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

From: David Holland <dholland-tech@xxxxxxxxxx>
Date: Wed, 18 Jul 2012 20:39:55 +0000

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
concerned about.

David A. Holland

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