Forum for asking questions related to block device drivers

[neha naik]

Yes. Interestingly my direct write i/o performance is better than my direct
read i/o performance for my passthrough device... And that doesn't make any
kind of sense to me.

pdev0 = pass through device on top of lvm

root at voffice-base:/home/neha/sbd# time dd if=/dev/pdev0 of=/dev/null
bs=4096 count=1024 iflag=direct
1024+0 records in
1024+0 records out
4194304 bytes (4.2 MB) copied, 4.09488 s, 1.0 MB/s

real    0m4.100s
user    0m0.028s
sys    0m0.000s

root at voffice-base:/home/neha/sbd# time dd if=/dev/shm/image of=/dev/pdev0
bs=4096 count=1024 oflag=direct
1024+0 records in
1024+0 records out
4194304 bytes (4.2 MB) copied, 0.0852398 s, 49.2 MB/s

real    0m0.090s
user    0m0.004s
sys    0m0.012s

Thanks,
Neha

On Thu, Apr 11, 2013 at 11:53 AM, Rajat Sharma <fs.rajat at gmail.com> wrote:

> 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
> >>
> >> >
> >> > _______________________________________________
> >> > Kernelnewbies mailing list
> >> > Kernelnewbies at kernelnewbies.org
> >> > http://lists.kernelnewbies.org/mailman/listinfo/kernelnewbies
> >> >
> >
> >
>
-------------- next part --------------
An HTML attachment was scrubbed...
URL: http://lists.kernelnewbies.org/pipermail/kernelnewbies/attachments/20130411/8e26c147/attachment.html