Re: [PATCH v2 00/10] evacuate struct page from the block layer, introduce __pfn_t

[Ingo Molnar <mingo@kernel.org>]

* Dan Williams <dan.j.williams@intel.com> wrote:

> On Thu, May 7, 2015 at 9:18 AM, Christoph Hellwig <hch@lst.de> wrote:
> > On Wed, May 06, 2015 at 05:19:48PM -0700, Linus Torvalds wrote:
> >> What is the primary thing that is driving this need? Do we have a very
> >> concrete example?
> >
> > FYI, I plan to to implement RAID acceleration using nvdimms, and I 
> > plan to ue pages for that.  The code just merge for 4.1 can easily 
> > support page backing, and I plan to use that for now.  This still 
> > leaves support for the gigantic intel nvdimms discovered over EFI 
> > out, but given that I don't have access to them, and I dont know 
> > of any publically available there's little I can do for now.  But 
> > adding on demand allocate struct pages for the seems like the 
> > easiest way forward.  Boaz already has code to allocate pages for 
> > them, although not on demand but at boot / plug in time.
> 
> Hmmm, the capacities of persistent memory that would be assigned for 
> a raid accelerator would be limited by diminishing returns.  I.e. 
> there seems to be no point to assign more than 8GB or so to the 
> cache? [...]

Why would that be the case?

If it's not a temporary cache but a persistent cache that hosts all 
the data even after writeback completes then going to huge sizes will 
bring similar benefits to using a large, fast SSD disk on your 
desktop... The larger, the better. And it also persists across 
reboots.

It could also host the RAID write intent bitmap (the dirty 
stripes/chunks bitmap) for extra speedups. (This bitmap is pretty 
small, but important to speed up resyncs after crashes or power loss.)

Thanks,

	Ingo