Re: [PATCH 2.6.20-rc6] md: expose uuid and degraded attributes in sysfs

[Bill Davidsen <davidsen@tmr.com>]

Neil Brown wrote:
> On Saturday February 10, davidsen@tmr.com wrote:
>   
>> Iustin Pop wrote:
>>     
>>> On Sat, Jan 27, 2007 at 02:59:48AM +0100, Iustin Pop wrote:
>>>   
>>>       
>>>> From: Iustin Pop <iusty@k1024.org>
>>>>
>>>> This patch exposes the uuid and the degraded status of an assembled
>>>> array through sysfs.
>>>>     
>>>>         
>>> [...]
>>>
>>> Sorry to ask, this was my first patch and I'm not sure what is the
>>> procedure to get it considered for merging... I was under the impression
>>> that just sending it to this list is enough. What do I have to do?
>>>       
>> Normally I would expect Neil to pick it up and forward it, but I would 
>> have expected an ack from him. He's been busy with other problems, NFS 
>> as I recall, and may have missed it, so I cc'd him this time.
>>
>> Neil, I would think this is 2.6.21 material unless you see a problem I 
>> missed.
>>
>>     
>
> yeh - sorry about that. mail is somewhat of a lossy protocol for me.
> I usually try to reply, but sometimes it just doesn't happen.
> Resending after a suitable pause (1-2 weeks) is never a bad idea.
>
> Exposing the 'degraded' status is probably a good idea.  I'll take
> that.
>
> Exposing the UUID isn't - and if it were it should be in
> "md_default_attrs" rather than "md_redundancy_attrs".
>   
I don't think moving it would get much argument, but having it visible 
has advantages as noted in the original post, and opens the door to 
someone writing code to allow the uuid to be changed with a write to a 
sys file. We had a discussion of setting uuid a few months ago, I think 
you agreed it was a reasonable thing to do.
> The UUID isn't an intrinsic aspect of the array.  It is simply part of
> the metadata that is used to match up different devices from the same
> array.
> I plan to add support for the 'DDF' metadata format (an 'industry
> standard') and that will be managed entirely in user-space.  The
> kernel won't know the uuid at all.
>   
Outside of forcing changes for all of us using uuid, what does this 
standard compliance buy us as users? Or you as a maintainer? Does this 
let Linux get run on a million computers which can't now because of lack 
or standard compliance? I'm always worried when things change without a 
visible benefit, so feel free to make the benefits visible. Managed in 
user space is not a big item for me, it means there will be more more 
places for errors to creep in.
> So any solution for easy access to uuids should be done in user-space.
> Maybe mdadm could create a link
>    /dev/md/by-uuid/xxxxxxxx -> /dev/whatever.
> ??
>
>   
Which would be kept in sync how when the uuid is changed?

I'm not trying to argue with you, just trying to cover the benefits and 
possible exposures from the change.

-- 
bill davidsen <davidsen@tmr.com>
  CTO TMR Associates, Inc
  Doing interesting things with small computers since 1979