* Dan Williams <dan.j.willi...@intel.com> wrote:
> On Thu, May 7, 2015 at 9:18 AM, Christoph Hellwig <h...@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
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