[gpfsug-discuss] pool block allocation algorithm

[Jan-Frode Myklebust]

Don’t have documentation/whitepaper, but as I recall, it will first
allocate round-robin over failureGroup, then round-robin over nsdServers,
and then round-robin over volumes. So if these new NSDs are defined as
different failureGroup from the old disks, that might explain it..


-jf
lør. 13. jan. 2018 kl. 00:15 skrev Aaron Knister <aaron.s.knister at nasa.gov>:

> 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
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