RE: [PATCH] sd: preserve sysfs updates to max_sectors_kb

Don Brace
2017-08-29T22:32:09Z

________________________________________
From: Bart Van Assche [Bart.VanAssche@wdc.com]
Sent: Tuesday, August 29, 2017 3:25 PM
To: hch@infradead.org; Viswas G; Gerry Morong; Mahesh Rajashekhara; POSWALD@suse.com; Scott Benesh; Don Brace; Bader Ali - Saleh; Kevin Barnett; joseph.szczypek@hpe.com; Scott Teel; jejb@linux.vnet.ibm.com; Justin Lindley; John Hall
Cc: linux-scsi@vger.kernel.org
Subject: Re: [PATCH] sd: preserve sysfs updates to max_sectors_kb

EXTERNAL EMAIL

On Tue, 2017-08-29 at 19:41 +0000, Don Brace wrote:
> BLKRRPART ends up in the sd driver function sd_revalidate_disk, which resets
> q->limits.max_sectors = min(rw_max, queue_max_hw_sectors(q));
> back to the value from VPD page 0xb0.
>
> When you cat out max_sectors_kb, it obtains it's valuse from q->limits.max_sectors
>
> Adding a udev rule... Will there be a delay between the watch rule and the new rule?

Hello Don,

In your original e-mail you wrote that max_sectors_kb was tuned because
of performance reasons. So does it really matter that for a very short
time max_sectors_kb does not have the optimal value?

> One more point, what will I change the value back to? The watch rule
> changed it and I do not have the original value.

To the value you want to set max_sectors_kb to.

Bart.

Users do set max_sectors_kb for performance reasons. They can increase
performance by setting max_sectors_kb to different values depending on
their I/O needs.

And, if they set the value, and sd_revalidate_disk changes this value,
there is no history of what they set the value to.

Thanks,
Don Brace
Microsemi Corp.