bug#6131: [PATCH]: fiemap support for efficient sparse file copy

[Tao Ma]

Hi Jim

On 05/27/2010 06:30 PM, Jim Meyering wrote:
> jeff.liu wrote:
> > This is the revised version, it fixed the fiemap-start offset calculation
> > approach to remove it out
> > of the 'for (i = 0; i<  fiemap->fm_mapped_extents; i++)' loop.
>
> Hi Jeff,
>
> I've included below the state of my local changes.
> Unfortunately, with that 5-patch series, there is always a test failure
> on F13/ext4.  Maybe someone who knows more about extents can provide an
> explanation?
Just want to clarify why ocfs2 didn't work here. I guess the reason also 
works for ext4 since both ext4 and ocfs2 use block group to organize 
their blocks in the volume.

I checked the perl test script to create sparse src file, it will create 
contiguous bytes(around 20-24k) at an interval of around 40k.So in 
general, these 20-24k should be contiguous. But that does exist some 
scenario that they could be separately into 2 extents. Consider one 
block group is used to allocate blocks to this file, when the block 
group only has 10K left while you are requiring 20K, it will use the 
left 10K in this group and allocate 10K from another block group. That 
would become 2 extents since they can't be contiguous.

So I guess the right step is to check the holes by using filefrag if you 
prefer this tool and want to make sure cp doesn't copy holes(I get this 
point from another e-mail written by you). How to find holes with 
filefrag? I guess it is quite simple since it also use fiemap and we can 
calculating holes easily by comparing the 2 consecutive records. I guess 
we can get what you want with ext4 after this update.

Regards,
Tao