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 _______________________________________________ Libguestfs mailing list [email protected] https://www.redhat.com/mailman/listinfo/libguestfs
