Passthrough device driver performance is low on reads compared to writes

[neha naik]

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
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