linux-kernel - Re: NVM Mapping API

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Message-ID: <CANGUGtAstHXLW-_kQJYMtrh35htpnGa2wXGtgC-PZnJJausTLg@mail.gmail.com>
Date:	Fri, 18 May 2012 14:07:22 +0200
From:	Marco Stornelli <marco.stornelli@...il.com>
To:	Matthew Wilcox <willy@...ux.intel.com>
Cc:	Greg KH <greg@...ah.com>, linux-fsdevel@...r.kernel.org,
	linux-kernel@...r.kernel.org
Subject: Re: NVM Mapping API

2012/5/16 Matthew Wilcox <willy@...ux.intel.com>:
> 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

For example?

> 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.
>

The idea could be good but I have doubt about it. Any fs is designed
for a specific environment, to provide VFS api to manage NVM is not
enough. I mean, a fs designed to reduce the seek time on hd, it adds
not needed complexity for this kind of environment. Maybe the goal
could be only for a "specific" support, for the journal for example.

Marco
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