[HDFS-12601] Implement new hdfs balancer's threshold units

• Type: Improvement
• Status: Open
• Priority: Major
• Resolution: Unresolved
• Affects Version/s: 2.6.5, 2.7.4, 3.0.0-alpha3
• Component/s: balancer & mover
• Assignee: Unassigned
• Reporter: Ruslan Dautkhanov
• Created: 05/Oct/17 20:34
• Updated: 05/Oct/17 20:35

Description

Balancer threshold unit is inappropriate in most cases for new clusters, that have a lot of capacity and small used%.

For example, in one of our new clusters HDFS capacity is 2.2 Pb and only 160Tb used (across all DNs). So 1% threshold equals to 0.55 Tb (there are 40 nodes in this cluster) for `hdfs balancer -threshold` parameter.
Now we have some DNs that have as low as *3.5*Tb
and other DNs as high as 4.6 Tb.

So the actual disbalance is more like 24%.
`hdfs balancer -threshold 1` command says there is nothing to balance (and I can't put smaller value than 1).
Balancer now thinks that the disbalance is less than 1% (based on full capacity),
when it's actually 24%.

We see that those nodes with more data actually getting more processing tasks
(because of data locality).

It would be great to introduce a suffix for -threshold balancer parameter:

• 10c ('c' for `c`apacity) would mean 10% from DN's capacity (current behavior, will default to 'c' if not specified so this change is backward compatible);
• 10u ('u' for `u`sed space variance across all DNs) would be measured as %min_used / %max_used. For the example above, the cluster would get rebalanced correctly as current disbalance is 24%.

Issue Links

• is related to: HDFS-12077 Implement a remaining space based balancer policy