linux-ext4 - Re: how to disable readahead

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Date:   Mon, 30 Jul 2018 11:10:37 -0400
From:   "Theodore Y. Ts'o" <tytso@....edu>
To:     "Bean Huo (beanhuo)" <beanhuo@...ron.com>
Cc:     "linux-ext4@...r.kernel.org" <linux-ext4@...r.kernel.org>,
        "beanhuo.linux@...gle.com" <beanhuo.linux@...gle.com>
Subject: Re: how to disable readahead

On Mon, Jul 30, 2018 at 09:10:52AM +0000, Bean Huo (beanhuo) wrote:
> Hi, ext4 experts 
> For some cases, the storage device inside(FW) already has readahead feature.
> I want to disable ext4 readahead and do some test. And see what is the difference
> between these situations.
> But I tried "inode_readahead_blks" parameter to disable readahead, but it doesn't work.
> What else I need to set for disabling ext4 readahead?

inode_readahead_blks, as it is documented, only controls readahead of
inode table blocks.

If you want to control the data readahead, that's a general setting
that applies for all file systems, and it can be found by adjusting
the value in:

	/sys/block/<dev>/queue/read_ahead_kb

(where <dev> might be sda, sdb, etc.)

See https://cromwell-intl.com/open-source/performance-tuning/disks.html
for more details.

						- Ted