Auto-assembling arrays using mdadm

Auto-assembling arrays using mdadm

[Sean Puttergill]

Hi.

I am looking to script auto-assembly of all arrays
detected on an arbitrary set of drives using mdadm.

The way I am thinking of doing this is as follows:

mdadm.conf =

DEVICE /dev/sd*

----

1. Use "mdadm -E" on each drive in turn to get the
UUID for the array to which it belongs and assemble a
list of unique UUIDs.

2. Run "mdadm -A --uuid=<UUID> --auto=md --scan" for
each unique UUID.


My question is, is it currently possible for me to get
mdadm to do this automatically?  Ideally I would
invoke  mdadm once to discover all RAID arrays that
exist on the devices listed in the mdadm.conf file and
assemble them, automatically creating the require md
devices.

I know that I could use kernel RAID autodetect, but I
hear that this is to be deprecated soon.  Also, the
RAID arrays I am using live on bare drives, not
partitions (i.e. no partition to set to type 0xFD).


Thanks,
Sean


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Re: Auto-assembling arrays using mdadm

[dean gaudet]

On Thu, 9 Mar 2006, Sean Puttergill wrote:

> Hi.
> 
> I am looking to script auto-assembly of all arrays
> detected on an arbitrary set of drives using mdadm.
> 
> The way I am thinking of doing this is as follows:
> 
> mdadm.conf =
> 
> DEVICE /dev/sd*

i prefer to use "DEVICE partitions"

> 1. Use "mdadm -E" on each drive in turn to get the
> UUID for the array to which it belongs and assemble a
> list of unique UUIDs.
> 
> 2. Run "mdadm -A --uuid=<UUID> --auto=md --scan" for
> each unique UUID.
> 
> 
> My question is, is it currently possible for me to get
> mdadm to do this automatically?

the man page has an example... it's just:

	mdadm --assemble --scan

-dean

Re: Auto-assembling arrays using mdadm

[Sean Puttergill]

As far as I know, "mdadm --assemble --scan" requires
that there be ARRAY lines in the mdadm.conf file. 
These lines must list the device node and the UUID of
the array at a minimum.

What I want to achieve is to automatically inspect a
set of disks and start all the RAID arrays found
without the need for any a priori information in the
mdadm.conf file.  I.e. with mdadm.conf as follows:

"
DEVICE /dev/sd*
"

This is the kind of functionality provided by kernel
RAID autodetect.  You don't have to have any config
information provided in advance.  The kernel finds and
assembles all arrays on disks with RAID autodetect
partition type.  I want to do the same thing, but with
mdadm.

Perhaps I am confused.  Has this changed in 2.x?  I am
using version 1.11.0


Sean

--- dean gaudet <dean@arctic.org> wrote:

> On Thu, 9 Mar 2006, Sean Puttergill wrote:
> 
> > Hi.
> > 
> > I am looking to script auto-assembly of all arrays
> > detected on an arbitrary set of drives using
> mdadm.
> > 
> > The way I am thinking of doing this is as follows:
> > 
> > mdadm.conf =
> > 
> > DEVICE /dev/sd*
> 
> i prefer to use "DEVICE partitions"
> 
> > 1. Use "mdadm -E" on each drive in turn to get the
> > UUID for the array to which it belongs and
> assemble a
> > list of unique UUIDs.
> > 
> > 2. Run "mdadm -A --uuid=<UUID> --auto=md --scan"
> for
> > each unique UUID.
> > 
> > 
> > My question is, is it currently possible for me to
> get
> > mdadm to do this automatically?
> 
> the man page has an example... it's just:
> 
> 	mdadm --assemble --scan
> 
> -dean
> 


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Re: Auto-assembling arrays using mdadm

[dean gaudet]

On Thu, 9 Mar 2006, Sean Puttergill wrote:

> This is the kind of functionality provided by kernel
> RAID autodetect.  You don't have to have any config
> information provided in advance.  The kernel finds and
> assembles all arrays on disks with RAID autodetect
> partition type.  I want to do the same thing, but with
> mdadm.

you know i've been bitten by that several times... the kernel sees the 
autodetect partition type of a disk which used to belong in another box 
(which for various reasons i've not yet been able to zero the old raid 
superblock)... and brings up a raid minor which conflicts with another 
array already present in the system... which causes device renaming to 
occur and can mess up the boot.  (although if i'm using UUIDs or labels 
for mounting the filesystems it can almost work -- there are still a few 
cases where those don't help... such as xfs external log partition.)

i suppose what i suggested doesn't do what you want... but i prefer not 
using kernel autoassembly these days because of the above problem.

the 1.12 man page has more examples which can help you...

	echo "DEVICE partitions" >tmp.mdadm.conf
	mdadm --detail --scan --config=tmp.mdadm.conf >>tmp.mdadm.conf
	mdadm --assemble --scan --config=tmp.mdadm.conf

i think that'll do what you want...

-dean

Re: Auto-assembling arrays using mdadm

[Sean Puttergill]

That did the trick, except you must do:

mdadm --examine --scan --config=tmp.mdadm.conf
>>tmp.mdadm.conf

instead of:

mdadm --detail --scan --config=tmp.mdadm.conf
>>tmp.mdadm.conf

I assume that this method would have the same problems
with conflicting minor number.  Will have to add an
intermediate step to process the tmp.mdadm.conf file
to uniquify the RAID array device names (i.e. remove
duplicate /dev/md0, etc.).


