Re: [PATCH] md: allow changing set_name of running array

[NeilBrown <neilb@suse.com>]

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On Mon, Sep 04 2017, Michał Mirosław wrote:

> On Mon, Sep 04, 2017 at 12:22:33PM +1000, NeilBrown wrote:
>> On Fri, Sep 01 2017, Michał Mirosław wrote:
>> 
>> > On Fri, Sep 01, 2017 at 10:07:29AM +1000, NeilBrown wrote:
>> >> On Wed, Aug 30 2017, Michał Mirosław wrote:
>> >> > Allow changing active array's set_name. This is the only way to
>> >> > safely update superblock on an array which carries a mounted fs.
>> >> 
>> >> Do you really need to change the set_name of an active array?
>> >> 
>> >> The name is only used when the array is actived, so wait until the next
>> >> time the array is stopped, and change the name then.
>> >> 
>> >> You can boot with a rescue CD or similar and use "--assemble
>> >> --update=name", or with a bit of effort you could get the normal boot
>> >> sequence to change the name.
>> >> 
>> >> I wouldn't object to adding something to mdadm so that it would read
>> >> something from mdadm.conf, and update the set name at boot time.
>> >> 
>> >> What is the underlying problem that you are trying to solve here?
>> >
>> > I had to fix /dev/md* naming on a system with no physical access.
>> >
>> > The problem was that despite matching mdadm.conf entries, arrays started
>> > with random 127-i indexes (because superblocks' set_names didn't match
>> > hostname, I guess).
>> 
>> That's odd.  If an array is listed in mdadm.conf, that is enough to tell
>> mdadm that it is "local" so that it doesn't need the hostname to match.
>> Can you show me exactly what was in your mdadm.conf?
>
> This is what was I had when it would reassign arrays on boot. System
> hostname is 'rere' - it was renamed some time after creating arrays.
>
> ---8<---
> HOMEHOST <system>
> MAILADDR root
> ARRAY /dev/md/0  metadata=1.2 UUID=d5ea39a9:5ec27c36:c83296d4:70f879d4 name=rere2:0
> ARRAY /dev/md/1  metadata=1.2 UUID=9bf69b92:2e7ba905:5f959c09:c8fecb99 name=rere2:1
> ---8<---
>
> I don't have old versions of mdadm.conf, so had to recreate this one.
> IIRC I verified UUIDs back then, but don't remember whether I modified
> names or not. Thinking of it, how would mdadm behave if it had matching
> UUIDs and mismatching array names in mdadm.conf?

That's what I was wondering and part of why I asked to see the
mdadm.conf.

If a name is given in mdadm.conf and it doesn't match the name in the
metadata, that line will be ignored.  If mdadm doesn't find a match in
mdadm.conf and the hostname doesn't match, the the array will be treated
as "foreign" and will be assembled as e.g. /dev/md127 with a symlink
from /dev/md/0_1.
If the name and uuid in mdadm.conf do match the metadata, it will be
treat as "local" and will be given the name that you would expect
(/dev/md0, /dev/md/0).

So my guess is that the mdadm.conf you had at the time doesn't match
what you have provided above.

It would be nice to add an option to "mdadm --incremental" to tell it to
update various details like you can with "mdadm --assemble"....

NeilBrown

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