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

[Eric Blake]

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.
> 

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

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