Re: Issue 15653 and imbalanced clusters

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On Tue, 22 Nov 2016, Dan Van Der Ster wrote:
> Hi,
> I have a couple questions about
> In the ticket Sage discusses small/big drives, and the small drives get 
> more data than expected.
> But we observe this at the rack level: our cluster has four racks, with 
> 7, 8, 8, 4 hosts respectively. The rack with 4 hosts is ~35% more full 
> than the others.
> So AFAICT, because of #15653, CRUSH does not currently work well if you 
> try to build a pool which is replicated rack/host-wise when your 
> rack/hosts are not all ~identical in size.

Right--it's not about devices, but items within a CRUSH bucket.  
Unfortunately we don't have a good technical solution for this yet.  The 
best proposal so far is Adam's PR at

but it leaves much to be desired.  I think we can do better, hopefully in 
time for lumninous.

In the meantime, you can underweight (devices in) small racks.  :(


> Are others noticing this pattern? Or are we unusual in that our clusters 
> are not flat/uniform in structure?
> Cheers, Dan
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