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    SubjectRe: [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@google.com> wrote:
    > > On Fri, Mar 26, 2010 at 8:20 AM, Vivek Goyal <vgoyal@redhat.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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