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From: Douglas Gilbert <dougg@torque.net>
To: Alan Stern <stern@rowland.harvard.edu>
Cc: Joerg Schilling <Joerg.Schilling@fokus.fraunhofer.de>,
	linux-kernel@vger.kernel.org, jens.axboe@oracle.com,
	James.Bottomley@steeleye.com
Subject: Re: [PATCH] Block layer: separate out queue-oriented ioctls
Date: Mon, 19 Feb 2007 23:47:51 -0500	[thread overview]
Message-ID: <45DA7D77.9020709@torque.net> (raw)
In-Reply-To: <Pine.LNX.4.44L0.0702192238110.4237-100000@netrider.rowland.org>

Alan Stern wrote:
> On Mon, 19 Feb 2007, Douglas Gilbert wrote:
> 
>> Alan,
>> The SG_GET_RESERVED_SIZE ioctl is also defined in
>> the block layer, see block/scsi_ioctl.c .
> 
> Ah, I didn't know that.  (Or more likely, I used to know and have since 
> forgotten.)  Thanks for pointing it out.
> 
>> I suspect it is just a kludge to fool cdrecord that it
>> is talking to a sg device. [One of many kludges in the
>> block SG_IO ioctl implementation to that end.]
>> So perhaps the block layer versions of SG_SET_RESERVED_SIZE
>> and SG_GET_RESERVED_SIZE need to be similarly capped.
> 
> Yes.  In fact one of them already is, but the other should be too.
> 
>> Actually I think that I would default SG_GET_RESERVED_SIZE to
>> request_queue->max_sectors * 512 in the block layer
>> implementation (as there is no "reserve buffer" associated
>> with a block device).
> 
> Okay.
> 
> 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


# 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).

Doug Gilbert



  reply	other threads:[~2007-02-20  4:48 UTC|newest]

Thread overview: 17+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2007-02-16 19:37 Alan Stern
2007-02-17  6:28 ` Jens Axboe
2007-02-17 21:18   ` Joerg Schilling
2007-02-18  3:43   ` Douglas Gilbert
2007-02-18 12:37     ` Joerg Schilling
2007-02-18 16:44     ` Alan Stern
2007-02-18 18:27       ` Joerg Schilling
2007-02-19 16:06         ` Alan Stern
2007-02-19 16:08           ` Joerg Schilling
2007-02-19 17:06             ` Alan Stern
2007-02-19 22:25               ` Douglas Gilbert
2007-02-20  3:48                 ` Alan Stern
2007-02-20  4:47                   ` Douglas Gilbert [this message]
2007-02-20 15:55                     ` Alan Stern
2007-04-26  9:19                 ` Joerg Schilling
2007-04-26 15:04                   ` Alan Stern
2007-04-26 15:08                     ` James Bottomley

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