Re: Files full of zeros with coreutils-8.11 and xfs (FIEMAP related?)

[Andreas Dilger]

On 2011-04-15, at 11:26 AM, Christoph Hellwig wrote:
> On Fri, Apr 15, 2011 at 11:24:19AM -0600, Eric Blake wrote:
>> Would it be worth borrowing from Solaris' semantics and adding SEEK_HOLE
>> and SEEK_DATA to lseek(2), as a higher level (less-detailed, but easier
>> to define and easier to use) interface for discovering the regions of a
>> file that only contain NUL bytes?
> 
> Yes, I've already suggested that both in this thread and on IRC.
> 
> For efficient copies it's the only usable interface.

I suspect that these bugs would have still existed whether the interface is 
SEEK_HOLE/SEEK_DATA, or FIEMAP.  The main problem is that the delalloc pages 
were not accounted for correctly during layout traversal.

For ext4 I think it is sufficient to add another case to ext4_ext_fiemap_cb() 
to check the pagecache for unmapped pages when it finds an uninitialized extent 
on disk.  This will be very similar to the fix for finding holes in the on-disk 
mapping.

Cheers, Andreas