[Lustre-discuss] questions about an OST content

[Aurélien Degrémont]

Le 08/11/2010 21:04, Andreas Dilger a écrit :
> Looks like you didn't copy the old "CONFIGS/mountdata" file over the new one.  You can also use "--writeconf" (described in the manual and several times on the list) to have the MGS re-generate the configuration, which should fix this as well.
>    

Tell me if I'm wrong regarding this OST update.
AFAIK, there is two ways to replace an OST by a new one:

Hot replace:
1 - Disable your OST on MDT (lctl deactivate)
2 - Empty your OST
3 - Backup the magic files (last_rcvd, LAST_ID, CONFIG/*)
4 - Deactivate the OST on all clients also.
5 - Unmount the OST
6 - Replace, reformat using same index
7 - Put back the backup magic files.
8 - Restart the OST.
9 - Activate the OST everywhere.

Cold replace:
1 - Empty your OST
2 - Stop your filesystem
3 - Replace/reformat using the same index
4 - Restart using --writeconf
5 - Remount the clients

Did I miss something ?

As far as i understand this, the important point here is to have the OST 
internal information in sync with what the MGS (CONFIG/*) and the MDT 
(last_rcvd, LAST_ID) knows.
What is currently preventing, a freshly formatted OST with the same 
index, to register itself properly (using first_time flag) to MGS and 
MDT when remounting and:
  - refreshing its CONFIG from MGS internal cache
  - telling MDT to reset last_rcvd/LAST_ID it knows for this OST.
That way, we could have an easy way to hot replace an OST.
How do you think this can be achieved ?

Thanks

Aurélien