Hi Dan,
Assume you have 2 pools with the same used capacity and the same number of PGs, one gets 10x the IOs than the other. From capacity balancing perspectives all the PGs look identical, but devices with PGs from one pool will get 10% of the IOs as devices with PGs only from the second pool. Under load almost all the load will go to the latter devices while the former will be almost idle, which makes very bad use of the cluster bandwidth.
This is an extreme case, but even in the case that the PGs are blended but not ideally (even one device has more PGs from the loaded pool and it is not split 50-50) we get weakest link in the chain effect on that pool and under load it will provide less than optimal bandwidth from the cluster.
IMHO it should be correct also when the cluster is almost full and not limited to half full clusters.
This is an extreme case, but even in the case that the PGs are blended but not ideally (even one device has more PGs from the loaded pool and it is not split 50-50) we get weakest link in the chain effect on that pool and under load it will provide less than optimal bandwidth from the cluster.
IMHO it should be correct also when the cluster is almost full and not limited to half full clusters.
I do agree with the observation of bad +1 PG splits among the OSDs and I believe this should be fixed. I am not sure I fully understood the huge node use case, if every PG has an OSD in a single node and still it is under utilized, I don't see how we can improve on this without sacrificing the reliability (by putting 2 copies on the same node).
Josh
On Wed, Oct 20, 2021 at 10:56 PM Dan van der Ster <dan@xxxxxxxxxxxxxx> wrote:
Hi Josh,That's another interesting dimension...Indeed a cluster that has plenty of free capacity could indeed be balanced by workload/iops, but once it reaches maybe 60 or 70% full, then I think capacity would need to take priority.But to be honest I don't really understand the workload/iops balancing use-case. Can you describe some of the scenarios you have in mind?.. DanOn Wed, 20 Oct 2021, 20:45 Josh Salomon, <jsalomon@xxxxxxxxxx> wrote:Just another point of view:The current balancer balances the capacity but this is not enough. The balancer should also balance the workload and we plan on adding primary balancing for Quincy. In order to balance the workload you should work pool by pool because pools have different workloads. So while the observation about the +1 PGs is correct, I believe the correct solution should be talking this into consideration while still balancing capacity pool by pool.Capacity balancing is a functional requirement, while workload balancing is a performance requirement so it is important only for very loaded systems (loaded in terms of high IOPS not nearly full systems)I would appreciate comments on this thought.On Wed, 20 Oct 2021, 20:57 Dan van der Ster, <dan@xxxxxxxxxxxxxx> wrote:Hi Jonas,From your readme:"the best possible solution is some OSDs having an offset of 1 PG to the ideal count. As a PG-distribution-optimization is done per pool, without checking other pool's distribution at all, some devices will be the +1 more often than others. At worst one OSD is the +1 for each pool in the cluster."That's an interesting observation/flaw which hadn't occurred to me before. I think we don't ever see it in practice in our clusters because we do not have multiple large pools on the same osds.How large are the variances in your real clusters? I hope the example in your readme isn't from real life??Cheers, Dan_______________________________________________On Wed, 20 Oct 2021, 15:11 Jonas Jelten, <jelten@xxxxxxxxx> wrote:Hi!
I've been working on this for quite some time now and I think it's ready for some broader testing and feedback.
https://github.com/TheJJ/ceph-balancer
It's an alternative standalone balancer implementation, optimizing for equal OSD storage utilization and PG placement across all pools.
It doesn't change your cluster in any way, it just prints the commands you can run to apply the PG movements.
Please play around with it :)
Quickstart example: generate 10 PG movements on hdd to stdout
./placementoptimizer.py -v balance --max-pg-moves 10 --only-crushclass hdd | tee /tmp/balance-upmaps
When there's remapped pgs (e.g. by applying the above upmaps), you can inspect progress with:
./placementoptimizer.py showremapped
./placementoptimizer.py showremapped --by-osd
And you can get a nice Pool and OSD usage overview:
./placementoptimizer.py show --osds --per-pool-count --sort-utilization
Of course there's many more features and optimizations to be added,
but it served us very well in reclaiming terrabytes of until then unavailable storage already where the `mgr balancer` could no longer optimize.
What do you think?
Cheers
-- Jonas
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