linux-kernel - Re: Question : internal processing of block I/O queue

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Message-ID: <4e5e476b0904220917p38be0cb3t665b11fe3ac7d765@mail.gmail.com>
Date:	Wed, 22 Apr 2009 18:17:32 +0200
From:	Corrado Zoccolo <czoccolo@...il.com>
To:	ohyama_sec@...el-networks.com
Cc:	linux-kernel@...r.kernel.org
Subject: Re: Question : internal processing of block I/O queue

Hi,
I think you should read: Documentation/block/barrier.txt
Without explicitly asking for barriers, block I/O doesn't guarantee
FIFO ordering (or any other ordering), since it would be too
inefficient on some devices.

Corrado


On Wed, Apr 22, 2009 at 4:50 PM,  <ohyama_sec@...el-networks.com> wrote:
> From: Hiroyasu OHYAMA
>
> I'm a student of Japan, and I diddle the Linux for joyment.
> Could I ask you a question about block I/O.
>
> The question is that "Does The linux block I/O processing follow a first-in first-out order"
>
> Now, I'm making layer under low level filesystem. And I wonna make it work as Cache of data like page cache.
> I could understood if I emit block I/O request with submit_bio(), the kernel delay it, and I could notice the result by .bi_end_io() method that I set.
>
> But I have something on my mind. "Does the kernel assure the order of block I/O ?"
>
> Once I issue a block I/O request to common I/O layer, the I/O processing is delayed. And the actual block I/O processing is executed by kernel thread.
> So user who commit block I/O never have to mind the sequence of delayed block I/O requests.
> But I want to know that "The block I/O processing executed first is executed earlient, or may not".
>
> If a I/O request issued back is issued earlier than before, I have to protect the target block that is issued block I/O request.
>
> Could you please give me the actuall operation of block I/O component, that is "It may happen, or not".
>
> Thank you in advance.
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