Thanks for the tip.  I personally think mdadm should
be modified to do this all in one step (including
dealing with minor number conflicts).


Sean

--- dean gaudet <dean@arctic.org> wrote:

> you know i've been bitten by that several times...
> the kernel sees the 
> autodetect partition type of a disk which used to
> belong in another box 
> (which for various reasons i've not yet been able to
> zero the old raid 
> superblock)... and brings up a raid minor which
> conflicts with another 
> array already present in the system... which causes
> device renaming to 
> occur and can mess up the boot.  (although if i'm
> using UUIDs or labels 
> for mounting the filesystems it can almost work --
> there are still a few 
> cases where those don't help... such as xfs external
> log partition.)
> 
> i suppose what i suggested doesn't do what you
> want... but i prefer not 
> using kernel autoassembly these days because of the
> above problem.
> 
> the 1.12 man page has more examples which can help
> you...
> 
> 	echo "DEVICE partitions" >tmp.mdadm.conf
> 	mdadm --detail --scan --config=tmp.mdadm.conf
> >>tmp.mdadm.conf
> 	mdadm --assemble --scan --config=tmp.mdadm.conf
> 
> i think that'll do what you want...
> 
> -dean
> -
> To unsubscribe from this list: send the line
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> More majordomo info at 
> http://vger.kernel.org/majordomo-info.html
> 


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Re: Auto-assembling arrays using mdadm

[Neil Brown]

On Thursday March 9, puttergi@yahoo.ca wrote:

It is not clear to me that this is something that you would ever want
to do, just as you would not want an automatic process to mount every
filesystem that it finds on any disk drive. (I agree with dean here).

Maybe if you explained why you wanted to do this, a more focussed
response could be provided.


> That did the trick, except you must do:
> 
> mdadm --examine --scan --config=tmp.mdadm.conf
> >>tmp.mdadm.conf
> 
> instead of:
> 
> mdadm --detail --scan --config=tmp.mdadm.conf
> >>tmp.mdadm.conf
> 
> I assume that this method would have the same problems
> with conflicting minor number.  Will have to add an
> intermediate step to process the tmp.mdadm.conf file
> to uniquify the RAID array device names (i.e. remove
> duplicate /dev/md0, etc.).
> 
> 
> Thanks for the tip.  I personally think mdadm should
> be modified to do this all in one step (including
> dealing with minor number conflicts).

This is unlikely to happen.

It is, however, possible that "mdadm --examine --scan" could grow an option
whereby the device names are given as
  /dev/md/UUID:OF:THE:ARRAY:THAT:WAS:FOUND

This, toogether with giving 'auto=mdp' could avoid and issues with
minor number conflicts.  Then:
  echo DEV partitions > /etc/mdadm.conf
  mdadm -Es --new-flag >> /etc/mdadm.conf
  mdadm -As --auto-mdp
would assemble all arrays, which is what you claim to want.

NeilBrown

Re: Auto-assembling arrays using mdadm

[Sean Puttergill]

I definitely wish to do this, that's why I'm asking. 
I assume your statement refers to the general
usefulness of what I am asking for.

As to why I wish to implement this functionality, well
you can tell that I am not targetting this at your
average user's PC.  Not to be too secretive, but think
along the lines of something more appliance like. 
Amongst the requirements that I have for the system
that I am constructing, I must deal with installing
RAID sets that come from other boxes.  I need to be
able to automatically start up existing RAID arrays on
drive sets that have migrated over without needing to
have any a priori knowledge of the RAID arrays that
might exist on the drives.

In any case, your suggestion of a new flag to output
unique device names from mdadm --examine (using UUID
of array) sounds like an acceptable solution.  Please
consider adding it.

I assume that using "--auto=md" when assembling will
currently not take care of minor number conflicts in
the config file?


Thanks,
Sean

--- Neil Brown <neilb@suse.de> wrote:

> On Thursday March 9, puttergi@yahoo.ca wrote:
> 
> It is not clear to me that this is something that
> you would ever want
> to do, just as you would not want an automatic
> process to mount every
> filesystem that it finds on any disk drive. (I agree
> with dean here).
> 
> Maybe if you explained why you wanted to do this, a
> more focussed
> response could be provided.
> 
> 
> > That did the trick, except you must do:
> > 
> > mdadm --examine --scan --config=tmp.mdadm.conf
> > >>tmp.mdadm.conf
> > 
> > instead of:
> > 
> > mdadm --detail --scan --config=tmp.mdadm.conf
> > >>tmp.mdadm.conf
> > 
> > I assume that this method would have the same
> problems
> > with conflicting minor number.  Will have to add
> an
> > intermediate step to process the tmp.mdadm.conf
> file
> > to uniquify the RAID array device names (i.e.
> remove
> > duplicate /dev/md0, etc.).
> > 
> > 
> > Thanks for the tip.  I personally think mdadm
> should
> > be modified to do this all in one step (including
> > dealing with minor number conflicts).
> 
> This is unlikely to happen.
> 
> It is, however, possible that "mdadm --examine
> --scan" could grow an option
> whereby the device names are given as
>   /dev/md/UUID:OF:THE:ARRAY:THAT:WAS:FOUND
> 
> This, toogether with giving 'auto=mdp' could avoid
> and issues with
> minor number conflicts.  Then:
>   echo DEV partitions > /etc/mdadm.conf
>   mdadm -Es --new-flag >> /etc/mdadm.conf
>   mdadm -As --auto-mdp
> would assemble all arrays, which is what you claim
> to want.
> 
> NeilBrown
> 


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