path: root/doc/FAQ

This file gives answers to frequently asked questions about dmsdos.


What is dmsdos ?

Dmsdos is a Linux kernel module that allows access to msdos/win95 compressed
disk partitions. It supports different doublespace, drivespace and stacker
versions.
See file dmsdos.doc for an actual list of supported dos/win95 configurations. 



How do I mount a compressed partition?

You need to mount the uncompressed host partition as usual. In this
partition, you can see the compressed partiton as large file (the CVF,
Compressed Volume File). It usually has names like 'dblspace.NNN',
'drvspace.NNN' or 'stacvol.XXX'. The whole compressed partition is inside
this file.

You need the loopback block device driver (enable the loopback device during
kernel configuration):

   *
   * Additional Block Devices
   *
   Loopback device support (CONFIG_BLK_DEV_LOOP) [M/n/y/?] Y or M
 
The loopback block device allows mounting a file as a filesystem (read some
Linux docs about this for more information). To avoid confusion, note
that this has absolutely nothing in common with the well-known network 
loopback interface.

Example: 'mount -t msdos -o loop /DOS/dblspace.001 /mnt'

This mounts your compressed filesystem that in fact resides in the file 
/DOS/dblspace.001 under /mnt.



It doesn't work!

Well, nothing simply doesn't work. It fails at a specific point and gives
some errors or warnings before failure which are usually an important hint
for finding out what exactly went wrong. Please have a look at file
troubleshooting.doc. If you still cannot solve the problem you can send an
email to the current dmsdos maintainer. The email address and a checklist
what to include in a bug report can be found in file dmsdos.doc.



I have compressed partitions, but I'm using umsdos. Can I use dmsdos without 
losing umsdos features?

No problem. It should work out of the box. If you want to use umsdos style
long filenames inside the CVF, you can even mount it as type umsdos instead 
of msdos. (Uhh. Use kernel 2.0.xx or get at least 2.1.94 for that).



My compressed partition has long filenames under Win95. How can I see 
them with dmsdos ?

Mount as type vfat instead of msdos.



Can I compress my Linux data under dmsdos?

You can (via umsdos), but it's not recommended.

Just to let you know: There's some other compression software available that
may be a better choice for Linux data. See file dmsdos.doc.



Can I boot Linux from a compressed partition?

No. (Older dmsdos versions supported it, but this feature has been given up
when the CVF-FAT interface was introduced. Well, the documentation always
warned ... )



Uhhh. Write access is so ssssssslow.....

What can I say. You decided to use compressed partitions - now you are
experiencing one of their greatest disadvantages. But dmsdos has some
special tricks for you in this case. You can... (in recommended order)

    - run the dmsdos daemon which does delayed compression in the background
      when the system is idle, or use the internal daemon 

    - set speedup bit #2 (allocation code skips exact search)

    - set speedup bit #6 (allocation code doesn't search at all but takes
      any hole free hole) (Really Not Recommended if you don't know what
      you are doing). Read the comments in the documentation, please.

See the dmsdos documentation for details and a discussion about their
advantages and disadvantages.



Is it safe to use dmsdos?

This question is something like that one: 'is it safe to drive a car?'.

There's no warrenty. I really can't promise that there aren't any bugs in
the driver code. I think you just like to feel safe:

 - If you don't have to write to your compressed filesystems, mount
   read-only.

 - Run the dos filesystem checker every time you boot dos and backup from
   time to time.

 - Backup your compressed filesystem before trying to mount read-write.



How do I enable/disable long filename support?

For long filename support use a filesystem type that knows long filenames,
e.g. vfat instead of msdos.



What happens if the compressed filesystem gets full during write access?

Don't ask. That has become too complex. :(

Well, in the worst case the same thing happens as under Dos: you get a
strange error (Dos: "sector not found") and might lose data. It is difficult
because some applications do not check the return values of write access
functions (AAARGHHH!!!). On a compressed filesystem not only cluster
allocation, but also usual write access may fail (i.e. when you replace
data that compress well by data that don't compress well and thus need more
physical space). So just keep in mind that you shouldn't fill a compressed
partition up to the last byte :)



What's the difference between win95 doublespace and drivespace 3 ?

Please note that win95 has two different drivespace programs. One of them
is included in win95 and it's in fact an old dos doublespace version. I
still refer to it as 'win95 doublespace' though M$ call it 'drivespace'.
This one is supported without restrictions.

The other one is sold seperately from win95 in a 'M$ Plus' package (is this
still true?) M$ have called it Drivespace 3. Meanwhile it should also
be supported quite well.

If you aren't sure, watch the filename of the large, hidden CVF. If it is
something like 'dblspace.xxx' it's the old version. If it's 'drvspace.xxx'
instead and you are very sure you created it under win95, it's Drivespace 3.
Dmsdos also tells when it detects a drivespace 3 format CVF. 

The main difference between these two versions is that drivespace 3 uses a
larger cluster size (32KB instead of 8KB) and can compress up to 2GB of data 
instead of only 512KB. It has also a more advanced compression scheme 
(SQ-0-0).



When do I have to defragment my compressed partitions?

Watch the dutil output (see file dmsdos.doc for a dutil description). It
displays a fragmentation value. You should keep it below 75% to feel safe. 
When dutil even displays a warning about high fragmentation, you should
immediately boot Dos and defragment the CVF.



Does dmsdos work with FAT32 filesystems?

Yes. All earlier problems have been avoided by moving dmsdos one layer down
in the filesystem interface. Well, it does not support *compressed* FAT32
filesystems. Win95 doesn't do either :) But, if some day they show up to 
exist, I might be persuaded to implement it also.



What about the relationship between dmsdos and software patents ?

I'm still receiving mails about this question. At my best knowledge dmsdos 
does not violate any software patents. (Well, I don't hope so, but my best 
knowledge might be wrong. I'm not a lawyer.) If you are interested in
details take a look at file patent.doc.



Do I have to recompile dmsdos when I upgrade my kernel ?

The safe answer is yes. It's best to recompile *all* modules if you change
something in your kernel though not always necessary. But who knows.

Some clever people invented a 'module version information' trick. You can
enable it during kernel configuration. This makes your modules less kernel
version dependent e.g. a module compiled under kernel 2.0.33 may also work 
under kernel 2.0.34. In that case, you may share one precompiled dmsdos 
module with several kernels. But please don't overuse it. You should not,
for example, use the same compiled object code of a dmsdos module for
kernel 2.0.34 and 2.1.105 - this is very likely to cause problems :)



Where does the name "DMSDOS" come from ?

It has historical reasons. The first piece of dmsdos code was written in 
times of MSDos 6.2 when Doublespace became very popular. Thus, dmsdos was
meant as something like "doublespaced msdos". Today, it supports a lot of 
more compressed Dos filesystems, and a better name would be "cmsdos" for 
"compressed msdos". But you never change the name of a successful project :)