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DragonFly kernel List (threaded) for 2012-07
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Re: Quota on tmpfs

From: Francois Tigeot <ftigeot@xxxxxxxxxxxx>
Date: Wed, 18 Jul 2012 09:18:49 +0200

On Tue, Jul 17, 2012 at 09:26:44PM -0400, Matthew Mondor wrote:
> On Tue, 17 Jul 2012 20:54:28 +0000 (UTC)
> mlelstv@serpens.de (Michael van Elst) 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.

Francois Tigeot

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