Re: [RFC PATCH 0/4] Persistent device name using alias name

[James Bottomley]

On Fri, 2011-07-08 at 09:04 -0700, Greg KH wrote:
> On Fri, Jul 08, 2011 at 10:54:11AM -0500, James Bottomley wrote:
> > On Fri, 2011-07-08 at 08:47 -0700, Greg KH wrote:
> > > On Fri, Jul 08, 2011 at 05:41:36PM +0200, Kay Sievers wrote:
> > > > On Fri, Jul 8, 2011 at 16:54, Greg KH <greg@xxxxxxxxx> wrote:
> > > > > On Fri, Jul 08, 2011 at 05:45:47PM +0900, Nao Nishijima wrote:
> > > > >> This patch series provides an "alias name" of the disk into kernel and procfs
> > > > >> messages. The user can assign a preferred name to an alias name of the device.
> > > > >>
> > > > >> Based on previous discussion (*), I changed patches as follows
> > > > >> - This is "alias name"
> > > > >> - An "alias name" is stored in gendisk struct
> > > > >> - Add document to Documentation/ABI/testing/sysfs-block
> > > > >> - When the user changes an "alias name", kernel notifies udev
> > > > >>
> > > > >> (*) http://marc.info/?l=linux-scsi&m=130812625531219&w=2
> > > > >
> > > > > I don't like it and I don't think it will really solve the root problem
> > > > > you are trying to address, but as the patches don't touch any code I
> > > > > maintain, there's not much I can do to object to it.
> > > > 
> > > > I can only repeat what I already wrote in detail earlier:
> > > > 
> > > > This approach seems to papers over the problem which emitting and
> > > > parsing free-text printk() messages with much-too-dumb tools cause. It
> > > > seems to fix the symptoms not the cause.
> > > > 
> > > > You can already write a udev rule today that logs _all_ symlinks of a
> > > > device at discovery time, and any later kernel message can safely be
> > > > associated with all possible names of the blockdev. No kernel changes
> > > > needed, all possible names are covered. That also works good enough
> > > > with our current stone-age tools for anybody who is able to scroll
> > > > back to the last log udev message in that same log file.
> > > > 
> > > > There can be by-definition no default udev rules assigning a proper
> > > > single name to a block device. There is never a valid single name for
> > > > a disks, so udev can not ship anything like that in the default setup,
> > > > so this stays as a custom hack.
> > > > 
> > > > We absolutely need _structured_ data for logging and error reporting,
> > > > not only to solve this problem. Along with the current free-text
> > > > printk(), we would be able to attach classifications, device
> > > > error/sense data, firmware register dumps and anything
> > > > interesting-for-debug to the messages.
> > > > 
> > > > We can't solve that problem in the kernel alone. Structured data from
> > > > the kernel will need to feed a smarter userspace logger that can index
> > > > and classify messages, merge current userspace data into it, and
> > > > provides hooks for the system management to act on critical failures
> > > > and raise notifications.
> > > > 
> > > > Structured logging seems like the solution for this and also to many
> > > > other problems in this area. Single custom names pushed into the
> > > > kernel might cover some rather exotic use cases, but I think, is not
> > > > what we are looking for.
> > > 
> > > I totally agree, but hey, no one listens to us :)
> > 
> > Yes, we did.  Everyone agrees structured logging would be the best long
> > term solution.  However, it's at least 10x the work presented here, plus
> > it would be a long process getting everyone to agree.  This looks like a
> > good 95% interim solution and it can be removed when structured logging
> > makes everything "just work(tm)".
> 
> Do you seriously think that this sysfs attribute can be removed someday
> in the future if we get structured logging implemented?  It can't,
> sorry, people will start depending on this and their jury-rigged
> implementations will require it no matter what we do otherwise.

Yes, it can.  The way we do that is to make udev the interface to this
and the sysfs file becomes an internal udev implementation, not an
external ABI.  Any program using it rather than udev that breaks, well,
tough, it used the wrong interface.

> But that's still not the main issue here.
> 
> The main issue that I see is that this really doesn't solve the problem
> that is trying to be addressed, and the authors admit that.  So for that
> very reason alone I feel it should be rejected, but again, it's not my
> subsystem to maintain or control.
> 
> best of luck,

Thanks,

James


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