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Posted to commits@cassandra.apache.org by "Adam Hattrell (JIRA)" <ji...@apache.org> on 2017/01/13 13:42:26 UTC
[jira] [Created] (CASSANDRA-13120) Trace and Histogram output
misleading
Adam Hattrell created CASSANDRA-13120:
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Summary: Trace and Histogram output misleading
Key: CASSANDRA-13120
URL: https://issues.apache.org/jira/browse/CASSANDRA-13120
Project: Cassandra
Issue Type: Bug
Components: Core
Reporter: Adam Hattrell
Priority: Minor
If we look at the following output:
{noformat}
[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
{noformat}
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:
{noformat}
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
{noformat}
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:
{noformat}
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
{noformat}
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.
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