---------- Forwarded message ----------
From: neha naik <nehanaik27@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@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);
}

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