Re: VFS caching of file extents

[Christoph Hellwig <hch@lst.de>]

> Wouldn't readplus (and maybe a sparse copy program) rather have
> something that is "SEEK_DATA, fill the buffer with data from that file
> position, and tell me what pos the data came from"?

Or rather a read operation that returns a length but no data if there
is a hole.  Either way a potentially incoherent VFS cache is the wrong
way to implement it.

> I also suspect that devising a "simple" mapping tree for simple
> filesystems will quickly devolve into a mess of figuring out their adhoc
> locking and making that work.

Heh.  If simple really is the file systems just using the buffer_head
helpers without any magic it might actually not be that bad, but once
it gets a little more complicated I tend to agree.

But the first thing would be to establish if we actually need it at all,
or if the buffer_head caching of their metadata is actually enough for
the file systems.