[lustre-discuss] MDT expansion

[Peter Grandi]

 > I want to use Data on Metadata. Before DOM I need to expand Metadata 
pool size. Backend file System is ZFS.

 > What is the best strategy for expansion of MDT pool?

Somewhat approximately, Lustre (like most other shared filesystem types) 
does different layouts by *directory*. So you actually have two options:

* Expand in-place the existing MDT.
* Add a new MDT for DOM associated with a specific directory.

 > ZFS supports online expansion. Do you recommend online expansion

ZFS surely supports online expansion of "zpools" by adding "vdevs", 
which is very solid (and very recent ZFS versions allow even adding *to* 
"vdevs", which is a bit hairy).

Note: adding "vdevs" to a "zpool" may not be necessarily what you want 
to do, unless it is "zpool" of *mirror* "vdevs".

Note: expanding a "zpool" by adding "vdevs" does not re-balance the data.

The question is whether mounting a ZFS filesystem with '-t lustre' 
somewhat means that "zpool" containing that filesystem should not be 
touched.

 > or do you recommend to stop Lustre beforehand?

I am about to post a question related to that. I think that the answer 
is "not necessary", but I would "lct barrier_freeze" just in case.
Also even if one wants to stop, there is no need to stop the OSTs, just 
un-mount the MDT filesystem.

 > [...] Zfs 0.7

That's a bit ancient...