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