linux-kernel - Re: [PATCH v10 00/21] Support ext4 on NV-DIMMs

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Date:	Thu, 28 Aug 2014 00:04:40 +0100
From:	One Thousand Gnomes <gnomes@...rguk.ukuu.org.uk>
To:	Andrew Morton <akpm@...ux-foundation.org>
Cc:	Christoph Lameter <cl@...ux.com>,
	Matthew Wilcox <matthew.r.wilcox@...el.com>,
	linux-fsdevel@...r.kernel.org, linux-mm@...ck.org,
	linux-kernel@...r.kernel.org, willy@...ux.intel.com
Subject: Re: [PATCH v10 00/21] Support ext4 on NV-DIMMs

On Wed, 27 Aug 2014 14:30:55 -0700
Andrew Morton <akpm@...ux-foundation.org> wrote:

> On Wed, 27 Aug 2014 16:22:20 -0500 (CDT) Christoph Lameter <cl@...ux.com> wrote:
> 
> > > Some explanation of why one would use ext4 instead of, say,
> > > suitably-modified ramfs/tmpfs/rd/etc?
> > 
> > The NVDIMM contents survive reboot and therefore ramfs and friends wont
> > work with it.
> 
> See "suitably modified".  Presumably this type of memory would need to
> come from a particular page allocator zone.  ramfs would be unweildy
> due to its use to dentry/inode caches, but rd/etc should be feasible.

If you took one of the existing ramfs types you would then need to

- make it persistent in its storage, and put all the objects in the store
- add journalling for failures mid transaction. Your dimm may retain its
  bits but if your CPU reset mid fs operation its got to be recovered
- write an fsck tool for it
- validate it

at which point it's probably turned into ext4 8)

It's persistent but that doesn't solve the 'my box crashed' problem. 

Alan
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