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

Mikulas Patocka
2008-01-11T08:04:32Z

> 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.

Mikulas