Re: md question re: max_hw_sectors_kb

[NeilBrown <neilb@suse.de>]

On Tue, 03 May 2011 15:20:46 -0500 Michael Reed <mdr@sgi.com> 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@sgi.com>
> 
> 
> --- 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