Date: Mon, 21 Feb 2005 16:41:51 -0800
From: Pete Zaitcev <zaitcev@redhat.com>
To: Linus Torvalds <torvalds@osdl.org>
Cc: Jens Axboe <axboe@suse.de>, linux-kernel@vger.kernel.org,
       zaitcev@redhat.com
Subject: Re: Merging fails reading /dev/uba1
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On Mon, 21 Feb 2005 12:00:48 -0800 (PST), Linus Torvalds <torvalds@osdl.org> wrote:

> That said, I'm surprised that the difference in performance is _that_ 
> large. Regardless of whether the disk blocksize is 512 bytes or 4096 
> bytes, you should be getting IO merging - it might use more CPU time, but 
> the actual IO should still be done in much larger blocks.

I am surprised too. Jens says "ub effectively disables merging by setting
max hw/phys segment limit of 1." But surely this ought not to be a problem
for reads within the same page.

> 	int size = 4096;
> 	ioctl(fd, BLKBSZSET, &size);

Thank you for the tip. This works fine, 4KB I/O is restored for dd.
However, I still have this problem with people who use ub to read CF sticks
from their cameras, mounted as FAT or VFAT. I verified that the effect of
this ioctl disappears at mount time, just as you said.

I'll think what I can do about it.

-- Pete
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