Re: NVM Mapping API

[Andy Lutomirski <luto@amacapital.net>]

On 05/15/2012 06:34 AM, Matthew Wilcox wrote:
> 
> There are a number of interesting non-volatile memory (NVM) technologies
> being developed.  Some of them promise DRAM-comparable latencies and
> bandwidths.  At Intel, we've been thinking about various ways to present
> those to software.  This is a first draft of an API that supports the
> operations we see as necessary.  Patches can follow easily enough once
> we've settled on an API.
> 
> We think the appropriate way to present directly addressable NVM to
> in-kernel users is through a filesystem.  Different technologies may want
> to use different filesystems, or maybe some forms of directly addressable
> NVM will want to use the same filesystem as each other.

> What we'd really like is for people to think about how they might use
> fast NVM inside the kernel.  There's likely to be a lot of it (at least in
> servers); all the technologies are promising cheaper per-bit prices than
> DRAM, so it's likely to be sold in larger capacities than DRAM is today.
> 
> Caching is one obvious use (be it FS-Cache, Bcache, Flashcache or
> something else), but I bet there are more radical things we can do
> with it.  What if we stored the inode cache in it?  Would booting with
> a hot inode cache improve boot times?  How about storing the tree of
> 'struct devices' in it so we don't have to rescan the busses at startup?
> 

I would love to use this from userspace.  If I could carve out a little
piece of NVM as a file (or whatever) and mmap it, I could do all kinds
of fun things with that.  It would be nice if it had well-defined, or at
least configurable or discoverable, caching properties (e.g. WB, WT, WC,
UC, etc.).

(Even better would be a way to make a clone of an fd that only allows
mmap, but that's a mostly unrelated issue.)

--Andy