Message-ID: <20100327002814.GC9280@redhat.com>
Date:	Fri, 26 Mar 2010 20:28:14 -0400
From:	Vivek Goyal <vgoyal@...hat.com>
To:	Divyesh Shah <dpshah@...gle.com>
Cc:	Chad Talbott <ctalbott@...gle.com>,
	Gui Jianfeng <guijianfeng@...fujitsu.com>,
	jens.axboe@...cle.com, mrubin@...gle.com,
	Li Zefan <lizf@...fujitsu.com>, linux-kernel@...r.kernel.org,
	Nauman Rafique <nauman@...gle.com>
Subject: Re: [PATCH 0/4] io-controller: Use names rather than major:minor

On Fri, Mar 26, 2010 at 04:21:41PM -0700, Divyesh Shah wrote:
> On Fri, Mar 26, 2010 at 3:54 PM, Chad Talbott <ctalbott@...gle.com> wrote:
> > On Fri, Mar 26, 2010 at 8:20 AM, Vivek Goyal <vgoyal@...hat.com> wrote:
> >> On Fri, Mar 26, 2010 at 09:31:41AM +0800, Gui Jianfeng wrote:
> >>> +int blk_lookup_devname(dev_t devt, char *name)
> >>> +{
> >
> > [ snip... loop through all block devices for devt ...snip ]
> >
> >>> So we can keep dev_t in blkio layer, and export to user a device name by calling
> >>> this function. Also, we retrive device number by calling blk_lookup_devt().
> >>> This change might keep things much simple. Jens, do you have any thoughts?
> >>>
> >> I agree with Gui that lets keep the dev_t the core in blkio layer. Keeping
> >> a pointer to gendisk in request queue is becoming little messy.
> >
> > Agreed on leaving gendisk pointer out of request_queue.  In doing
> > further investigation, I've found that it's up to the driver to
> > maintain the association between gendisk and request_queue, and some
> > drivers put multiple gendisk behind a single request_queue, so the
> > back pointer would be ill-specified.
> >
> >> But if that does not work for you, then I would also like to keep things
> >> simple and translate dev_t to diskname during read routine. Similiarly,
> >> while somebody is putting policy, use blk_lookup_devt().
> >
> > I like the simplicity of blk_lookup_devt(), but I don't like the idea
> > of iterating through all block devices on every lookup of the name.
> > Perhaps we could cache the name somewhere?
> >
> > Actually, the name is the name of the *queue* (or the key in
> > blk-cgroup), because as I mentioned above there can be a many to one
> > relationship between disks and queues in general.
> >
> > The more I think about it, the more it seems to make sense to extend
> > blkio_policy_ops to include a function to get the name of the key.
> > blk-cgroup makes no current use of the dev, except to invent a name
> > for the request_queue whose policy is being set or printed.  It could
> > be argued that the thing being scheduled has a better idea of the name
> > of that thing.
> >
> >> But this will lead to issue of how do you now display both device number
> >> and disk name in the output. May be following.
> >>
> >> major:minor  diskname  data
> >>
> >> I am not sure if people are fond of multiple values in a single file. At
> >> the same time for setting the rules or deleting the rules, it will make
> >> syntax complicated/confusing. Also will require breaking ABI for existing
> >> blkio.time, blkio.sectors, blkio.dequeue files.
> >
> > I don't like this, either.  It breaks ABI and is more confusing for users.
> >
> >> So I would prefer to keep the major/minor number based interface for
> >> follwing reasons.
> >>
> >> - Chaning it now breaks ABI.
> >> - Other cgroup controller "device" is also using major/minor number based
> >>  interface for device access policy. So it is consistent with other
> >>  controller.
> >
> > Which controllers are these?
> >
> >> - Displaying both device major/minor and diskname is an option but that
> >>  makes the file format syntax little complicated and new rule setting
> >>  or removoal confusing.
> >
> > A few messages back you mentioned that you preferred device names
> > because they would be better for users of the system.  If there was a
> > simple implementation, would you still be behind a new name-based
> > interface?  We could go that direction and maintain ABI by deprecating
> > current interface and making a new interface with names.
> >
> > If you can't tell, I'm a big fan of using the name! :)  It's *much*
> > more consistent with the interfaces in /sys.
> 
> I agree with Chad here. The major/minor number interface to me seems
> like a departure from convention as /proc/diskstat,

Both /proc/diskstats and /proc/partitions list first major/minor and
then diskname. So why do you think it is departuture from convention?

> /sys/block all use
> the device names at the kernel-user interface.

/sys provides multiple ways to access samve device. Both using disknames
as well as major:minor number (/sys/dev/block/major:minor).

Vivek

>About deprecating the
> current ABI, we could do that but do we expect a lot of user tools to
> be built around this interface since the 2.6.33 release already?
> 
> -Divyesh
> 
> >
> > Chad
> >
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