Re: thin provisioned LUN support

Matthew Wilcox
2008-11-06T17:15:28Z

On Thu, Nov 06, 2008 at 03:24:05PM +0000, David Woodhouse wrote:
> I think we should be content to declare such devices 'broken'.
>
> They have to keep track of individual sectors _anyway_, and dropping
> information for small discard requests is just careless.

As an implementor of such a device, I say "ya, boo, sucks to you".
ata_ram simply ignores the bits of the trim which don't line up with the
page size chunks it's allocated. Sure, it'd be possible to add a bitmap
to indicate which 512-byte chunks of the block contain data and which
don't, but I haven't done that yet. I think there's even space in the
struct page that I can abuse to do that.

I think this really is a QoI thing. Vendors who don't track individual
sectors will gradually get less and less efficient. Hopefully users
will buy from vendors who don't cheat. We can even write a quick
program to allocate the entire drive then trim sectors in a chessboard
patterns. That'll let users see who's got a crap implementation and
who's got a good one.

--
Matthew Wilcox Intel Open Source Technology Centre
"Bill, look, we understand that you're interested in selling us this
operating system, but compare it to ours. We can't possibly take such
a retrograde step."