Re: [Qemu-devel] [RFC] qemu-img: Drop BLK_ZERO from convert

[Paolo Bonzini <pbonzini@redhat.com>]

On 07/03/2018 16:57, Max Reitz wrote:
>>>> (2) For sparse raw images, this is absolutely devastating.  Reading them
>>>> now takes more than (ext4) or nearly (xfs) twice as much time as reading
>>>> a fully allocated image.  So much for "if a filesystem driver has any
>>>> sense".
>> Are you sure that only the filesystem is the problem? Checking for every
>> single byte of an image whether it is zero has to cost some performance.
> Well, yes, but "read data location from FS metadata" + "realize it's a
> hole" + memset() + "repe scasb" shouldn't take twice as much time as
> "read data location from FS metadata" + "read data from SSD".
> 
> I expected the "realize it's a hole" part to fall out for free, so this
> would that memset() + repe scasb take much longer than reading data from
> the SSD -- and that's just pretty much impossible.
> 

This makes a lot of sense, but just to double-check, what does profiling
say?

Paolo