On Wed, 1 Feb 2012 15:10:17 -0500
Vivek Goyal [off-list ref] wrote:

On Wed, Feb 01, 2012 at 04:18:07AM -0500, Christoph Hellwig wrote:

quoted

On Tue, Jan 31, 2012 at 10:36:53PM -0500, Vivek Goyal wrote:

quoted

I still see that IO is being submitted one page at a time. The only
real difference seems to be that queue unplug happening at random times
and many a times we are submitting much smaller requests (40 sectors, 48
sectors etc).

This is expected given that the block device node uses
block_read_full_page, and not mpage_readpage(s).

What is the difference between block_read_full_page() and
mpage_readpage().

block_read_full_page() will attach buffer_heads to the page and will
perform IO via those buffer_heads.  mpage_readpage() feeds the page
directly to the BIO layer and leaves it without attached buffer_heads.

IOW, why block device does not use mpage_readpage(s)
interface?

We've tried it in the past and problems ensued.  A quick google search
for blkdev_readpages turns up stuff like
http://us.generation-nt.com/answer/patch-add-readpages-support-block-devices-help-201462802.html

Applying following patch improved the speed from 110MB/s to more than
230MB/s.

Yeah.  It should be doable - it would be a matter of hunting down and
squishing the oddball corner cases.

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