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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 :)
1.1.1.2 ! root 206:
! 207:
! 208:
! 209: Where does the name "DMSDOS" come from ?
! 210:
! 211: It has historical reasons. The first piece of dmsdos code was written in
! 212: times of MSDos 6.2 when Doublespace became very popular. Thus, dmsdos was
! 213: meant as something like "doublespaced msdos". Today, it supports a lot of
! 214: more compressed Dos filesystems, and a better name would be "cmsdos" for
! 215: "compressed msdos". But you never change the name of a successful project :)
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