linux-kernel - Re: [PATCH v2 00/10] evacuate struct page from the block layer, introduce __pfn

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Message-ID: <20150507184037.GA22952@gmail.com>
Date:	Thu, 7 May 2015 20:40:37 +0200
From:	Ingo Molnar <mingo@...nel.org>
To:	Dan Williams <dan.j.williams@...el.com>
Cc:	Christoph Hellwig <hch@....de>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Boaz Harrosh <boaz@...xistor.com>, Jan Kara <jack@...e.cz>,
	Mike Snitzer <snitzer@...hat.com>, Neil Brown <neilb@...e.de>,
	Benjamin Herrenschmidt <benh@...nel.crashing.org>,
	Dave Hansen <dave.hansen@...ux.intel.com>,
	Heiko Carstens <heiko.carstens@...ibm.com>,
	Chris Mason <clm@...com>, Paul Mackerras <paulus@...ba.org>,
	"H. Peter Anvin" <hpa@...or.com>, Alasdair Kergon <agk@...hat.com>,
	"linux-nvdimm@...ts.01.org" <linux-nvdimm@...ts.01.org>,
	Mel Gorman <mgorman@...e.de>,
	Matthew Wilcox <willy@...ux.intel.com>,
	Ross Zwisler <ross.zwisler@...ux.intel.com>,
	Rik van Riel <riel@...hat.com>,
	Martin Schwidefsky <schwidefsky@...ibm.com>,
	Jens Axboe <axboe@...nel.dk>, Theodore Ts'o <tytso@....edu>,
	"Martin K. Petersen" <martin.petersen@...cle.com>,
	Julia Lawall <Julia.Lawall@...6.fr>, Tejun Heo <tj@...nel.org>,
	linux-fsdevel <linux-fsdevel@...r.kernel.org>,
	Andrew Morton <akpm@...ux-foundation.org>
Subject: Re: [PATCH v2 00/10] evacuate struct page from the block layer,
 introduce __pfn_t


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

> On Thu, May 7, 2015 at 9:18 AM, Christoph Hellwig <hch@....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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