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From "Phil Yang (JIRA)" <j...@apache.org>
Subject [jira] [Comment Edited] (CASSANDRA-8938) Full Row Scan does not count towards Reads
Date Thu, 12 Mar 2015 08:05:38 GMT

    [ https://issues.apache.org/jira/browse/CASSANDRA-8938?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=14358257#comment-14358257
] 

Phil Yang edited comment on CASSANDRA-8938 at 3/12/15 8:04 AM:
---------------------------------------------------------------

I'm not an expert. Do you mean "full row scan" is selecting all rows in a table? The full
table scan seems need to read all sstables, so compaction that reduces the number of sstables
will not reduce the disk IO while scanning the whole row. I think it is the reason that sstables
will not be marked as hot after the full table scan.

For your first question, it seems that there are two read counter, one is "readLatency" the
other is "rangeLatency", and nodetool only returns the count of the first one so your scan
query does not affect it.


was (Author: yangzhe1991):
I'm not an expert. Do you mean "full row scan" is selecting all rows in a table? The full
table scan seems need to read all sstables, so compaction that reduces the number of sstables
will not reduce the disk IO while scanning the whole row. I think it is the reason that sstables
will not be marked as hot after the full row scan.

For your first question, it seems that there are two read counter, one is "readLatency" the
other is "rangeLatency", and nodetool only returns the count of the first one so your scan
query does not affect it.

> Full Row Scan does not count towards Reads
> ------------------------------------------
>
>                 Key: CASSANDRA-8938
>                 URL: https://issues.apache.org/jira/browse/CASSANDRA-8938
>             Project: Cassandra
>          Issue Type: Bug
>          Components: API, Core, Tools
>         Environment: Unix, Cassandra 2.0.3
>            Reporter: Amit Singh Chowdhery
>            Priority: Minor
>              Labels: none
>
> When a CQL SELECT statement is executed with WHERE clause, Read Count is incremented
in cfstats of the column family. But, when a full row scan is done using SELECT statement
without WHERE clause, Read Count is not incremented. 
> Similarly, when using Size Tiered Compaction, if we do a full row scan using Hector RangeslicesQuery,
Read Count is not incremented in cfstats, Cassandra still considers all sstables as cold and
does not trigger compaction for them. If we fire MultigetSliceQuery, Read Count is incremented
and sstables becomes hot, triggering compaction of these sstables. 
> Expected Behavior:
> 1. Read Count must be incremented by number of rows read during a full row scan done
using CQL SELECT statement or Hector RangeslicesQuery.
> 2. Size Tiered compaction must consider all sstables as Hot after a full row scan.



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