Stijn Hoop <[EMAIL PROTECTED]> wrote:

> On Wed, Jun 09, 2004 at 02:21:40PM -0500, Scott wrote:
> > As a newbie to FreeBSD, I may be way off base, but it seems 
> > very logical to me that the size of your drive or partition 
> > would make a difference on at what percentage full one would 
> > start to notice problems.
> > 
> > In terms of megs/gigs 80% of 120 gigs still has a lot of 
> > work space left. 80% of 4 gigs is not much. I would think 
> > with a larger drive/partition, one could run at a higher 
> > percentage before trouble started.
> > 
> > It makes sense to me anyway :)
> That's what one would like, but UFS doesn't work that way.  It's allocation
> algorithm assumes 10% of the disk is free -- regardless of actual size. Or so
> I've been told (multiple times).
> IMHO this is a bit ridiculous -- I mean, given 1 TB of space (nearly feasible
> for a home server right now), why would an FS allocator need 10% of that if
> the files on the volume are averaging 10 MB?
> But then again, and this is worth noting -- I'm certainly nowhere near as
> clueful as others on how to design a stable & fast file system.  Seeing as
> UFS1 is still in use, and has been for the last 20 years (think about it!), I
> think maybe the tradeoff might make sense to an expert...
> BTW, note that you really need to consider the perfomance drop for yourself
> -- like others said, if the files on the volume change infrequently,
> performance matters little, and space more so.

I think you've missed the point.

The designers of UFS/FFS did not design the filesystem to require 10% free space
in order to perform well.

They developed the best, fastest (thus the name "fast file system") filesystem
algorithms they could come up with.

Then, during testing, they found that these algorithms started to perform really
poorly when the filesystem got really full.  Thinking this might be important,
they tested further until they knew exactly what point the performance started
to drop off at.  They then went one step further and developed another
algorithm in an attempt to maintain as much performance as possible even when
the filesystem got very full.  This is why you'll occasionally see the
"switching from time to space" message when your filesystem starts to fill up.
The filesystem drivers are doing their best to degrade gracefully.

Now, I'm not going to say that there is no more that can be done.  I think the
fact is that the two algorithms work well enough that nobody has bothered to
invest the research into improving them.  (That combined with the fact that
disk space keeps getting cheaper and cheaper, makes it unlikely that anyone will
invest much $$$ into researching how to use that last 10% while still
maintaining top performance).

Bill Moran
Potential Technologies
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