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Date: Thu, 7 May 2015 12:44:17 -0700
Message-ID: <CAPcyv4ghDSB=s7HgCzT+j-x3YiLu5QLxWziMybwVCBbWijBGdg@mail.gmail.com>
Subject: Re: [PATCH v2 00/10] evacuate struct page from the block layer,
 introduce __pfn_t
From: Dan Williams <dan.j.williams@intel.com>
To: Ingo Molnar <mingo@kernel.org>
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>,
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On Thu, May 7, 2015 at 11:40 AM, Ingo Molnar <mingo@kernel.org> wrote:
>
> * 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.

True, that's more "dm-cache" than "RAID accelerator", but point taken.
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