Safest way to renumber RAID-5 devices

Safest way to renumber RAID-5 devices

[Ian Pilcher]

Hi all!

I have several RAID-5 devices that I would like to renumber
(incrementing each minor number by 1).

Is it possible to do this without re-creating them?  I'd like to avoid
unnecessary re-syncs as well as the possibility of screwing up.

Thanks!
-- 
========================================================================
Ian Pilcher                                           pilchman@attbi.com
========================================================================

Re: Safest way to renumber RAID-5 devices

[Neil Brown]

On Saturday June 28, pilchman@attbi.com wrote:
> Hi all!
> 
> I have several RAID-5 devices that I would like to renumber
> (incrementing each minor number by 1).
> 
> Is it possible to do this without re-creating them?  I'd like to avoid
> unnecessary re-syncs as well as the possibility of screwing up.
> 

mdadm -A --update=superminor /dev/mdNEWNUMBER  /dev/sd...

should work with mdadm 1.2.0.

Obviously you have to stop the array first as "mdadm -A" starts a
stopped array.

Check the manpage to make sure I haven't mispelled somthing.

NeilBrown

Re: Safest way to renumber RAID-5 devices

[Ian Pilcher]

Neil Brown wrote:
> 
> mdadm -A --update=superminor /dev/mdNEWNUMBER  /dev/sd...
> 
> should work with mdadm 1.2.0.
> 

Works like a charm.  I didn't realize that Red Hat was shipping a
back level version (in addition to still using the old raidtools in
their init scripts).

> 
> Check the manpage to make sure I haven't mispelled somthing.
> 

You forgot the hyphen in super-minor.  :-)

Thanks!
-- 
========================================================================
Ian Pilcher                                           pilchman@attbi.com
========================================================================

Re: Really need RESYNC at creation of array???

[Neil Brown]

On Monday June 30, bo@anthologysolutions.com wrote:
> Hi Neil & Raiders,
> 
> 
> 
> I am not quite sure if we really need to resync the array at the time of 
> 
> RAID 1 or RAID 5 array creation. I tried and tested without RESYNC by
> 
> modifying "case1" and "case2" from MDADM source, I could not notice
> 
> any problems or issues.
> 
> 
> 
> So I want to know from you what is the reason for these codes, and 
> 
> what you experieced from without RESYNC.
> 
> 

Others have answered with very relevant arguments for an initial sync.

For raid1 it is not absolutely essential, but for the reasons others
have mentioned it can be a good idea.

For raid5 it is essential:  you do not have any guarantee of data
integrity after a drive fails without it.

Consider:
   You start with a raid5 array where all the parity blocks are wrong.
   You write to a block on disk A and update the parity using a 
   read-modify-write cycle.  i.e.
      pre-read A.  pre-read Parity.
      subtract old A from parity.  Add new A into parity
      write new A, write new parity.
    The parity block is still wrong with respect to all other data
    blocks in that strip.
   Now drive A fails.  You have lost the block that you wrote there.


I would not be against adding a flag to mdadm which caused it to
create a raid1 array without any initial resync.  I would be against
allowing it for raid5.

NeilBrown