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

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Message-Id: <20090422145050.C00333780DD@ns.ariel-networks.com>
Date:	Wed, 22 Apr 2009 23:50:50 +0900 (JST)
From:	ohyama_sec@...el-networks.com
To:	linux-kernel@...r.kernel.org
Subject: Question : internal processing of block I/O queue

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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