Date: Thu, 7 May 2015 20:40:37 +0200
From: Ingo Molnar <mingo@kernel.org>
To: Dan Williams <dan.j.williams@intel.com>
Cc: Christoph Hellwig <hch@lst.de>,
        Linus Torvalds <torvalds@linux-foundation.org>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
        Boaz Harrosh <boaz@plexistor.com>, Jan Kara <jack@suse.cz>,
        Mike Snitzer <snitzer@redhat.com>, Neil Brown <neilb@suse.de>,
        Benjamin Herrenschmidt <benh@kernel.crashing.org>,
        Dave Hansen <dave.hansen@linux.intel.com>,
        Heiko Carstens <heiko.carstens@de.ibm.com>, Chris Mason <clm@fb.com>,
        Paul Mackerras <paulus@samba.org>, "H. Peter Anvin" <hpa@zytor.com>,
        Alasdair Kergon <agk@redhat.com>,
        "linux-nvdimm@lists.01.org" <linux-nvdimm@ml01.01.org>,
        Mel Gorman <mgorman@suse.de>, Matthew Wilcox <willy@linux.intel.com>,
        Ross Zwisler <ross.zwisler@linux.intel.com>,
        Rik van Riel <riel@redhat.com>,
        Martin Schwidefsky <schwidefsky@de.ibm.com>,
        Jens Axboe <axboe@kernel.dk>, "Theodore Ts'o" <tytso@mit.edu>,
        "Martin K. Petersen" <martin.petersen@oracle.com>,
        Julia Lawall <Julia.Lawall@lip6.fr>, Tejun Heo <tj@kernel.org>,
        linux-fsdevel <linux-fsdevel@vger.kernel.org>,
        Andrew Morton <akpm@linux-foundation.org>
Subject: Re: [PATCH v2 00/10] evacuate struct page from the block layer,
 introduce __pfn_t
Message-ID: <20150507184037.GA22952@gmail.com>
References: <20150506200219.40425.74411.stgit@dwillia2-desk3.amr.corp.intel.com>
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* 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
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