linux-kernel - Re: NVM Mapping API

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Date:	Wed, 16 May 2012 11:57:28 -0400
From:	Matthew Wilcox <willy@...ux.intel.com>
To:	Greg KH <greg@...ah.com>
Cc:	linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: NVM Mapping API

On Tue, May 15, 2012 at 10:46:39AM -0700, Greg KH wrote:
> On Tue, May 15, 2012 at 09:34:51AM -0400, Matthew Wilcox wrote:
> > 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?
> 
> Rescanning the busses at startup are required anyway, as devices can be
> added and removed when the power is off, and I would be amazed if that
> is actually taking any measurable time.  Do you have any numbers for
> this for different busses?

Hi Greg,

I wasn't particularly serious about this example ... I did once time
the scan of a PCIe bus and it took a noticable number of milliseconds
(which is why we now only scan the first device for the downstream "bus"
of root ports and downstream ports).

I'm just trying to stimulate a bit of discussion of possible usages for
persistent memory.

> What about pramfs for the nvram?  I have a recent copy of the patches,
> and I think they are clean enough for acceptance, there was no
> complaints the last time it was suggested.  Can you use that for this
> type of hardware?

pramfs is definitely one filesystem that's under investigation.  I know
there will be types of NVM for which it won't be suitable, so rather
than people calling pramfs-specific functions, the notion is to get a
core API in the VFS that can call into the various different filesystems
that can handle the vagaries of different types of NVM.

Thanks.
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