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

[Eric Blake]

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