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Annotation of dmsdos/doc/FAQ, revision 1.1

1.1     ! root        1: This file gives answers to frequently asked questions about dmsdos.
        !             2: 
        !             3: 
        !             4: What is dmsdos ?
        !             5: 
        !             6: Dmsdos is a Linux kernel module that allows access to msdos/win95 compressed
        !             7: disk partitions. It supports different doublespace, drivespace and stacker
        !             8: versions.
        !             9: See file dmsdos.doc for an actual list of supported dos/win95 configurations. 
        !            10: 
        !            11: 
        !            12: 
        !            13: How do I mount a compressed partition?
        !            14: 
        !            15: You need to mount the uncompressed host partition as usual. In this
        !            16: partition, you can see the compressed partiton as large file (the CVF,
        !            17: Compressed Volume File). It usually has names like 'dblspace.NNN',
        !            18: 'drvspace.NNN' or 'stacvol.XXX'. The whole compressed partition is inside
        !            19: this file.
        !            20: 
        !            21: You need the loopback block device driver (enable the loopback device during
        !            22: kernel configuration):
        !            23: 
        !            24:    *
        !            25:    * Additional Block Devices
        !            26:    *
        !            27:    Loopback device support (CONFIG_BLK_DEV_LOOP) [M/n/y/?] Y or M
        !            28:  
        !            29: The loopback block device allows mounting a file as a filesystem (read some
        !            30: Linux docs about this for more information). To avoid confusion, note
        !            31: that this has absolutely nothing in common with the well-known network 
        !            32: loopback interface.
        !            33: 
        !            34: Example: 'mount -t msdos -o loop /DOS/dblspace.001 /mnt'
        !            35: 
        !            36: This mounts your compressed filesystem that in fact resides in the file 
        !            37: /DOS/dblspace.001 under /mnt.
        !            38: 
        !            39: 
        !            40: 
        !            41: It doesn't work!
        !            42: 
        !            43: Well, nothing simply doesn't work. It fails at a specific point and gives
        !            44: some errors or warnings before failure which are usually an important hint
        !            45: for finding out what exactly went wrong. Please have a look at file
        !            46: troubleshooting.doc. If you still cannot solve the problem you can send an
        !            47: email to the current dmsdos maintainer. The email address and a checklist
        !            48: what to include in a bug report can be found in file dmsdos.doc.
        !            49: 
        !            50: 
        !            51: 
        !            52: I have compressed partitions, but I'm using umsdos. Can I use dmsdos without 
        !            53: losing umsdos features?
        !            54: 
        !            55: No problem. It should work out of the box. If you want to use umsdos style
        !            56: long filenames inside the CVF, you can even mount it as type umsdos instead 
        !            57: of msdos. (Uhh. Use kernel 2.0.xx or get at least 2.1.94 for that).
        !            58: 
        !            59: 
        !            60: 
        !            61: My compressed partition has long filenames under Win95. How can I see 
        !            62: them with dmsdos ?
        !            63: 
        !            64: Mount as type vfat instead of msdos.
        !            65: 
        !            66: 
        !            67: 
        !            68: Can I compress my Linux data under dmsdos?
        !            69: 
        !            70: You can (via umsdos), but it's not recommended.
        !            71: 
        !            72: Just to let you know: There's some other compression software available that
        !            73: may be a better choice for Linux data. See file dmsdos.doc.
        !            74: 
        !            75: 
        !            76: 
        !            77: Can I boot Linux from a compressed partition?
        !            78: 
        !            79: No. (Older dmsdos versions supported it, but this feature has been given up
        !            80: when the CVF-FAT interface was introduced. Well, the documentation always
        !            81: warned ... )
        !            82: 
        !            83: 
        !            84: 
        !            85: Uhhh. Write access is so ssssssslow.....
        !            86: 
        !            87: What can I say. You decided to use compressed partitions - now you are
        !            88: experiencing one of their greatest disadvantages. But dmsdos has some
        !            89: special tricks for you in this case. You can... (in recommended order)
        !            90: 
        !            91:     - run the dmsdos daemon which does delayed compression in the background
        !            92:       when the system is idle, or use the internal daemon 
        !            93: 
        !            94:     - set speedup bit #2 (allocation code skips exact search)
        !            95: 
        !            96:     - set speedup bit #6 (allocation code doesn't search at all but takes
        !            97:       any hole free hole) (Really Not Recommended if you don't know what
        !            98:       you are doing). Read the comments in the documentation, please.
        !            99: 
        !           100: See the dmsdos documentation for details and a discussion about their
        !           101: advantages and disadvantages.
        !           102: 
        !           103: 
        !           104: 
        !           105: Is it safe to use dmsdos?
        !           106: 
        !           107: This question is something like that one: 'is it safe to drive a car?'.
