[gpfsug-discuss] File system descriptor quorum

[Simon Thompson]

I’ve been trying to get my head around how GPFS file-system descriptor quorum is placed. I read the docs and I can see:

Based on the number of failure groups and disks, GPFS creates one to five replicas of the descriptor:
* If there are at least five different failure groups, five replicas are created.
* If there are at least three different disks, three replicas are created.
* If there are only one or two disks, a replica is created on each disk.

And I also know I can have a disk type of descOnly.

Now what I'm not clear on is, if I have three disks with descOnly (and different failure groups), will GPFS prefer to place descriptors on those disks vs other "data" or "metadata" disks with shared failure groups with the descOnly failure groups.

Basically I want to do:

Site 1:
Lots of storage, FG100
Quorum/manager node, FG100 (with descOnly)

Site 2:
Some storage, FG200
Quorum/manager node, FG200 (with descOnly)

Site 3:
Quorum/manager node, FG300 (with descOnly)

(I'm assuming I have networking such that if a site is lost the other two sites can continue to talk).

i.e. can I be sure that GPFS will place the descriptors on the quorum nodes with the descOnly disks and be sure about the placement? Or in site 1, might it place the descriptor on "lots of storage" in FG100.

I did wonder about using more FGs, but I'm a little hesitant as the docs are unclear if there are more than 5 ... (and us accidentally ending up with too many replicas in say Site 1)

Thanks

Simon