Re: [libvirt] [PATCH] Use posix_fallocate() to allocate disk space
On (Wed) Mar 04 2009 [12:55:22], Daniel P. Berrange wrote:
On Mon, Mar 02, 2009 at 03:02:11PM +0530, Amit Shah wrote:
>
> Inspite of this if some feedback is needed for a non-extents-based file
> system, a run-time probe for the underlying file system can be made and
> we could default to a chunk-based allocation in that case.
These results are quite impressive. It is better by orders of magnitude
for modern filesystems, and even on ext3 it has a slight edge. So I'm
inclined to say we should use posix_fallocate() by default at all
times.
When we introduce the API for incremental feedback, we could do something
like call fallocate() in 1GB chunks so we can get some reasonable amount
of feedback while still keeping it very fast & well allocated on disk.
Oh btw fallocate() seems to be an O(1) operation (at least on new file
systems). Allocating a 5G file also gave me 0s:few microseconds delay.
And of course, just fallocate() the whole thing upfront if the user
does
not provide a callback for requesting feedback.
Amit