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