[Lustre-discuss] MDT and MGS

[Brian J. Murrell]

On Mon, 2008-09-29 at 11:28 -0400, Jeff Darcy wrote:
>    
> Actually they demonstrate setting up a separate MDT to use an existing 
> MGS.

Are we talking about the same thing here?  In the commands:

mgsnode# mkfs.lustre --mgs /dev/sda
mdtfoonode# mkfs.lustre --fsname=foo --mdt --mgsnode=mgsnode at tcp0 /dev/sda
ossfoonode# mkfs.lustre --fsname=foo --ost --mgsnode=mgsnode at tcp0 /dev/sda
ossfoonode# mkfs.lustre --fsname=foo --ost --mgsnode=mgsnode at tcp0 /dev/sdb
mdtbarnode# mkfs.lustre --fsname=bar --mdt --mgsnode=mgsnode at tcp0 /dev/sda
ossbarnode# mkfs.lustre --fsname=bar --ost --mgsnode=mgsnode at tcp0 /dev/sda
ossbarnode# mkfs.lustre --fsname=bar --ost --mgsnode=mgsnode at tcp0 /dev/sdb

The MGS is completely separate from any MDT.  It is in fact on a
separate node even.

>  There's even a note that says "specify --mdt --mgs on one, and 
> --mdt --mgsnode=/<mgsnodenid>/ on the others"

Indeed.

> which would still run into 
> the same problem should one ever need to reformat that first MGS/MDT.  

Right.

> That won't direct somebody who has "even a notion of wanting more than 
> one filesystem" toward creating a separate MGS.  If it's something you 
> guys feel should be recommended for all or nearly all cases, it needs to 
> be presented that way in even the early examples.

Fair enough.  Can you file a documentation ticket on this?

> That's why I said an *existing* filesystem - i.e. one existing before 
> and therefore separate from any MDTs you create.

So this would be a non-lustre filesystem?  If you want failover MGS
capability, this filesystem will need to be accessible from multiple
(i.e. MGS) nodes.

> Sure, if you reformat 
> the filesystem containing the MGS data you'll still have to do a 
> (trivial) save and restore, but that will always be the case no matter 
> where the MGS data goes.  At least customers wouldn't find themselves 
> facing the problem that started the thread, where reformatting the MDT 
> will rather unexpectedly mean reformatting their MGS as well.

I agree that this is a problem.  I'm just trying to propose a(n
unnecessarily complicated) solution.

> Yes, it is, but no more so than the MGS-on-private-storage case 
> currently.

If the MGS is on separate storage, there is no action required (as
opposed to the idea of moving it around depending on what you want to
format) so, yes, it does seem to me to be less cumbersome.

>  More importantly, it avoids the cumbersome requirement to 
> devote a whole separate block device to this role.

Hrm.  That doesn't seem "cumbersome".  Maybe wasteful, yes.

Anyway, I've proposed how one can resolve this problem with current
implementations of Lustre.  Anyone is of course free to request any
further features or enchantments one wishes via our bugzilla system.

b.

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