[Lustre-discuss] MDT and MGS

[Brian J. Murrell]

On Mon, 2008-09-29 at 10:34 -0400, Jeff Darcy wrote:
>    
> I just checked, and the current Lustre operations manual (v1.14, updated 
> September 19) does not seem to reflect that belief; even the "more 
> complex configurations" offer mkfs.lustre commands that give a combined 
> MGS/MDT.

But you will notice that in the specific section 4.2.2.6 "Running
Multiple Lustre Filesystems" they do demonstrate setting up a separate
MGS:

An installation with two filesystems could look like this:

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

Certainly it's not an absolute requirement, but for the corner case
being discussed here, the need to reformat a filesystem -- the one
co-located with the MGS, there is a problem.

> In any case, my experience has been that customers are often 
> reluctant to carve a whole LUN out of their disk arrays for so little 
> data,

Agreed.

> and partitioning can have undesirable performance impacts.

Also agreed.

> Has 
> any consideration been given to ways of storing the MGS information 
> within a file on an existing filesystem instead of requiring on a 
> separate block device?

How would that be any better than the co-located MGS/MDT situation where
you want to reformat the filesystem that has the configuration
information stored on it in a file?

> I'm tempted to suggest doing this via the 
> loopback driver, since the MGS data-change rate is so low anyway.

But you still have the problem of formatting that particular filesystem.
I suppose you could umount the loopback device, copy the file to a
different filesystem and remount the loopback device.  That just seems
cumbersome.

I think a better proposal would be to enhance mkfs.lustre to save and
restore the configuration data of the filesystems not being reformatted
when a combination MGS/MDT is being formatted.  Feel free to file a
bugzilla ticket requesting that.

b.

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