linux-kernel - Re: [PATCH] mm: readahead: remove redundant ra_pages in file_ra

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Message-ID: <20121025023808.GA23462@localhost>
Date:	Thu, 25 Oct 2012 10:38:08 +0800
From:	Fengguang Wu <fengguang.wu@...el.com>
To:	YingHang Zhu <casualfisher@...il.com>
Cc:	Dave Chinner <david@...morbit.com>, akpm@...ux-foundation.org,
	linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org,
	linux-mm@...ck.org
Subject: Re: [PATCH] mm: readahead: remove redundant ra_pages in file_ra_state

Hi YingHang,

> Actually I've talked about it with Fengguang, he advised we should unify the
> ra_pages in struct bdi and file_ra_state and leave the issue that
> spreading data
> across disks as it is.
> Fengguang, what's you opinion about this?

Yeah the two ra_pages may run out of sync for already opened files,
which could be a problem for long opened files. However as Dave put
it, a device's max readahead size is typically a static value that can
be set at mount time. So, the question is: do you really hurt from the
old behavior that deserves this code change?

I agree with Dave that the multi-disk case is not a valid concern.  In
fact, how can the patch help that case? I mean, if it's two fuse files
lying in two disks, it *was* not a problem at all. If it's one big
file spreading to two disks, it's a too complex scheme to be
practically manageable which I doubt if you have such a setup. 

Thanks,
Fengguang
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