Return to fat_cvf.txt CVS log

Up to [GPL Stacker compression] / dmsdos / patches

Annotation of dmsdos/patches/fat_cvf.txt, revision 1.1

1.1     ! root        1: This is the main documentation for the CVF-FAT filesystem extension.  18Nov1998
        !             2: 
        !             3: 
        !             4: Table of Contents:
        !             5: 
        !             6: 1. The idea of CVF-FAT
        !             7: 2. Restrictions
        !             8: 3. Mount options
        !             9: 4. Description of the CVF-FAT interface
        !            10: 5. CVF Modules
        !            11: 
        !            12: ------------------------------------------------------------------------------
        !            13: 
        !            14: 
        !            15: 1. The idea of CVF-FAT
        !            16: ------------------------------------------------------------------------------
        !            17: 
        !            18: CVF-FAT is a FAT filesystem extension that provides a generic interface for
        !            19: Compressed Volume Files in FAT partitions. Popular CVF software, for
        !            20: example, are Microsoft's Doublespace/Drivespace and Stac's Stacker.
        !            21: Using the CVF-FAT interface, it is possible to load a module that handles
        !            22: all the low-level disk access that has to do with on-the-fly compression
        !            23: and decompression. Any other part of FAT filesystem access is still handled
        !            24: by the FAT, MSDOS or VFAT or even UMSDOS driver.
        !            25: 
        !            26: CVF access works by redirecting certain low-level routines from the FAT
        !            27: driver to a loadable, CVF-format specific module. This module must fake
        !            28: a normal FAT filesystem to the FAT driver while doing all the extra stuff
        !            29: like compression and decompression silently.
        !            30: 
        !            31: 
        !            32: 2. Restrictions
        !            33: ------------------------------------------------------------------------------
        !            34: 
        !            35: - BMAP problems
        !            36: 
        !            37:   CVF filesystems cannot do bmap. It's impossible in principle. Thus
        !            38:   all actions that require bmap do not work (swapping, writable mmapping).
        !            39:   Read-only mmapping works because the FAT driver has a hack for this
        !            40:   situation :) Well, writable mmapping should now work using the readpage
        !            41:   interface function which has been hacked into the FAT driver just for 
        !            42:   CVF-FAT :)
        !            43:   
        !            44: - attention, DOSEmu users 
        !            45: 
        !            46:   You may have to unmount all CVF partitions before running DOSEmu depending 
        !            47:   on your configuration. If DOSEmu is configured to use wholedisk or 
        !            48:   partition access (this is often the case to let DOSEmu access 
        !            49:   compressed partitions) there's a risk of destroying your compressed 
        !            50:   partitions or crashing your system because of confused drivers.
        !            51:   
        !            52:   Note that it is always safe to redirect the compressed partitions with 
        !            53:   lredir or emufs.sys. Refer to the DOSEmu documentation for details.
        !            54: 
        !            55: 
        !            56: 3. Mount options
        !            57: ------------------------------------------------------------------------------
        !            58: 
        !            59: The CVF-FAT extension currently adds the following options to the FAT
        !            60: driver's standard options:
        !            61: 
        !            62:   cvf_format=xxx
        !            63:     Forces the driver to use the CVF module "xxx" instead of auto-detection.
        !            64:     Without this option, the CVF-FAT interface asks all currently loaded
        !            65:     CVF modules whether they recognize the CVF. Therefore, this option is
        !            66:     only necessary if the CVF format is not recognized correctly
        !            67:     because of bugs or incompatibilities in the CVF modules. (It skips
        !            68:     the detect_cvf call.) "xxx" may be the text "none" (without the quotes)
        !            69:     to inhibit using any of the loaded CVF modules, just in case a CVF
        !            70:     module insists on mounting plain FAT filesystems by misunderstanding.
        !            71:     "xxx" may also be the text "autoload", which has a special meaning for
        !            72:     kerneld, but does not skip auto-detection.
