linux-ext4 - Re: [PATCH V8 1/8] mm/fs: cleancache documentation

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Date:	Sat, 16 Apr 2011 03:54:39 +0900
From:	OGAWA Hirofumi <hirofumi@...l.parknet.co.jp>
To:	Dan Magenheimer <dan.magenheimer@...cle.com>
Cc:	chris.mason@...cle.com, viro@...iv.linux.org.uk,
	akpm@...ux-foundation.org, adilger.kernel@...ger.ca, tytso@....edu,
	mfasheh@...e.com, jlbec@...lplan.org, matthew@....cx,
	linux-btrfs@...r.kernel.org, linux-kernel@...r.kernel.org,
	linux-fsdevel@...r.kernel.org, linux-ext4@...r.kernel.org,
	ocfs2-devel@....oracle.com, linux-mm@...ck.org, hch@...radead.org,
	ngupta@...are.org, jeremy@...p.org, JBeulich@...ell.com,
	kurt.hackel@...cle.com, npiggin@...nel.dk,
	dave.mccracken@...cle.com, riel@...hat.com, avi@...hat.com,
	konrad.wilk@...cle.com, mel@....ul.ie, yinghan@...gle.com,
	gthelen@...gle.com, torvalds@...ux-foundation.org
Subject: Re: [PATCH V8 1/8] mm/fs: cleancache documentation

Dan Magenheimer <dan.magenheimer@...cle.com> writes:

> [PATCH V8 1/8] mm/fs: cleancache documentation
>
> This patchset introduces cleancache, an optional new feature exposed
> by the VFS layer that potentially dramatically increases page cache
> effectiveness for many workloads in many environments at a negligible
> cost.  It does this by providing an interface to transcendent memory,
> which is memory/storage that is not otherwise visible to and/or directly
> addressable by the kernel.
>
> Instead of being discarded, hooks in the reclaim code "put" clean
> pages to cleancache.  Filesystems that "opt-in" may "get" pages 
> from cleancache that were previously put, but pages in cleancache are 
> "ephemeral", meaning they may disappear at any time. And the size
> of cleancache is entirely dynamic and unknowable to the kernel.
> Filesystems currently supported by this patchset include ext3, ext4,
> btrfs, and ocfs2.  Other filesystems (especially those built entirely
> on VFS) should be easy to add, but should first be thoroughly tested to
> ensure coherency.
>
> Details and a FAQ are provided in Documentation/vm/cleancache.txt
>
> This first patch of eight in this cleancache series only adds two
> new documentation files.

Another question: why can't this enable/disable per sb, e.g. via mount
options? (I have the interest the cache stuff like this by SSD on
physical systems like dragonfly's swapcache.)

Well, anyway, I guess force enabling this for mostly unused sb can just
add cache-write overhead and call for unpleasing reclaim to backend
(because of limited space of backend) like updatedb.

And already there is in FAQ though, I also have interest about async
interface because of SDD backend (I'm not sure for now though). Is there
any plan like SSD backend?

Thanks.
-- 
OGAWA Hirofumi <hirofumi@...l.parknet.co.jp>
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