Re: [PATCH] Block layer: separate out queue-oriented ioctls

[Alan Stern <stern@rowland.harvard.edu>]

On Mon, 19 Feb 2007, Douglas Gilbert wrote:

> > Come to think of it, the reserved_size value used when a new sg device is
> > created should also be capped at max_sectors * 512.  Agreed?  I can't see
> > any reason for ever having a larger buffer -- it would be impossible to
> > make use of the extra space.
> 
> Alan,
> That depends whether or not max_sectors can be changed
> (via sysfs) subsequent to a sg device being created.
> And I think it can.
> 
> # ls -l /sys/block/sdc/queue/
> total 0
> drwxr-xr-x 2 root root    0 Feb 19 18:29 iosched
> -r--r--r-- 1 root root 4096 Feb 19 23:41 max_hw_sectors_kb
> -rw-r--r-- 1 root root 4096 Feb 19 23:41 max_sectors_kb
> -rw-r--r-- 1 root root 4096 Feb 19 23:41 nr_requests
> -rw-r--r-- 1 root root 4096 Feb 19 23:41 read_ahead_kb
> -rw-r--r-- 1 root root 4096 Feb 19 23:41 scheduler

Yes, it definitely can be changed.

> # cat max_hw_sectors_kb > max_sectors_kb
> 
> ... is the real maximum if the LLD that set max_hw_sectors_kb
> is to be believed (actually it is often a finger in
> the wind).

That's why my patch computes the minimum value every time the 
GET_RESERVED_SIZE ioctl runs -- in case max_sectors has changed.

If the user decides to increase max_sectors, then the reserved_size can be 
increased immediately afterward.  This shouldn't cause any problems.

I will submit a revised patch shortly, on the linux-scsi list.

Alan Stern