR) open-osd
  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     $(KER_DIR) above pointing to the Kernel you compile against. See the file
  44     open-osd/top-level-Makefile for an example.
  462. Get the OSD initiator and target set up properly, and login to the target.
  47  See Documentation/scsi/osd.txt for farther instructions. Also see ./do-osd
  48  for example script that does all these steps.
  503. Insmod the exofs.ko module:
  51   [exofs]$ insmod exofs.ko
  534. Make sure the directory where you want to mount exists. If not, create it.
  54   (For example, mkdir /mnt/exofs)
  565. At first run you will need to invoke the mkfs.exofs application
  58   As an example, this will create the file system on:
  59   /dev/osd0 partition ID 65536
  61   mkfs.exofs --pid=65536 --format /dev/osd0
  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)
  68   If pid already exists, it will be deleted and a new one will be created in
  69   its place. Be careful.
  71   An exofs lives inside a single OSD partition. You can create multiple exofs
  72   filesystems on the same device using multiple pids.
  74   (run mkfs.exofs without any parameters for usage help message)
  766. Mount the file system.
  78   For example, to mount /dev/osd0, partition ID 0x10000 on /mnt/exofs:
  80        mount -t exofs -o pid=65536 /dev/osd0 /mnt/exofs/
  827. 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.
  878. Extra compilation flags (uncomment in fs/exofs/Kbuild):
  88        CONFIG_EXOFS_DEBUG - for debug messages and extra checks.
  91exofs mount options
  93Similar to any mount command:
  94        mount -t exofs -o exofs_options /dev/osdX mount_exofs_directory
  97    -t exofs: specifies the exofs file system
  99    /dev/osdX: X is a decimal number. /dev/osdX was created after a successful
 100               login into an OSD target.
 102    mount_exofs_directory: The directory to mount the file system on
 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. integer can be
 108                                Hex by pre-pending an 0x to the number.
 109                osdname=<id>  - Mount by a device's osdname.
 110                                osdname is usually a 36 character uuid of the
 111                                form "d2683732-c906-4ee1-9dbd-c10c27bb40df".
 112                                It is one of the device's uuid specified in the
 113                                mkfs.exofs format command.
 114                                If this option is specified then the /dev/osdX
 115                                above can be empty and is ignored.
 116                to=<integer>  - Timeout in ticks for a single command.
 117                                default is (60 * HZ) [for debugging only]
 123* The file system control block (AKA on-disk superblock) resides in an object
 124  with a special ID (defined in common.h).
 125  Information included in the file system control block is used to fill the
 126  in-memory superblock structure at mount time. This object is created before
 127  the file system is used by mkexofs.c. It contains information such as:
 128        - The file system's magic number
 129        - The next inode number to be allocated
 131* Each file resides in its own object and contains the data (and it will be
 132  possible to extend the file over multiple objects, though this has not been
 133  implemented yet).
 135* A directory is treated as a file, and essentially contains a list of <file
 136  name, inode #> pairs for files that are found in that directory. The object
 137  IDs correspond to the files' inode numbers and will be allocated according to
 138  a bitmap (stored in a separate object). Now they are allocated using a
 139  counter.
 141* Each file's control block (AKA on-disk inode) is stored in its object's
 142  attributes. This applies to both regular files and other types (directories,
 143  device files, symlinks, etc.).
 145* Credentials are generated per object (inode and superblock) when they are
 146  created in memory (read from disk or created). The credential works for all
 147  operations and is used as long as the object remains in memory.
 149* Async OSD operations are used whenever possible, but the target may execute
 150  them out of order. The operations that concern us are create, delete,
 151  readpage, writepage, update_inode, and truncate. The following pairs of
 152  operations should execute in the order written, and we need to prevent them
 153  from executing in reverse order:
 154        - The following are handled with the OBJ_CREATED and OBJ_2BCREATED
 155          flags. OBJ_CREATED is set when we know the object exists on the OSD -
 156          in create's callback function, and when we successfully do a
 157          read_inode.
 158          OBJ_2BCREATED is set in the beginning of the create function, so we
 159          know that we should wait.
 160                - create/delete: delete should wait until the object is created
 161                  on the OSD.
 162                - create/readpage: readpage should be able to return a page
 163                  full of zeroes in this case. If there was a write already
 164                  en-route (i.e. create, writepage, readpage) then the page
 165                  would be locked, and so it would really be the same as
 166                  create/writepage.
 167                - create/writepage: if writepage is called for a sync write, it
 168                  should wait until the object is created on the OSD.
 169                  Otherwise, it should just return.
 170                - create/truncate: truncate should wait until the object is
 171                  created on the OSD.
 172                - create/update_inode: update_inode should wait until the
 173                  object is created on the OSD.
 174        - Handled by VFS locks:
 175                - readpage/delete: shouldn't happen because of page lock.
 176                - writepage/delete: shouldn't happen because of page lock.
 177                - readpage/writepage: shouldn't happen because of page lock.
 182The exofs file system is based on ext2 v0.5b (distributed with the Linux kernel
 183version 2.6.10).  All files include the original copyrights, and the license
 184is GPL version 2 (only version 2, as is true for the Linux kernel).  The
 185Linux kernel can be downloaded from
 186 kindly hosted by Redpill Linpro AS, provider of Linux consulting and operations services since 1995.