Re: Block-level access

[Bart Van Assche]

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@xxxxxxxxxxxxxxxxxxx> 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.