Forum for asking questions related to block device drivers

[Rajat Sharma]

Hi,

On Thu, Apr 11, 2013 at 2:23 AM, neha naik <nehanaik27 at gmail.com> wrote:
> Hi All,
>    Nobody has replied to my query here. So i am just wondering if there is a
> forum for block device driver where i can post my query.
> Please tell me if there is any such forum.
>
> Thanks,
> Neha
>
> ---------- Forwarded message ----------
> From: neha naik <nehanaik27 at gmail.com>
> Date: Tue, Apr 9, 2013 at 10:18 AM
> Subject: Passthrough device driver performance is low on reads compared to
> writes
> To: kernelnewbies at kernelnewbies.org
>
>
> Hi All,
>   I have written a passthrough block device driver using 'make_request'
> call. This block device driver simply passes any request that comes to it
> down to lvm.
>
> However, the read performance for my passthrough driver is around 65MB/s
> (measured through dd) and write performance is around 140MB/s for dd block
> size 4096.
> The write performance matches with lvm's write performance more or less but,
> the read performance on lvm is around 365MB/s.
>
> I am posting snippets of code which i think are relevant here:
>
> static int passthrough_make_request(
> struct request_queue * queue, struct bio * bio)
> {
>
>         passthrough_device_t * passdev = queue->queuedata;
>         bio->bi_bdev = passdev->bdev_backing;
>         generic_make_request(bio);
>         return 0;
> }
>
> For initializing the queue i am using following:
>
> blk_queue_make_request(passdev->queue, passthrough_make_request);
> passdev->queue->queuedata = sbd;
> passdev->queue->unplug_fn = NULL;
> bdev_backing = passdev->bdev_backing;
> blk_queue_stack_limits(passdev->queue, bdev_get_queue(bdev_backing));
> if ((bdev_get_queue(bdev_backing))->merge_bvec_fn) {
>         blk_queue_merge_bvec(sbd->queue, sbd_merge_bvec_fn);
> }
>

What is the implementation for sbd_merge_bvec_fn? Please debug through
it to check requests are merging or not? May be that is the cause of
lower performance?

> Now, I browsed through dm code in kernel to see if there is some flag or
> something which i am not using which is causing this huge performance
> penalty.
> But, I have not found anything.
>
> If you have any ideas about what i am possibly doing wrong then please tell
> me.
>
> Thanks in advance.
>
> Regards,
> Neha
>

-Rajat

>
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