DragonFly kernel List (threaded) for 2012-07
Re: Quota on tmpfs
On Wed, Jul 18, 2012 at 09:18:49AM +0200, Francois Tigeot wrote:
> > > I would also guess that sparse files are very rarely used. But for
> > > disk usage purposes you want to consider real disk usage including
> > > overhead because the quotas are mostly used to partition the available
> > > space. That doesn't work if your quotas allow you to write a few
> > > thousand files of 1 byte length that account together as a single
> > > single block when they really occupy a few thousand blocks.
> > A scenario in which they're frequently used is block-based file system
> > transfer protocols (especially distributed ones where blocks may
> > download in random order, including bittorrent), also by download
> > managers that support "download optimization" where multiple
> > connections will be made to transfer multiple file sections at a time
> > (i.e. the DownloadThemAll Firefox extension).
> > Another common usage of sparse files is for live file system images.
> > The cost of creation (open/creat + trunk/lseek + newfs) is small
> > compared to writing a full image of zeros, then the blocks can be
> > lazily allocated and written when needed.
> And this last usage is one good reason to only count seek sizes and not
> holes in files. Disk quotas can fill up suddenly when data is written to
> what appears to be a perfectly sized device.
> The potential for disruption of virtualized systems is very high in this
> situation and it was deemed best to count the full file size at creation
> time and avoid bad surprises.
It seems like a better way to address that problem would be to provide
a scheme to allocate all the blocks in the image files you're
David A. Holland