Re: thin provisioned LUN support & file system allocation policy

Matthew Wilcox
2008-11-07T14:55:51Z

On Fri, Nov 07, 2008 at 09:43:11AM -0500, Theodore Tso wrote:
> The one thing we might need for SSD-friendly allocation policies is to
> tell the allocators to not try so hard to make sure allocations are
> contiguous, but there are other reasons why you want contiguous
> extents anyway (such as reducing the size of your extent tree and
> reducing the number of block allocation data structures that need to
> be updated). And, I think to some extent SSD's do care to some level
> about contiguous extents, from the point of view of reducing scatter
> gather operations if nothing else, right?

It's not so much s-g operations as it is that you can only have 32
commands outstanding with the drive at any given time. Each read/write
command can specify only one extent. So if you can ask for one 256k
extent rather than have to ask for a 4k extent 64 times, you're going
to get your data faster.

--
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."