write latency is shown as NaN

(4 posts) (2 voices)

• Started 7 months ago by ravi.videla
• Latest reply from thobbs
• This topic is not a support question

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1. 

   ravi.videla
   Member

   Hi,

   We are using Apache Cassandra 1.0.5. For some column families, write latency is shown as NaN even though write count is present. See below. How can i make it show correct latency value. Appreciate your help.

   Keyspace: Test1
   Read Count: 389056
   Read Latency: 0.03497599831386741 ms.
   Write Count: 43413
   Write Latency: 0.024415106074217402 ms.
   Pending Tasks: 0
   Column Family: test_cf
   SSTable count: 2
   Space used (live): 1007557
   Space used (total): 1007557
   Number of Keys (estimate): 1408
   Memtable Columns Count: 54
   Memtable Data Size: 27125
   Memtable Switch Count: 20
   Read Count: 389056
   Read Latency: 0.104 ms.
   Write Count: 43413
   Write Latency: NaN ms.
   Pending Tasks: 0
   Bloom Filter False Postives: 0
   Bloom Filter False Ratio: 0.00000
   Bloom Filter Space Used: 2344
   Key cache capacity: 200000
   Key cache size: 802
   Key cache hit rate: 1.0
   Row cache: disabled
   Compacted row minimum size: 643
   Compacted row maximum size: 924
   Compacted row mean size: 847

   Posted 7 months ago #
2. 

   thobbs
   Administrator

   The write latency measured there is the "recent" write latency, which only covers writes since the last time the latency value was checked (basically, the last time nodetool cfstats was run or something like OpsCenter checked the value). If there haven't been any new writes since the last time it was checked, you will see NaN.

   Posted 7 months ago #
3. 

   ravi.videla
   Member

   Thanks Thobbs. Is there any way to get overall read/write latency for a column family? What about latency values shown at keyspace level - are they for all transactions since cassandra started?

   Posted 7 months ago #
4. 

   thobbs
   Administrator

   Is there any way to get overall read/write latency for a column family?

   You will have to go through JMX (e.g. use jconsole), but you can get the Read/WriteCount value and the TotalRead/WriteLatencyMicros value from the column family MBean and divide the second by the first.

   What about latency values shown at keyspace level - are they for all transactions since cassandra started?

   Yes, they are for all transactions since Cassandra started. Nodetool basically does what I describe above.

   Posted 7 months ago #

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