Re: [PATCH 0/4] Fiemap, an extent mapping ioctl - round 2

[Jamie Lokier <jamie@shareable.org>]

Dave Chinner wrote:

> If you know the offset and length of the xattr, then you can get it
> specifically, bu to do that you need to know about the internals of
> the filesystem....
>
> FWIW, this is exactly the same case as getting the extent map of a
> directory data (I use xfs_bmap all the time for this) - you know
> where the blocks are, but without completely decoding the directory
> structure you have no idea where inside that map a given entry is.

There's a thought.  What does "offset" mean for directory data and
FIEMAP?  (And xattrs, but let's ignore that, it's less important).

What's the appropriate thing to return for FIEMAP on a directory, on a
filesystem which doesn't store directories as a blob of data with
contiguous offsets?

E.g. directory offsets (readdir) do mean something, but there's no
guarantee that directory offsets 0..N-1 corresponds with extents
covering N bytes exactly.

Would it return extent data similar to a file with large holes?

-- Jamie