Date:	Thu, 14 Apr 2011 13:07:42 -0700
From:	Greg KH <greg@...ah.com>
To:	Nao Nishijima <nao.nishijima.xt@...achi.com>
Cc:	linux-kernel@...r.kernel.org, linux-scsi@...r.kernel.org,
	linux-hotplug@...r.kernel.org, Kay Sievers <kay.sievers@...y.org>,
	James Bottomley <James.Bottomley@...e.de>,
	Jon Masters <jcm@...hat.com>, 2nddept-manager@....hitachi.co.jp
Subject: Re: [PATCH 1/2] SCSI: Add a SCSI option for persistent device names
 in Kernel.

On Thu, Apr 14, 2011 at 05:15:36PM +0900, Nao Nishijima wrote:
> Hi,
> 
> (2011/04/09 1:12), Greg KH wrote:
> > On Fri, Apr 08, 2011 at 11:07:41PM +0900, Nao Nishijima wrote:
> >> Hi,
> >>
> >> (2011/04/06 1:14), Greg KH wrote:
> >>> On Tue, Apr 05, 2011 at 09:49:46PM +0900, Nao Nishijima wrote:
> >>>> This patch series provides a SCSI option for persistent device
> >>>> names in kernel. With this option, user can always assign a
> >>>> same device name (e.g. sda) to a specific device according to
> >>>> udev rules and device id.
> >>>>
> >>>> Issue:
> >>>> Recently, kernel registers block devices in parallel. As a result,
> >>>> different device names will be assigned at each boot time. This
> >>>> will confuse file-system mounter, thus we usually use persistent
> >>>> symbolic links provided by udev. However, dmesg and procfs outputs
> >>>> show device names instead of the symbolic link names. This causes
> >>>> a serious problem when managing multiple devices (e.g. on a
> >>>> large-scale storage), because usually, device errors are output
> >>>> with device names on dmesg. We also concern about some commands
> >>>> which output device names, as well as kernel messages.
> >>>>
> >>>> Solution:
> >>>> To assign a persistent device name, I create unnamed devices (not
> >>>> a block device, but an intermediate device. users cannot read/write
> >>>> this device). When scsi device driver finds a LU, the LU is registered
> >>>> as an unnamed device and inform to udev. After udev finds the unnamed
> >>>> device, udev assigns a persistent device name to a specific device
> >>>> according to udev rules and registers it as a block device. Hence,
> >>>> this is just a naming, not a renaming.
> >>>>
> >>>> Some users are satisfied with current udev solution. Therefore, my
> >>>> proposal is implemented as an option.
> >>>>
> >>>> If using this option, add the following kernel parameter.
> >>>>
> >>>> 	scsi_mod.persistent_name=1
> >>>>
> >>>> Also, if you want to assign a device name with sda, add the
> >>>> following udev rules.
> >>>>
> >>>> SUBSYSTEM=="scsi_unnamed", ATTR{disk_id}=="xxx", PROGRAM="/bin/sh
> >>>> -c 'echo -n sda > /sys/%p/disk_name'"
> >>>
> >>> Also, where is the "real" program you have created to properly name
> >>> devices from userspace?  You need that to properly test this patch,
> >>> right?
> >>>
> >>
> >> In the udev rule described above, notation “xxx” indicated by
> >> ATTR(disk_id) is scsi id given by disk. Then, when udev finds this rule,
> >> "/bin/sh -c 'echo -n sda>  /sys/%p/disk_name'" indicated by PROGRAM is
> >> operated using xxx (scsi id) if udev find the disk with scic id xxx.
> >> Thus, persistent device name is assigned.
> >>
> >> I have tested this patch using the udev rule. and It works well.
> > 
> > That's nice, but it's a "toy".  What have you used to test this with
> > 20000 scsi devices to properly work like it does today?  Where is the
> > program that will name them in a deterministic manner?
> > 
> > We have that functionality today, you can't break it.
> > 
> > thanks,
> 
> Indeed, I have not (can not, of course) tested 20000 scsi devices to
> properly work.

Why not?  You should be able to do this easily with a laptop these days
with the dummy scsi device code.

> This feature targets only scsi disk/cdrom/tape devices.
> Not all devices. We expect the target users are system administrators
> who need to control all devices for maintenance. They know all devices
> connected to the server and those devices will tested before connecting.

But that's not the large majority of the people using Linux.  You need a
sane default, which this does not provide.

> We expect that admin will enable this option after installation. This
> means that device names are assigned by current mechanism when
> installation. Admin enables the option and makes udev rules (we are
> planning to make shell script which generate udev rules use by-id and
> assigned device names). In other word, this feature will be used just
> for "fixing" the names.

So admins are going to write their own udev rules by hand?  I find that
very unlikely.

> In our scenario, admin appends a new rule manually when a new LU is
> added. In the future, we are planning to enhance udev to append a new
> rule automatically as like as NIC.

I recommend doing the other options that have been proposed please.

thanks,

greg k-h
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