[CASSANDRA-13120] Trace and Histogram output misleading

• Type: Bug
• Status: Resolved
• Priority: Low
• Resolution: Fixed
• Fix Version/s: 3.0.14, 3.11.0, 4.0-alpha1, 4.0
• Component/s: Legacy/Core
• Severity: Low
• Assignee: Benjamin Lerer
• Reporter: Adam Hattrell

Authors

Benjamin Lerer

Reviewers

Stefania Alborghetti

• Created: 13/Jan/17 13:41
• Updated: 15/May/20 08:00
• Resolved: 01/Jun/17 08:26

Description

If we look at the following output:

[centos@cassandra-c-3]$ nodetool getsstables -- keyspace table 60ea4399-6b9f-4419-9ccb-ff2e6742de10
/mnt/cassandra/data/data/keyspace/table-62f30431acf411e69a4ed7dd11246f8a/mc-647146-big-Data.db
/mnt/cassandra/data/data/keyspace/table-62f30431acf411e69a4ed7dd11246f8a/mc-647147-big-Data.db
/mnt/cassandra/data/data/keyspace/table-62f30431acf411e69a4ed7dd11246f8a/mc-647145-big-Data.db
/mnt/cassandra/data/data/keyspace/table-62f30431acf411e69a4ed7dd11246f8a/mc-647152-big-Data.db
/mnt/cassandra/data/data/keyspace/table-62f30431acf411e69a4ed7dd11246f8a/mc-647157-big-Data.db
/mnt/cassandra/data/data/keyspace/table-62f30431acf411e69a4ed7dd11246f8a/mc-648137-big-Data.db

We can see that this key value appears in just 6 sstables. However, when we run a select against the table and key we get:

