Re: Quota on tmpfs

[Francois Tigeot <ftigeot%wolfpond.org@localhost>]

On Tue, Jul 17, 2012 at 06:14:50AM -0400, Thor Lancelot Simon wrote:
> On Fri, Jul 13, 2012 at 08:24:49AM +0200, Francois Tigeot wrote:
> > 
> > This is not really an issue for me; I don't care about low-level storage
> > and only manage visible file sizes.
> 
> This seems like a severe design bug, then.
> 
> Sparse files are a basic building block of many Unix applications,
> particularly ones using databases.  Counting them as if they used
> as much space on disk as they have bytes of readable length will
> either just plain break stuff (due to spurious quota violations)
> or render your new quota system unuseful to a large number of users.

Besides sparse files, the issue of visible size and different real
disk usage is also present with historical data and deduplicated
blocks.

There's not much an individual user can do about all this; having
a deduplication batch reduce disk usage without any work can be welcome
but on the other hand even removing files won't reduce disk usage
on a HAMMER filesystem with history retention enabled.

Having a quota system based on visible file sizes gives at least consistent
results with what a regular user sees when listing files or using du(1).

Choosing the limits to set is the admin's burden; not having sparse file
support doesn't preclude him/her from making the right choices, which
will be system-specific anyway.

> Is there really a consensus among the Dragonfly developers that
> a new quota system without sparse file support is a good thing?

I remember some discussions about it; having sparse file support would be
nice but even without it a quota system is already useful.

-- 
Francois Tigeot