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Re: [Libguestfs] [PATCH v2 2/4] file: Support zero without ZERO_RANGE



On 08/03/2018 02:28 PM, Nir Soffer wrote:
File systems not supporting FALLOC_FL_ZERO_RANGE yet fall back to manual
zeroing.

We can avoid this by combining two fallocate calls:

     fallocate(FALLOC_FL_PUNCH_HOLE)
     fallocate(0)

Based on my tests this is much more efficient compared to manual
zeroing. The idea came from this qemu patch:
https://github.com/qemu/qemu/commit/1cdc3239f1bb



Note: the image is sparse, but nbdkit creates a fully allocated image.
This may be a bug in nbdkit or qemu-img.

Calling fallocate(0) forces allocation; so anything explicitly written to 0 won't be sparse when this mode is used. There's also a question of whether your source file accurately reports holes to begin with (poor tmpfs SEEK_HOLE performance is still a common problem). But I don't see that as getting in the way of this patch going in.

---
  plugins/file/file.c | 32 ++++++++++++++++++++++++++++++++
  1 file changed, 32 insertions(+)


LGTM

--
Eric Blake, Principal Software Engineer
Red Hat, Inc.           +1-919-301-3266
Virtualization:  qemu.org | libvirt.org


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