[CASSANDRA-11226] nodetool tablestats' keyspace-level metrics are wrong/misleading

• Type: Bug
• Status: Resolved
• Priority: Low
• Resolution: Fixed
• Fix Version/s: 3.6
• Component/s: Tool/nodetool
• Labels: lhf
• Severity: Low
• Assignee: Sylvain Lebresne
• Reporter: Tom Hobbs

Authors

Sylvain Lebresne

Reviewers

Marcus Eriksson

• Created: 24/Feb/16 21:59
• Updated: 16/Apr/19 09:30
• Resolved: 17/Mar/16 13:47

Description

In the nodetool tablestats output (formerly cfstats), we display "keyspace" level metrics before the table-level metrics:

Keyspace: testks
        Read Count: 14772528
        Read Latency: 0.14456651623879135 ms.
        Write Count: 4761283
        Write Latency: 0.062120404521218336 ms.
        Pending Flushes: 0
                Table: processes
                SSTable count: 7
                Space used (live): 496.76 MB
                Space used (total): 496.76 MB
                Space used by snapshots (total): 0 bytes
                Off heap memory used (total): 285.76 KB
                SSTable Compression Ratio: 0.2318241570710227
                Number of keys (estimate): 3027
                Memtable cell count: 2140
                Memtable data size: 1.66 MB
                Memtable off heap memory used: 0 bytes
                Memtable switch count: 967
                Local read count: 14772528
                Local read latency: 0.159 ms
                Local write count: 4761283
                Local write latency: 0.068 ms

However, the keyspace-level metrics are misleading, at best. They are aggregate metrics for every table in the keyspace that is included in the command line filters. So, if you run tablestats for a single table, the keyspace-level stats will only reflect that table's stats.

I see two possible fixes:

1. If the command line options don't include the entire keyspace, skip the keyspace-level stats
2. Ignore the command line options, and always make the keyspace-level stats an aggregate of all tables in the keyspace

My only concern with option 2 is that performance may suffer a bit on keyspaces with many tables. However, this is a command line tool, so as long as the response time is reasonable, I don't think it's a big deal.