> So I looked at the code - it seems you build a full extent of the blocks
quoted text
> 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.

Mikulas
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