On Fri, 6 May 2011 14:24:34 +1000 NeilBrown <neilb@xxxxxxx> wrote:
> On Tue, 03 May 2011 15:20:46 -0500 Michael Reed <mdr@xxxxxxx> wrote:
>
> > Resending to linux-raid.
> >
> >
> > Hi Neil,
> >
> > My name is Mike Reed. I work at SGI. I've been looking at a performance
> > issue with md writes and have tracked it down to the md device not having
> > a high enough initial max_hw_sectors_kb setting.
> >
> > There is code in blk_queue_make_request() which lowers the default value
> > from INT_MAX to BLK_SAFE_MAX_SECTORS, which is 255. This is generally
> > lower than all the underlying devices with which I use md.
> >
> > As md appears to be a stacking driver, i.e., it calls disk_stack_limits()
> > for each member of a volume, it would seem reasonable for md to use the,
> > INT_MAX setting for max_hw_sectors_kb instead of BLK_SAFE_MAX_SECTORS.
> >
> > I have tried this, and have observed that md correctly limits the md device's
> > max_sectors_kb to the value of the underlying devices in my mirror volume.
> >
> > Is this the correct way to address this issue?
> >
> > Applies to linux-2.6.39-rc4.
> >
> > Signed-off-by: Michael Reed <mdr@xxxxxxx>
> >
> >
> > --- drivers/md/md.c 2011-04-18 21:26:00.000000000 -0700
> > +++ drivers/md/md.c.new 2011-04-21 15:53:11.452536201 -0700
> > @@ -4328,6 +4328,7 @@ static int md_alloc(dev_t dev, char *nam
> > mddev->queue->queuedata = mddev;
> >
> > blk_queue_make_request(mddev->queue, md_make_request);
> > + blk_queue_max_sectors(mddev->queue, INT_MAX);
> >
> > disk = alloc_disk(1 << shift);
> > if (!disk) {
> >
>
> Hi Mike,
> sorry for not responding to this earlier - it seemed to keep falling through
> cracks for some reason :-(
>
> Yes, I completely agree with your analysis and think you patch is correct and
> useful.
> I will queue it up for the next merge window.
>
> thanks,
> NeilBrown
...except of course that it should be
blk_queue_max_hw_sectors(......)
^^
because blk_queue_max_sectors was renamed back in 2.6.34 :-)
NeilBrown
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