Re: [PATCH 1/4] vfs: vfs-level fiemap interface

Josef 'Jeff' Sipek
2008-09-20T16:47:59Z

On Tue, Sep 16, 2008 at 05:31:07PM -0400, Theodore Tso wrote:
> On Mon, Sep 15, 2008 at 10:53:05AM -0700, Mark Fasheh wrote:
> > On Mon, Sep 15, 2008 at 10:49:48AM -0400, Christoph Hellwig wrote:
> > > I agree to you (or someone elses - don't remember anymore) suggestion
> > > to put in more padding so we can add fields later. I strongly disagree
> > > putting in features now that we neither have a user, nor a usecase or
> > > testcase for.
> >
> > So, how about another 64 bits of padding in struct fiemap_extent? That could
> > help cover future uses like compression, which might require another 64 bit
> > offset field - we only have 32 bits of reserved space there right now.
>
> What I'd recommend is a 56 byte structure:

Why not just make it 64 bytes? Sure, that's 8 extra bytes, but I find the
power-of-2 size (and the extra space) comforting. (AFAIK, slab allocators
will give you 64 bytes anyway; and I expect something similar on the
user-space side of things.)

...
> Yeah, it's a little extra memory per extent, but filesystems seem to
> always invent new things, and it seem spretty clear that we have at
> least two extensions on deck (compression, multiple storage devices)
> both of which have at least one implementation that are either in the
> kernel or will likely enter the kernel. So it's likely that there is
> something that we may have missed, and leaving a little extra space
> doesn't actually cost us that much.

Right.

Josef 'Jeff' Sipek.

--
Once you have their hardware. Never give it back.
(The First Rule of Hardware Acquisition)