linux-ext4 - Re: how to disable readahead

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Date:   Thu, 2 Aug 2018 13:55:31 +0000
From:   "Bean Huo (beanhuo)" <beanhuo@...ron.com>
To:     "Theodore Y. Ts'o" <tytso@....edu>
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

Hi, Ted

>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

I am newbie on ext4, I tried the above method to disable readahead, echo 0 > /sys/block/<dev>/queue/read_ahead_kb
Then I read by 128kB chunk size, ext4 will read the file by 4KB chunk size each time. that means ext4 splits 128KB into 32 4KB to read.
That's not my expectation.
Do you know how to still keep and let ext4 read by 128KB in case of disable readahead?
Thanks.

//bean huo