        !           108: 
        !           109: There's no warrenty. I really can't promise that there aren't any bugs in
        !           110: the driver code. I think you just like to feel safe:
        !           111: 
        !           112:  - If you don't have to write to your compressed filesystems, mount
        !           113:    read-only.
        !           114: 
        !           115:  - Run the dos filesystem checker every time you boot dos and backup from
        !           116:    time to time.
        !           117: 
        !           118:  - Backup your compressed filesystem before trying to mount read-write.
        !           119: 
        !           120: 
        !           121: 
        !           122: How do I enable/disable long filename support?
        !           123: 
        !           124: For long filename support use a filesystem type that knows long filenames,
        !           125: e.g. vfat instead of msdos.
        !           126: 
        !           127: 
        !           128: 
        !           129: What happens if the compressed filesystem gets full during write access?
        !           130: 
        !           131: Don't ask. That has become too complex. :(
        !           132: 
        !           133: Well, in the worst case the same thing happens as under Dos: you get a
        !           134: strange error (Dos: "sector not found") and might lose data. It is difficult
        !           135: because some applications do not check the return values of write access
        !           136: functions (AAARGHHH!!!). On a compressed filesystem not only cluster
        !           137: allocation, but also usual write access may fail (i.e. when you replace
        !           138: data that compress well by data that don't compress well and thus need more
        !           139: physical space). So just keep in mind that you shouldn't fill a compressed
        !           140: partition up to the last byte :)
        !           141: 
        !           142: 
        !           143: 
        !           144: What's the difference between win95 doublespace and drivespace 3 ?
        !           145: 
        !           146: Please note that win95 has two different drivespace programs. One of them
        !           147: is included in win95 and it's in fact an old dos doublespace version. I
        !           148: still refer to it as 'win95 doublespace' though M$ call it 'drivespace'.
        !           149: This one is supported without restrictions.
        !           150: 
        !           151: The other one is sold seperately from win95 in a 'M$ Plus' package (is this
        !           152: still true?) M$ have called it Drivespace 3. Meanwhile it should also
        !           153: be supported quite well.
        !           154: 
        !           155: If you aren't sure, watch the filename of the large, hidden CVF. If it is
        !           156: something like 'dblspace.xxx' it's the old version. If it's 'drvspace.xxx'
        !           157: instead and you are very sure you created it under win95, it's Drivespace 3.
        !           158: Dmsdos also tells when it detects a drivespace 3 format CVF. 
        !           159: 
        !           160: The main difference between these two versions is that drivespace 3 uses a
        !           161: larger cluster size (32KB instead of 8KB) and can compress up to 2GB of data 
        !           162: instead of only 512KB. It has also a more advanced compression scheme 
        !           163: (SQ-0-0).
        !           164: 
        !           165: 
        !           166: 
        !           167: When do I have to defragment my compressed partitions?
        !           168: 
        !           169: Watch the dutil output (see file dmsdos.doc for a dutil description). It
        !           170: displays a fragmentation value. You should keep it below 75% to feel safe. 
        !           171: When dutil even displays a warning about high fragmentation, you should
        !           172: immediately boot Dos and defragment the CVF.
        !           173: 
        !           174: 
        !           175: 
        !           176: Does dmsdos work with FAT32 filesystems?
        !           177: 
        !           178: Yes. All earlier problems have been avoided by moving dmsdos one layer down
        !           179: in the filesystem interface. Well, it does not support *compressed* FAT32
        !           180: filesystems. Win95 doesn't do either :) But, if some day they show up to 
        !           181: exist, I might be persuaded to implement it also.
        !           182: 
        !           183: 
        !           184: 
        !           185: What about the relationship between dmsdos and software patents ?
        !           186: 
        !           187: I'm still receiving mails about this question. At my best knowledge dmsdos 
        !           188: does not violate any software patents. (Well, I don't hope so, but my best 
        !           189: knowledge might be wrong. I'm not a lawyer.) If you are interested in
        !           190: details take a look at file patent.doc.
        !           191: 
        !           192: 
        !           193: 
        !           194: Do I have to recompile dmsdos when I upgrade my kernel ?
        !           195: 
        !           196: The safe answer is yes. It's best to recompile *all* modules if you change
        !           197: something in your kernel though not always necessary. But who knows.
        !           198: 
        !           199: Some clever people invented a 'module version information' trick. You can
        !           200: enable it during kernel configuration. This makes your modules less kernel
        !           201: version dependent e.g. a module compiled under kernel 2.0.33 may also work 
        !           202: under kernel 2.0.34. In that case, you may share one precompiled dmsdos 
        !           203: module with several kernels. But please don't overuse it. You should not,
        !           204: for example, use the same compiled object code of a dmsdos module for
        !           205: kernel 2.0.34 and 2.1.105 - this is very likely to cause problems :)

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