linux-kernel - Re: Block-level access

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Date:   Thu, 1 Sep 2016 15:16:06 -0700
From:   Bart Van Assche <bart.vanassche@...disk.com>
To:     Alex Austin <aaustin@...antechonline.com>,
        <linux-kernel@...r.kernel.org>, <linux-scsi@...r.kernel.org>,
        <linux-block@...r.kernel.org>
Subject: Re: Block-level access

On 09/01/2016 02:48 PM, Alex Austin wrote:
> CC'ing linux-scsi and linux-block.
>
> Also, please CC me in replies.
>
> On Thu, Sep 1, 2016 at 4:43 PM, Alex Austin <aaustin@...antechonline.com> wrote:
>> Hello,
>> What is the most performant way to directly interface with an attached hard
>> drive? I've so far used read()/write() on /dev/sd_ but I find error handling
>> exceedingly difficult, as I don't always even get errors reported, even if the
>> open() call includes O_DIRECT. I've also used ioctl(SG_IO), but find that it's
>> extremely slow due to the lack of queuing support in the API. Is there a
>> mid-level API that will get me decent error handling while allowing command
>> queuing, or do I just need to make multiple threads all doing separate SG_IO
>> ioctls?

What the most efficient way is to interface is with a hard drive depends 
on the I/O pattern. Are you aware that buffered I/O performs read-ahead? 
Have you already tried to tune the read-ahead setting (blockdev --getra 
/ blockdev --setra)?

BTW, if you need an example of how to queue SG_IO, you are welcome to 
have a look at the fio source code. You will probably be interested in 
the bsg API. See e.g. https://lwn.net/Articles/174469/.

Bart.