Re: [Qemu-devel] [PATCH v2 02/15] file-posix: support BDRV_REQ_ALLOCATE

[Anton Nefedov]

On 06/01/2017 10:54 PM, Eric Blake wrote:
> On 06/01/2017 02:49 PM, Eric Blake wrote:
> > On 06/01/2017 10:14 AM, Anton Nefedov wrote:
> > > Current write_zeroes implementation is good enough to satisfy this flag too
> > >
> > > Signed-off-by: Anton Nefedov <address@hidden>
> > > ---
> > >   block/file-posix.c | 9 ++++++++-
> > >   1 file changed, 8 insertions(+), 1 deletion(-)
> >
> > Are we sure that fallocate() is always fast, or are there some file
> > systems where it is no faster than manually writing zeroes?  I'm worried
> > that blindly claiming BDRV_REQ_ALLOCATE may fail if we encounter a libc
>
> not so much fail as in "break the guest", but fail as in "take far more
> time than we were expecting, pessimising our behavior to worse than if
> we had not tried the allocation at all"
>
> > or kernel-based fallback that takes a slow patch on our behalf.

I would expect such filesystems to not support fallocate.

Though I must admit I can't see anywhere in the documentation that it 
MUST be strictly faster than writing zeroes; it would look very strange
to me if there were a slowpath fallback somewhere past the libc.

/Anton