        !            73: 
        !            74:     If the kernel supports kerneld, the cvf_format=xxx option also controls
        !            75:     on-demand CVF module loading. Without this option, nothing is loaded
        !            76:     on demand. With cvf_format=xxx, a module "xxx" is requested automatically
        !            77:     before mounting the compressed filesystem (unless "xxx" is "none"). In 
        !            78:     case there is a difference between the CVF format name and the module 
        !            79:     name, setup aliases in your kerneld configuration. If the string "xxx" 
        !            80:     is "autoload", a non-existent module "cvf_autoload" is requested which 
        !            81:     can be used together with a special kerneld configuration (alias and 
        !            82:     pre-install statements) in order to load more than one CVF module, let 
        !            83:     them detect automatically which kind of CVF is to be mounted, and only 
        !            84:     keep the "right" module in memory. For examples please refer to the 
        !            85:     dmsdos documentation (ftp and http addresses see below).
        !            86: 
        !            87:   cvf_options=yyy
        !            88:     Option string passed to the CVF module. I.e. only the "yyy" is passed
        !            89:     (without the quotes). The documentation for each CVF module should 
        !            90:     explain it since it is interpreted only by the CVF module. Note that 
        !            91:     the string must not contain a comma (",") - this would lead to 
        !            92:     misinterpretation by the FAT driver, which would recognize the text 
        !            93:     after a comma as a FAT driver option and might get confused or print 
        !            94:     strange error messages. The documentation for the CVF module should 
        !            95:     offer a different separation symbol, for example the dot "." or the
        !            96:     plus sign "+", which is only valid inside the string "yyy".
        !            97: 
        !            98: 
        !            99: 4. Description of the CVF-FAT interface
        !           100: ------------------------------------------------------------------------------
        !           101: 
        !           102: Assuming you want to write your own CVF module, you need to write a lot of
        !           103: interface functions. Most of them are covered in the kernel documentation
        !           104: you can find on the net, and thus won't be described here. They have been
        !           105: marked with "[...]" :-) Take a look at include/linux/fat_cvf.h.
        !           106: 
        !           107: struct cvf_format
        !           108: { int cvf_version;
        !           109:   char* cvf_version_text;
        !           110:   unsigned long int flags;
        !           111:   int (*detect_cvf) (struct super_block*sb);
        !           112:   int (*mount_cvf) (struct super_block*sb,char*options);
        !           113:   int (*unmount_cvf) (struct super_block*sb);
        !           114:   [...]
        !           115:   void (*cvf_zero_cluster) (struct inode*inode,int clusternr);
        !           116: }
        !           117: 
        !           118: This structure defines the capabilities of a CVF module. It must be filled
        !           119: out completely by a CVF module. Consider it as a kind of form that is used
        !           120: to introduce the module to the FAT/CVF-FAT driver.
        !           121: 
        !           122: It contains...
        !           123:   - cvf_version:
        !           124:       A version id which must be unique. Choose one.
        !           125:   - cvf_version_text:
        !           126:       A human readable version string that should be one short word 
        !           127:       describing the CVF format the module implements. This text is used
        !           128:       for the cvf_format option. This name must also be unique.
        !           129:   - flags:
        !           130:       Bit coded flags, currently only used for a readpage/mmap hack that 
        !           131:       provides both mmap and readpage functionality. If CVF_USE_READPAGE
        !           132:       is set, mmap is set to generic_file_mmap and readpage is caught
        !           133:       and redirected to the cvf_readpage function. If it is not set,
        !           134:       readpage is set to generic_readpage and mmap is caught and redirected
        !           135:       to cvf_mmap. (If you want writable mmap use the readpage interface.)
        !           136:   - detect_cvf:
        !           137:       A function that is called to decide whether the filesystem is a CVF of
        !           138:       the type the module supports. The detect_cvf function must return 0
        !           139:       for "NO, I DON'T KNOW THIS GARBAGE" or anything >0 for "YES, THIS IS
        !           140:       THE KIND OF CVF I SUPPORT". The function must maintain the module
        !           141:       usage counters for safety, i.e. do MOD_INC_USE_COUNT at the beginning
        !           142:       and MOD_DEC_USE_COUNT at the end. The function *must not* assume that
        !           143:       successful recongition would lead to a call of the mount_cvf function
        !           144:       later. 
