Re: File Limits

[Bryan J Smith <b.j.smith@ieee.org>]

All DOS (including DOS 7/Win 4 releases running in 386Enhanced mode -- Windows 
95, 98, Me) only support 2GiB files IIRC.  Even Real86 mode DOS 7 supports 
FAT32, which allows bigger than 2GiB _file_systems_, but still has the 2GiB-1 
(signed 32-bit int) _individual_file_ size limitation IIRC.


In fact, I don't think the 4GiB-1 (unsigned 32-bit int) support is in any 
DOS/Win releases.  I only think it is an option in NT kernels (FAT16 in 3.1, 
3.5, 3.51 and 4.0, FAT16 and FAT32 in 5.0/2000, 5.1/XP-2003, 6.0-Vista and 
6.1/7-2008).  And any program that uses it _must_ know about the NT-only Win32 
function calls that offer this.

I.e., the "Chicago" (Win9x) and NT/Win32 teams used _different_ DOS Int 21h and 
Win32[s] function calls, the former completely ignoring the pre-existing of the 
latter.  "Chicago" _never_ supported the native NT/Win32 calls, hence why they  
likely _never_ supported the extensions for 4GiB-1 support IIRC.[1]

One quick trip to TechNet and you'll note this is an issue in "bare metal" 
DOS/Win and even NT.  The legacy "Chicago" DOS Int 21h file services don't 
handle growing beyond 2GiB-1 proper, _unless_ the NT/Win32 function extensions 
are utilized.  Unfortunately even NT 5.0 (Windows 2000) implements the "Chicago" 
calls bug-for-bug for compatibility reasons.

I.e., your program is likely instigating this, and the OS is just letting it do 
what it wants (and causing the issue).

-- Bryan

NOTES:  
[1] Like many libraries in NT/Win32, things just "didn't work" or "work well" 
under Win9x -- OpenGL being my personal favorite, and the whole "DirectMM -- aka 
Direct [DOS] Memory Map" lineage now known as "DirectX".  It wasn't until NT 5.0 
(Windows 2000) that the NT/Win32 team started also supporting the 
"Chicago"/Win32 DOS Int 21h changes.



----- Original Message ----

From: Xavier Tarifa <Xavier.Tarifa@adparts.com>

So here's my problem: Until now I was using dosemu with ms-dos 6.22 to run an 
old database. A little while ago we had some big changes to the tables and they 
grew a lot.
Yesterday I realized that we have a database file that is about to hit 2 Gb. I 
remembered that some time ago, we hit a bug on our program and it endlessly 
inserted rows on a table until it hit the 2 gb mark. Then the file either 
disappeared or became a 0 bytes file, I don't remember well.
So I started reading and found that ms-dos 6.22 only supports FAT16 and 
apparently it only allows 2 gb volumes, and 2 gb files.
So I thought of using ms-dos 7.10 which supports fat32 and would give us 4 gb 
files. But the same happens. The files disappear as soon as they hit the 2 gb 
mark. And when I execute the dir command, it always reports 2 gb of free disk 
space, which is odd.

I don't know if this is a limitation of dosemu or I'm doing something wrong. 
Right now, I have about 140 users. On each $HOME/.dosemu/drive_c/ of the users 
there are symlinks to each file in /usr/share/dosemu/drive_z/ , where ms-dos is 
located. And I have the database files in /datos/ . I use lredir to make it 
appear as f: to every user.
All of this is in the same ext3 partition.

Any ideas?