Re: SEUL: Partitioning

[Erik Walthinsen <omega@aracnet.com>]

> > The file is fixed in size+position, so it can be treated exactly like a
> > partition on the drive - this may require some kernel changes, but they
> > should be minor. I have never personally tried loopback, so I'll give it a
> > go tonight (I saw it when kernel v2.0 came out, thought "Cool!" and forgot
> > about it). I would not have thought the extra VFS step would cause too much
> > of a performance hit, but I will try it and report the findings later. I use
> > NFS over ethernet and find it fast enough.
> 
> Forgive my ignorance, for I ask out of ignorance, but I was wondering
> what would happen to the ext2 virtual-filesystem when the user ran
> Defrag under DOS or Win95?

Exactly.  If we can create the file on the FAT partition in a single 
contiguous block (after doing a defrag from Linux), then we can do the 
evil/disgusting hacks to the kernel necessary to eliminate the second trip 
through the VFS layer.  But once the user defrags from M$, the file goes 
nutsoid again.  This is where an intelligent log-structured filesystem might 
help, since some of them are designed to operate on less-than-ideal underling 
devices.  If we can tell the filesystem where it's components are relative to 
each other on the platters, it might be able to get decent performance out of 
it.

A solution to the defrag problem is to make the file -RHS (no archive, 
read-only, hidden, system), which triggers the unmovable flag in defrag, but 
then the user is wondering "what the heck is that???"  Messy, but it could 
work.

Oh, what happens to the ext2fs stuff?  Nothing, unless M$ has some special 
code to detect and mutliate unmovable files with an ext2fs magic cookie... ;-) 
 The file just gets reorganized like any other, basically moving the block 
references.  When the FAT layer in Linux gets to it next, it just has to look 
through and figure out where everything is again.  This breaks performance 
bigtime, but it's still operational.

        Erik Walthinsen - Programmer, webmaster, 3D artist, etc.   __
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