samba - Apr 2003 - vfat / ftruncate problem

I recently added a 60Gig HD to my little linux server at home with the intention
of sharing it via Samba to all my PCs which run Windows 98SE.  I really want to
use vfat file system on it.

The server is running Mandrake 8.1, so kernel 2.4.something, and the Samba that
came with it.

Much to my disappointment I quickly discovered poor performance copying files to
it, the copy is like slow to get going, and trying to copy big files to it (like
750Meg is the size of the file I keep testing with) just fails outright.

I tried it on a different system on my Lan, also running Mandrake 8.1, same
story copying to vfat from Win98SE.

I downloaded, compiled, and installed Samba 2.2.8a, same story.

I installed Mandrake 9.1, also kernel 2.4.something and used the samba it came
with it, 2.2.7 I think, same story.

It all works completely fine copying to ext2 or ext3 partitions from Windows
Explorer.

Here's a real kicker, it even works completely fine with no delays from
Win98SE copying to a Samba shared vfat partition, IF, I just open a DOS command
prompt and do the copy from there.  But if I just exit the command prompt and
attempt the exact same copy in Windows Explorer, it will fail.

I messed with a multitude of samba options and mount options for the vfat
partition all to no avail.  I finally think I've homed in on the problem
when I set Samba to log level 10, (log level 9 was not sufficient) at which
point I discovered its hanging up on a ftruncate command.

When I attempt to copy my 750Meg file to the samba shared vfat partition, the
last couple things logged are an fallocate IIRC, and then an ftruncate, the
server just sits there 90 seconds or so grinding and grinding then I guess
something times out and either Samba drops the connection to the Win98SE client
or Win98SE drops it itself, Win98SE gives an error for the copy that the
resource is no longer available, but the connection restablishes itself right
away then, but the copy attempt has been aborted.

I think its the same type of deal copying smaller files to the Samba shared vfat
partition that does work except there is a long pause before the copy really
gets going, I think Samba is doing the fallocate deal and it just takes quite a
while which is where the pause comes in before the copying gets going, but
doesn't take so long that the connection times out.

I imagine this could be easily be reproduced by anyone willing to try as I
reproduced it on two very different machines running samba (one is a Pentium II
system, the other an AMD Duron) two different versions of Mandrake and 3
different versions of Samba.

I did a bunch of googling and saw that in Samba 2.2.1a some kind of fix was
added in for the vfat/fallocate bug, but the impression I got from an archived
message on some Samba developers list was that what Samba actually does is go
ahead and attempt the fallocate on vfat partitions and if it fails, which
apparently it always will on vfat, then Samba does something else instead.  More
of a workaround than a fix.  Maybe thats the best Samba can do and this is
really something that should be handled by the kernel/file system people.  My
hunch is that the problem with the workaround Samba has implemented is that with
great big files the time it takes for fallocate to fail on a vfat partition is
just to long so the Samba workaround doesn't even kick in, something times
out before fallocate can return the failure code and the workaround can kick in.

Comments anyone?

Thankyou,
Tom Schaefer

I kept fighting with it and came up with a solution.  (I think)

My analysis in the previous posting was somewhat flawed.  It isn't that the
ftruncate command to the vfat partition is taking to long to fail.  Its that
after its failed the workaround implemented by Samba is taking to long.

I modified the the source code of vfs-wrap.c, in particular this section:
 
result = sys_ftruncate(fd, len);
        if (result == 0)
                goto done;

to 

/*result = sys_ftruncate(fd, len);
        if (result == 0)*/
result = 0;
                goto done;

recompiled smbd, put the new binary in place, restarted samba and lo and behold
it worked!  Now I can copy my 750Meg test file from Windows98SE Windows Explorer
to a samba shared vfat partition and the copy starts immediately and completes
successfully after a minute or two.

So, now my question is - What kind of problem(s) if any am I setting myself up
for by doing this?

Thankyou,
Tom Schaefer