The load balancer introduced the idea of badness, which is a measure of
how a tablet migration effects table balance on the source and
destination. This is an abbreviated definition of the badness struct:
struct migration_badness {
double src_shard_badness = 0;
double src_node_badness = 0;
double dst_shard_badness = 0;
double dst_node_badness = 0;
...
double node_badness() const {
return std::max(src_node_badness, dst_node_badness);
}
double shard_badness() const {
return std::max(src_shard_badness, dst_shard_badness);
}
};
A negative value for either of these 4 members signifies a good
migration (improves table balance), and a positive signifies a bad
migration.
In two places in the balancer, badness for source and destination is
computed independently in two objects of type migration_badness
(src_badness and dst_badness), and later combined into a single object
similar to this:
return migration_badness{
src_badness.shard_badness(),
src_badness.node_badness(),
dst_badness.shard_badness(),
dst_badness.node_badness()
};
This is a problem when, for instance, source shard badness is good
(less that 0), shard_badness() will return 0 because of std::max().
This way the actual computed badness is not set in the final object.
This can lead to incorrect decisions made later by the balancer, when it
searches for the best migration among a set of candidates.
Closesscylladb/scylladb#26091
d9f272dbdd