Re: RFC: new cp option: --efficient-sparse=HOW

[Eric Blake]

On 01/31/2011 03:17 PM, Eric Blake wrote:
>> The default is --efficient-sparse=auto, and for symmetry,
>> I've provided --efficient-sparse=never, in case someone finds
>> a reason to want to skip the ioctl.
> 
> Conversely, what happens if I have a file that contains large blocks of
> zeros but is NOT fully sparse (plausible, since we're still facing the
> fact that it is still not easy to punch holes into existing files when
> data in that portion of the file is no longer needed)?  Does all the new
> fiemap code still have the ability for me to request that the cp code
> specifically look for large blocks of zero in the source, rather than
> trusting the fiemap, so that I can create a copy that is more sparse
> than the original?  Does that also need a tunable; and if so, should we
> try to combine it into this tunable or is it orthogonal?

Maybe something like this matrix, introducing --sparse=search as a step
beyond --sparse=always to request that the (non-sparse) portions of the
input be specifically searched for possible further blocks of 0 that can
be made sparse in the destination:

--sparse=always --efficient-sparse=always
    require fiemap, destination will only be as sparse as input
--sparse=search --efficient-sparse=always
    require fiemap, destination might be more sparse than input
--sparse=always --efficient-sparse=auto
    if fiemap is supported, destination will be as sparse as input
    if fiemap is not supported, destination might be more sparse
--sparse=search --efficient-sparse=auto
    destination might be more sparse than input
--sparse=auto --efficient-sparse=always
    if input is sparse, require fiemap, destination will match input
sparseness
--sparse=auto --efficient-sparse=auto
    destination will be sparse if input was sparse; if fiemap is
supported then destination will be equally sparse, otherwise, it might
be more sparse than the input

-- 
Eric Blake   address@hidden    +1-801-349-2682
Libvirt virtualization library http://libvirt.org

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