[gpfsug-discuss] pool block allocation algorithm

[Aaron Knister]

Apologies if this has been covered elsewhere (I couldn't find it if it
has). I'm curious how GPFS decides where to allocate new blocks.

We've got a filesystem that we added some NSDs to a while back and it
hurt there for a little while because it appeared as though GPFS was
choosing to allocate new blocks much more frequently on the ~100% free
LUNs than the existing LUNs (I can't recall how free they were at the
time). Looking at it now, though, it seems GPFS is doing the opposite.
There's now a ~10% difference between the LUNs added and the existing
LUNs (20% free vs 30% free) and GPFS is choosing to allocate new writes
at a ratio of about 3:1 on the disks with *fewer* free blocks than on
the disks with more free blocks. That's completely inconsistent with
what we saw when we initially added the disks which makes me wonder how
GPFS is choosing to allocate new blocks (other than the obvious bits
about failure group, and replication factor). Could someone explain (or
point me at a whitepaper) what factors GPFS uses when allocating blocks,
particularly as it pertains to choosing one NSD over another within the
same failure group.

Thanks!

-Aaron

-- 
Aaron Knister
NASA Center for Climate Simulation (Code 606.2)
Goddard Space Flight Center
(301) 286-2776