linux-ext4 - Re: [PATCH, RFC 0/3] Introduce new O_HOT and O

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Date:	Sat, 21 Apr 2012 19:56:05 -0400
From:	KOSAKI Motohiro <kosaki.motohiro@...il.com>
To:	"Ted Ts'o" <tytso@....edu>
Cc:	James Bottomley <James.Bottomley@...senpartnership.com>,
	Lukas Czerner <lczerner@...hat.com>,
	Boaz Harrosh <bharrosh@...asas.com>,
	linux-fsdevel@...r.kernel.org,
	Ext4 Developers List <linux-ext4@...r.kernel.org>,
	linux-mm@...ck.org
Subject: Re: [PATCH, RFC 0/3] Introduce new O_HOT and O_COLD flags

On Fri, Apr 20, 2012 at 10:58 AM, Ted Ts'o <tytso@....edu> wrote:
> On Fri, Apr 20, 2012 at 06:42:08PM +0400, James Bottomley wrote:
>>
>> I'm not at all wedded to O_HOT and O_COLD; I think if we establish a
>> hint hierarchy file->page cache->device then we should, of course,
>> choose the best API and naming scheme for file->page cache.  The only
>> real point I was making is that we should tie in the page cache, and
>> currently it only knows about "hot" and "cold" pages.
>
> The problem is that "hot" and "cold" will have different meanings from
> the perspective of the file system versus the page cache.  The file
> system may consider a file "hot" if it is accessed frequently ---
> compared to the other 2 TB of data on that HDD.  The memory subsystem
> will consider a page "hot" compared to what has been recently accessed
> in the 8GB of memory that you might have your system.  Now consider
> that you might have a dozen or so 2TB disks that each have their "hot"
> areas, and it's not at all obvious that just because a file, or even
> part of a file is marked "hot", that it deserves to be in memory at
> any particular point in time.

So, this have intentionally different meanings I have no seen a reason why
fs uses hot/cold words. It seems to bring a confusion.

But I don't know full story of this feature and I might be overlooking
something.
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