Based on kernel version 4.2. Page generated on 2015-09-09 12:13 EST.
1 =============================================================================== 2 WHAT IS EXOFS? 3 =============================================================================== 4 5 exofs is a file system that uses an OSD and exports the API of a normal Linux 6 file system. Users access exofs like any other local file system, and exofs 7 will in turn issue commands to the local OSD initiator. 8 9 OSD is a new T10 command set that views storage devices not as a large/flat 10 array of sectors but as a container of objects, each having a length, quota, 11 time attributes and more. Each object is addressed by a 64bit ID, and is 12 contained in a 64bit ID partition. Each object has associated attributes 13 attached to it, which are integral part of the object and provide metadata about 14 the object. The standard defines some common obligatory attributes, but user 15 attributes can be added as needed. 16 17 =============================================================================== 18 ENVIRONMENT 19 =============================================================================== 20 21 To use this file system, you need to have an object store to run it on. You 22 may download a target from: 23 http://open-osd.org 24 25 See Documentation/scsi/osd.txt for how to setup a working osd environment. 26 27 =============================================================================== 28 USAGE 29 =============================================================================== 30 31 1. 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). 34 35 a. download open-osd including exofs source using: 36 [parent-directory]$ git clone git://git.open-osd.org/open-osd.git 37 38 b. Build the library module like this: 39 [parent-directory]$ make -C KSRC=$(KER_DIR) open-osd 40 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 $(KER_DIR) above pointing to the Kernel you compile against. See the file 44 open-osd/top-level-Makefile for an example. 45 46 2. Get the OSD initiator and target set up properly, and login to the target. 47 See Documentation/scsi/osd.txt for farther instructions. Also see ./do-osd 48 for example script that does all these steps. 49 50 3. Insmod the exofs.ko module: 51 [exofs]$ insmod exofs.ko 52 53 4. Make sure the directory where you want to mount exists. If not, create it. 54 (For example, mkdir /mnt/exofs) 55 56 5. At first run you will need to invoke the mkfs.exofs application 57 58 As an example, this will create the file system on: 59 /dev/osd0 partition ID 65536 60 61 mkfs.exofs --pid=65536 --format /dev/osd0 62 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) 67 68 If pid already exists, it will be deleted and a new one will be created in 69 its place. Be careful. 70 71 An exofs lives inside a single OSD partition. You can create multiple exofs 72 filesystems on the same device using multiple pids. 73 74 (run mkfs.exofs without any parameters for usage help message) 75 76 6. Mount the file system. 77 78 For example, to mount /dev/osd0, partition ID 0x10000 on /mnt/exofs: 79 80 mount -t exofs -o pid=65536 /dev/osd0 /mnt/exofs/ 81 82 7. 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. 86 87 8. Extra compilation flags (uncomment in fs/exofs/Kbuild): 88 CONFIG_EXOFS_DEBUG - for debug messages and extra checks. 89 90 =============================================================================== 91 exofs mount options 92 =============================================================================== 93 Similar to any mount command: 94 mount -t exofs -o exofs_options /dev/osdX mount_exofs_directory 95 96 Where: 97 -t exofs: specifies the exofs file system 98 99 /dev/osdX: X is a decimal number. /dev/osdX was created after a successful 100 login into an OSD target. 101 102 mount_exofs_directory: The directory to mount the file system on 103 104 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] 118 119 =============================================================================== 120 DESIGN 121 =============================================================================== 122 123 * 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 130 131 * 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). 134 135 * 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. 140 141 * 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.). 144 145 * 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. 148 149 * 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. 178 179 =============================================================================== 180 LICENSE/COPYRIGHT 181 =============================================================================== 182 The exofs file system is based on ext2 v0.5b (distributed with the Linux kernel 183 version 2.6.10). All files include the original copyrights, and the license 184 is GPL version 2 (only version 2, as is true for the Linux kernel). The 185 Linux kernel can be downloaded from www.kernel.org.