To Be Decided: April 2011

Command - Usage and Output

Cassandra provides nodetool cfhistograms command to print statistic histograms for a given column family. Following is the usage:

./nodetool -h -p cfhistograms

The output of the command has following 6 columns:

Interpreting the output

Offset: This represents the series of values to which the counts for below 5 columns correspond. This corresponds to the X axis values in histograms. The unit is determined based on the other columns.
SSTables: This represents the number of SSTables accessed per read. For eg if a read operation involved accessing 3 SSTables then you will find a +ve value against Offset 3. The values are recent i.e. for duration lapsed between two calls.
Write Latency: This shows the distribution of number of operations across the range of Offset values representing latency in microseconds. For eg. If 100 operations took say 5 ms then you will find a +ve value against offset 5.
Read Latency: This is similar to write latency. The values are recent i.e. for duration lapsed between two calls.
Row Size: This shows the distribution of rows across the range of Offset values representing size in bytes. For eg. If you have 100 rows of size 2000bytes then you will find a +ve value against offset 2000.
Column Count: This is similar to row size. The offset values represent column count.

Some additional details

Typically in a histogram the values are plotted over discrete intervals. Similarly Cassandra defines buckets. The number of buckets is 1 more than the bucket offsets. The last element is values greater than the last offset. The values you see in the Offset column in the output is bucket offsets.
The bucket offset starts at 1 and grows by 1.2 each time (rounding and removing duplicates). It goes from 1 to around 36M by default (creating 90+1 buckets), which will give us timing resolution from microseconds to 36 seconds, with less precision as the numbers get larger. (see EstimatedHistogram class)

To Be Decided