Re: [PATCH][RFC] fast file mapping for loop

[Jens Axboe <jens.axboe@oracle.com>]

On Fri, Jan 11 2008, Mikulas Patocka wrote:
> > So I looked at the code - it seems you build a full extent of the blocks
> > in the file, filling holes as you go along. I initally did that as well,
> > but that is to slow to be usable in real life.
> > 
> > You also don't support sparse files, falling back to normal fs
> > read/write paths. Supporting sparse files properly is a must, people
> > generally don't want to prealloc a huge disk backing.
> 
> How would you do sparse file support with passthrough loopback that 
> doesn't use pagecache?
> 
> Holes are allocated at get_block function provided by each filesystem and 
> the function gets a buffer that is supposed to be in the pagecache. Now if 
> you want to allocate holes without pagecache, there's a problem --- new 
> interface to all filesystems is needed.
> 
> It could be possible to use pagecache interface for filling holes and 
> passthrough interface for other requests --- but get_block is allowed to 
> move other blocks on the filesystem (and on UFS it really does), so 
> calling get_block to fill a hole could move other unrelated blocks which 
> would result in desychronized block map and corruption of both 
> filesystems.

Please read the posted patch and the posts from Chris as well, it
basically covers everything in your email.

The patch, as posted, doesn't work if the fs moves blocks around.

-- 
Jens Axboe