[Lustre-discuss] more on MGS and MDT separation

[Bernd Schubert]

On Thursday 22 October 2009, Ms. Megan Larko wrote:
> Greetings,
> 
> I am deactivating an older Lustre filesystem in favor of a newer one
> (already up and stable).
> The message in Lustre-discuss Digest Vol 45 Issue 22 stated (with two
> of my comments in-line):
> 
> Message: 2
> Date: Sun, 11 Oct 2009 19:05:07 +0200
> From: Bernd Schubert <bs_lists at aakef.fastmail.fm>
> Subject: Re: [Lustre-discuss] Moving MGS to separate device
> To: lustre-discuss at lists.lustre.org
> Message-ID: <200910111905.08076.bs_lists at aakef.fastmail.fm>
> Content-Type: Text/Plain;  charset="iso-8859-15"
> 
> Hello Wojciech,
> 
> I already did this several times, here are the steps I so far used:
> 
> 1) Remove MGS from MDT-device
> 
> tunefs.lustre --nogms /dec/mdt_device
>                         ^^^^
>           Megan: I am assuming "--nomgs"  here.

Yes, sorry a typo.

> 
> 2) Create new MGS
> 
> mkfs.lustre --mgs /dev/mgs_device
> 
> 3) Make sure OSTs and  MDTs re-register with the MGS:
> 
> tunefs.lustre --writeconf  /dev/device
> 
>          Megan:  Do I need to do this even if the MGS is being moved
> from a shared device with an MDT to its own device/hard drive on the
> same physical server (same MAC addr, IP, hostname etc.....)?

I did it to make sure MDT and OSTs re-register with the MGS, so to make sure 
the MGS really knows about them. In the end the MGS is only there to know 
about OSTs and MDTs and to provide those information to clients. I think on 
mounting of MDTs and OSTs they always contact the MGS, but if the MGS is not 
available they will give up, so it **might** happen that they wouldn't contact 
the MGS and so wouldn't be registered. --writeconf will ensure that.
As I also wrote, it might work to simply copy the CONFIGS directory, but I 
didn't test that yet.

> 
> I'm not sure if writeconf is really necessary, but so far I always did it
>  to make sure everything goes smoothly (clients shouldn't have the
>  filesystem mounted at this step).
> 
> 5) Mount MGS, MDT, OSTs
> 
> 4) Re-apply settings done with lctl.
> 
>          Megan: Why are the above ordered the way they are?  Shouldn't
> I mount first and then apply the settings?   (I didn't think I could
> "lctl" an unmounted OST/MDT etc.)

And another typo, actually even two. lctl settings can be applied only with 
the filesystem being mounted. So 

4) Mount MGS, MDT, OSTs

5) Re-apply settings done with lctl.



Cheers,
Bernd

-- 
Bernd Schubert
DataDirect Networks