Re: 2.6.15: mdrun, udev -- who creates nodes?

[Bill Davidsen <davidsen@tmr.com>]

linas wrote:

>On Tue, Jan 31, 2006 at 04:40:46PM +0000, Jason Lunz was heard to remark:
>  
>
>>md@Linux.IT said:
>>    
>>
>>>>-- kernel scans /dev/hda1, looking for md superblock
>>>>-- kernel assembles devices according to info found in the superblocks
>>>>-- udev creates /dev/md0, etc. 
>>>>        
>>>>
>>>The problem is that some users and distributions build the drivers as
>>>modules and/or disable in-kernel auto-assembly.
>>>      
>>>
>>Not only that, the raid developers themselves consider autoassembly
>>deprecated.
>>
>>http://article.gmane.org/gmane.linux.kernel/373620
>>    
>>
>
>Hmm. My knee-jerk, didn't-stop-to-think-about-it reaction is that 
>this is one of the finest features of linux raid, so why remove it?
>
>Speaking as a real-life sysadmin, with actual servers and actual failed
>disks, disk cables and disk controllers, this is a life-saving feature. 
>Persistant naming of devices in Linux has long been a problem, and in
>this case, it seemed to work.
>
><story>
>I once had an ide controller fail on an x86 board. I bought a new 
>controller at the local store, recabled the disks, and booted. 
>I was alarmed to find that the system was trying to mount /home 
>as /usr, and /usr as /lib, etc. Turned out that /dev/hdc had  
>gotten renamed as /dev/hde, etc. and  had to go through a long,
>painful, rocket-science (yes, I *do* have a PhD) boot-floppy rescue
>to restore the system to working order.
> 
>I shudder to think what would have happened if RAID reconstruction 
>had started based on faulty device names. Worse, as part of my rescue
>ops, I had to make multle copies of /etc/fstab, which resided on
>different disks (my root volume was raided), as well as the boot 
>floppy, and each contained inconsistent info (needed to bootstrap 
>my way back). Along the way, I made multiple errors in editing 
>the /etc/fstab since I could not keep them straight; twiddling 
>BIOS settings added to the confusion.  If this had been /etc/raid.conf 
>instead, with reconstruction triggered off of it, this could have 
>been an absolute disaster.
></story>
>
>Based on the above, real-life experience, my gut reaction is 
>raid assembly based on config files is a bad idea. I don't 
>understand how innocent, "minor" errors made by the sysadmin 
>won't result in catastrophic data loss.
>

I fear you don't understand how the auto detect and assemble works. Or 
more to the point what it does, since how it works is somewhat more 
complex. If you use partitions and UUID, you can just plug in the drives 
any old place and they will be found and recognised in spite of that. As 
long as you have a boot drive where the BIOS will use it, mdadm with 
find your stuff and put it together correctly. Neil does more magic than 
harry Potter!

I know someone who gave this a real life test, although I'd not say who.

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



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