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
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