linux-ext4 - Re: how to disable readahead

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Message-ID: <5965fa79-7e43-2c77-4d4b-9bf6cb5a18ca@thelounge.net>
Date:   Thu, 2 Aug 2018 17:22:09 +0200
From:   Reindl Harald <h.reindl@...lounge.net>
To:     "Bean Huo (beanhuo)" <beanhuo@...ron.com>,
        Jack Wang <jack.wang.usish@...il.com>
Cc:     "tytso@....edu" <tytso@....edu>,
        "linux-ext4@...r.kernel.org" <linux-ext4@...r.kernel.org>,
        "beanhuo.linux@...gle.com" <beanhuo.linux@...gle.com>
Subject: Re: how to disable readahead



Am 02.08.2018 um 17:01 schrieb Bean Huo (beanhuo):
>>> 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
>> I guess you're doing buffered IO, have you try to use direct IO?
>>
> Direct I/O is another case, it is true that will bypass readahead.
> But I want to disable readahead with page cache.

don't get me wrong but when you state "I am newbie on ext4" this all
sounds like "having solution, seeking for problem"