        !           145:   - mount_cvf:
        !           146:       A function that sets up some values or initializes something additional
        !           147:       to what has to be done when a CVF is mounted. This is called at the
        !           148:       end of fat_read_super and must return 0 on success. Definitely, this
        !           149:       function must increment the module usage counter by MOD_INC_USE_COUNT.
        !           150:       This mount_cvf function is also responsible for interpreting a CVF
        !           151:       module specific option string (the "yyy" from the FAT mount option
        !           152:       "cvf_options=yyy") which cannot contain a comma (use for example the
        !           153:       dot "." as option separator symbol).
        !           154:   - unmount_cvf:
        !           155:       A function that is called when the filesystem is unmounted. Most likely
        !           156:       it only frees up some memory and calls MOD_DEC_USE_COUNT. The return
        !           157:       value might be ignored (it currently is ignored).
        !           158:   - [...]:
        !           159:       All other interface functions are "caught" FAT driver functions, i.e.
        !           160:       are executed by the FAT driver *instead* of the original FAT driver
        !           161:       functions. NULL means use the original FAT driver functions instead.
        !           162:       If you really want "no action", write a function that does nothing and 
        !           163:       hang it in instead.
        !           164:   - cvf_zero_cluster:
        !           165:       The cvf_zero_cluster function is called when the fat driver wants to
        !           166:       zero out a (new) cluster. This is important for directories (mkdir).
        !           167:       If it is NULL, the FAT driver defaults to overwriting the whole
        !           168:       cluster with zeros. Note that clusternr is absolute, not relative
        !           169:       to the provided inode.
        !           170: 
        !           171: Notes:
        !           172:   1. The cvf_bmap function should be ignored. It really should never
        !           173:      get called from somewhere. I recommend redirecting it to a panic
        !           174:      or fatal error message so bugs show up immediately.
        !           175:   2. The cvf_writepage function is ignored. This is because the fat
        !           176:      driver doesn't support it. This might change in future. I recommend
        !           177:      setting it to NULL (i.e use default).
        !           178: 
        !           179: int register_cvf_format(struct cvf_format*cvf_format);
        !           180:   If you have just set up a variable containing the above structure,
        !           181:   call this function to introduce your CVF format to the FAT/CVF-FAT
        !           182:   driver. This is usually done in init_module. Be sure to check the
        !           183:   return value. Zero means success, everything else causes a kernel
        !           184:   message printed in the syslog describing the error that occurred.
        !           185:   Typical errors are:
        !           186:     - a module with the same version id is already registered or 
        !           187:     - too many CVF formats. Hack fs/fat/cvf.c if you need more.
        !           188: 
        !           189: int unregister_cvf_format(struct cvf_format*cvf_format);
        !           190:   This is usually called in cleanup_module. Return value =0 means
        !           191:   success. An error only occurs if you try to unregister a CVF format
        !           192:   that has not been previously registered. The code uses the version id
        !           193:   to distinguish the modules, so be sure to keep it unique.
        !           194: 
        !           195: 5. CVF Modules
        !           196: ------------------------------------------------------------------------------
        !           197: 
        !           198: Refer to the dmsdos module (the successor of the dmsdos filesystem) for a
        !           199: sample implementation.  It can currently be found at
        !           200: 
        !           201:   ftp://fb9nt.uni-duisburg.de/pub/linux/dmsdos/dmsdos-x.y.z.tgz
        !           202:   ftp://sunsite.unc.edu/pub/Linux/system/Filesystems/dosfs/dmsdos-x.y.z.tgz
        !           203:   ftp://ftp.uni-stuttgart.de/pub/systems/linux/local/system/dmsdos-x.y.z.tgz
        !           204: 
        !           205: (where x.y.z is to be replaced with the actual version number). Full
        !           206: documentation about dmsdos is included in the dmsdos package, but can also
        !           207: be found at
        !           208: 
        !           209:   http://fb9nt.uni-duisburg.de/mitarbeiter/gockel/software/dmsdos/index.html
        !           210:   http://www.yk.rim.or.jp/~takafumi/dmsdos/index.html (in Japanese).

This archive runs on limited infrastructure. Preserving old code on modern bandwidth. Automated agents are requested to crawl responsibly.