Forum for asking questions related to block device drivers

[Rajat Sharma]

so you mean direct I/O read of your passthrough device is lower than
direct I/O read of lvm?

On Thu, Apr 11, 2013 at 8:39 PM, neha naik <nehanaik27 at gmail.com> wrote:
> Hi,
>  I am calling the merge function of the block device driver below me(since
> mine is only pass through). Does this not work?
> When i tried seeing what read requests were coming then i saw that when i
> issue dd with count=1 it retrieves 4 pages,
> so i tried with 'direct' flag. But even with direct io my read performance
> is way lower than my write performance.
>
> Regards,
> Neha
>
>
> On Wed, Apr 10, 2013 at 11:15 PM, Rajat Sharma <fs.rajat at gmail.com> wrote:
>>
>> 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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>> > Kernelnewbies at kernelnewbies.org
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>> >
>
>