Re: Merging fails reading /dev/uba1

[Jeff Garzik <jgarzik@pobox.com>]

On Mon, Feb 21, 2005 at 10:24:31AM -0800, Pete Zaitcev wrote:
> On Mon, 21 Feb 2005 08:51:32 +0100, Jens Axboe <axboe@suse.de> wrote:
> 
> > > [root@lembas ~]# time dd if=/dev/uba of=/dev/null bs=10k count=10240
> > > real    0m22.731s
> 
> > > [root@lembas ~]# time dd if=/dev/uba1 of=/dev/null bs=10k count=10240
> > > real    1m42.622s
> 
> > > So, reading from a partition of the same device is 5 times slower than
> > > reading from the device itself. The question is, why?
> 
> > I can't explain why the replugging slows it down, maybe you were lucky
> > to get contigious pages in the first case? As far as I can see, ub
> > effectively disables merging by setting max hw/phys segment limit of 1.
> 
> If you mean physical replugging, it has nothing to do with the issue.
> I only mentioned it to show that old pages were purged.
> 
> Contiguous pages have nothing to do with it either. I forgot to mention
> that in the first case (whole device), all reads are done with length of
> 4KB, while in the second case (partition), all reads are 512 bytes long.
> 
> Basically, the key is reading from a partition or not. It causes the
> sub-page sized merging to fail.

Does setting the blkdev's block size change things?

	Jeff