per-directory quotas possible on 5.x ?

[Robert Watson]

On Mon, 30 Jun 2003, Josh Brooks wrote:

> On Mon, 30 Jun 2003, Robert Watson wrote:
> 
> > As you may have noticed in trying the vn-backed mechanism, there are some
> > inefficiencies that turn up in FreeBSD when have large numbers of
> > pseudo-devices, etc.  The resizing problem is real, also, since we don't
> > have online file system resizing.  FWIW, a file system like HFS+ (which
> > has a much more strict directory hierarchy) would lend itself to directory
> > quotas much more.  A port of HFS+ to FreeBSD was recently posted to
> > freebsd-fs.
> 
> Thank you for your very informative response.  I am curious, what sort
> of inefficiencies do turn up when you have large numebers of pseudo
> devices ? 
> 
> Do you have any comments on a system running, say, 100 vn-backed mounted
> filesystems ?  200 ?  (presume moderate to heavy activity in each ...) 

I've observed two problems when using large numbers of pseudo-devices:

(1) Hard upper limits on the number of such devices that can exist (I
    don't remember the current limits, but on -STABLE I seem to recall
    it's around 512).

(2) Increased chances of a deadlock if you have lots of vn backing files
    in the same directory and you perform directory operations on the
    directory.  This has been observed on -STABLE with a few hundred
    active chroots on vn-backed devices in jails.  There have been some
    mitigating changes in -CURRENT recently that decrease the chances of
    this happening.  The -STABLE workaround was to put each backing file
    in its own subdirectory if the problem occurs on your system with your
    workload.

On -CURRENT, you can also use the GPT partition layout which makes it
possible to have a lot more partitions on a disk than the BSD label format
or MBR.  Using disk partitions makes things a little more difficult to
resize, but not hugely more, and avoids going through a file system
loopback, improving performance (and eliminating the chances of (2) 
above).

Robert N M Watson             FreeBSD Core Team, TrustedBSD Projects
robert at fledge.watson.org      Network Associates Laboratories