Tracing session: a6c81330-d670-11e6-b00b-c1d403fd6e84

 activity                                                                                                          | timestamp                  | source         | source_elapsed
-------------------------------------------------------------------------------------------------------------------+----------------------------+----------------+----------------
                                                                                                Execute CQL3 query | 2017-01-09 13:36:40.419000 | 10.200.254.141 |              0
 Parsing SELECT * FROM keyspace.table WHERE id = 60ea4399-6b9f-4419-9ccb-ff2e6742de10; [SharedPool-Worker-2]       | 2017-01-09 13:36:40.419000 | 10.200.254.141 |            104
                                                                         Preparing statement [SharedPool-Worker-2] | 2017-01-09 13:36:40.419000 | 10.200.254.141 |            220
                                        Executing single-partition query on table [SharedPool-Worker-1]            | 2017-01-09 13:36:40.419000 | 10.200.254.141 |            450
                                                                Acquiring sstable references [SharedPool-Worker-1] | 2017-01-09 13:36:40.419000 | 10.200.254.141 |            477
                                                 Bloom filter allows skipping sstable 648146 [SharedPool-Worker-1] | 2017-01-09 13:36:40.419000 | 10.200.254.141 |            496
                                                 Bloom filter allows skipping sstable 648145 [SharedPool-Worker-1] | 2017-01-09 13:36:40.419001 | 10.200.254.141 |            503
                                                            Key cache hit for sstable 648140 [SharedPool-Worker-1] | 2017-01-09 13:36:40.419001 | 10.200.254.141 |            513
                                                 Bloom filter allows skipping sstable 648135 [SharedPool-Worker-1] | 2017-01-09 13:36:40.419001 | 10.200.254.141 |            520
                                                 Bloom filter allows skipping sstable 648130 [SharedPool-Worker-1] | 2017-01-09 13:36:40.419001 | 10.200.254.141 |            526
                                                 Bloom filter allows skipping sstable 648048 [SharedPool-Worker-1] | 2017-01-09 13:36:40.419001 | 10.200.254.141 |            530
                                                 Bloom filter allows skipping sstable 647749 [SharedPool-Worker-1] | 2017-01-09 13:36:40.419001 | 10.200.254.141 |            535
                                                 Bloom filter allows skipping sstable 647404 [SharedPool-Worker-1] | 2017-01-09 13:36:40.419001 | 10.200.254.141 |            540
                                                            Key cache hit for sstable 647145 [SharedPool-Worker-1] | 2017-01-09 13:36:40.419001 | 10.200.254.141 |            548
                                                            Key cache hit for sstable 647146 [SharedPool-Worker-1] | 2017-01-09 13:36:40.419001 | 10.200.254.141 |            556
                                                            Key cache hit for sstable 647147 [SharedPool-Worker-1] | 2017-01-09 13:36:40.419002 | 10.200.254.141 |            564
                                                 Bloom filter allows skipping sstable 647148 [SharedPool-Worker-1] | 2017-01-09 13:36:40.419002 | 10.200.254.141 |            570
                                                 Bloom filter allows skipping sstable 647149 [SharedPool-Worker-1] | 2017-01-09 13:36:40.419002 | 10.200.254.141 |            575
                                                 Bloom filter allows skipping sstable 647150 [SharedPool-Worker-1] | 2017-01-09 13:36:40.419002 | 10.200.254.141 |            580
                                                 Bloom filter allows skipping sstable 647151 [SharedPool-Worker-1] | 2017-01-09 13:36:40.419002 | 10.200.254.141 |            585
                                                            Key cache hit for sstable 647152 [SharedPool-Worker-1] | 2017-01-09 13:36:40.419002 | 10.200.254.141 |            591
                                                 Bloom filter allows skipping sstable 647153 [SharedPool-Worker-1] | 2017-01-09 13:36:40.419002 | 10.200.254.141 |            597
                                                 Bloom filter allows skipping sstable 647154 [SharedPool-Worker-1] | 2017-01-09 13:36:40.419002 | 10.200.254.141 |            601
                                                 Bloom filter allows skipping sstable 647155 [SharedPool-Worker-1] | 2017-01-09 13:36:40.419002 | 10.200.254.141 |            606
                                                 Bloom filter allows skipping sstable 647156 [SharedPool-Worker-1] | 2017-01-09 13:36:40.419002 | 10.200.254.141 |            611
                                                            Key cache hit for sstable 647157 [SharedPool-Worker-1] | 2017-01-09 13:36:40.419002 | 10.200.254.141 |            617
                                                 Bloom filter allows skipping sstable 647158 [SharedPool-Worker-1] | 2017-01-09 13:36:40.419003 | 10.200.254.141 |            623
                  Skipped 0/22 non-slice-intersecting sstables, included 0 due to tombstones [SharedPool-Worker-1] | 2017-01-09 13:36:40.419003 | 10.200.254.141 |            644
                                                 Merging data from memtables and 22 sstables [SharedPool-Worker-1] | 2017-01-09 13:36:40.419003 | 10.200.254.141 |            654
                                                           Read 9 live and 0 tombstone cells [SharedPool-Worker-1] | 2017-01-09 13:36:40.419003 | 10.200.254.141 |            732
                                                                                                  Request complete | 2017-01-09 13:36:40.419808 | 10.200.254.141 |            808

You'll note we claim not to have skipped any files due to bloom filters - even though we know the data is only in 6 files.

CFHistograms also report that we're hitting every sstables:

Percentile SSTables Write Latency Read Latency Partition Size Cell Count 
(micros) (micros) (bytes) 
50% 24.00 14.24 182.79 103 1 
75% 24.00 17.08 315.85 149 2 
95% 24.00 20.50 7007.51 372 7 
98% 24.00 24.60 10090.81 642 12 
99% 24.00 29.52 12108.97 770 14 
Min 21.00 3.31 29.52 43 0 
Max 29.00 1358.10 62479.63 1597 35

Code for the read is here:

https://github.com/apache/cassandra/blob/cassandra-3.0/src/java/org/apache/cassandra/db/SinglePartitionReadCommand.java#L561

We seem to iterate over all the sstables and increment the metric as part of that iteration.

Either the reporting is incorrect - or we should maybe check the bloom filters first and then iterate the tombstones after?

In this particular case we were using TWCS which makes the problem more apparent. TWCS guarantees that we'll keep more sstables in an un-merged state. With STCS we have to search them all, but most of them should be merged together if Compaction is keeping up. LCS the read path is restricted which will disguise the impact.