linux-kernel - Re: [PATCH] mm: use only per-device readahead limit

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Message-ID: <CA+55aFyc8bb=ASmQbhk72cFOOmGpNhowdWGtSn+biog69_f+LA@mail.gmail.com>
Date:	Fri, 21 Aug 2015 11:17:43 -0700
From:	Linus Torvalds <torvalds@...ux-foundation.org>
To:	Roman Gushchin <klamm@...dex-team.ru>
Cc:	linux-mm <linux-mm@...ck.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Raghavendra K T <raghavendra.kt@...ux.vnet.ibm.com>,
	Jan Kara <jack@...e.cz>, Wu Fengguang <fengguang.wu@...el.com>,
	David Rientjes <rientjes@...gle.com>,
	Andrew Morton <akpm@...ux-foundation.org>
Subject: Re: [PATCH] mm: use only per-device readahead limit

On Fri, Aug 21, 2015 at 10:12 AM, Roman Gushchin <klamm@...dex-team.ru> wrote:
>
> There are devices, which require custom readahead limit.
> For instance, for RAIDs it's calculated as number of devices
> multiplied by chunk size times 2.

So afaik, the default read-ahead size is 128kB, which is actually
smaller than the old 512-page limit.

Which means that you probably changed "ra_pages" somehow. Is it some
system tool that does that automatically, and if so based on what,
exactly?

I'm also slightly worried about the fact that now the max read-ahead
may actually be zero, and/or basically infinite (there's a ioctl to
set it that only tests that it's not negative). Does everything react
ok to that?

               Linus
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