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From aaron morton <aa...@thelastpickle.com>
Subject Re: help needed interpreting Read/Write latency in cfstats and cfhistograms output
Date Mon, 03 Oct 2011 20:40:43 GMT
Hi Rameash, 
	
	Both tools output the "recent" latency, and while they do this slightly differently, the
result is that it's the latency since the last time it was checked. Also the two tools use
different counters, so using cfstats will not update cfhistogram. 

	
	So when you see 
>  Read Latency: 5.086 ms.
>  Write Latency: 0.018 ms.
It means since you last checked the average latency for requests was 5.086 and 0.018ms

When you see 
> Offset      SSTables     Write Latency      Read Latency          Row Size      Column
Count
> 1            4148086                 9            198896                 0          
      0

it means that 198,896 read requests were completed in 1 *microsecond* and 9 write requests
completed n 1 microsecond. 

Cheers

  
-----------------
Aaron Morton
Freelance Cassandra Developer
@aaronmorton
http://www.thelastpickle.com

On 4/10/2011, at 4:58 AM, Ramesh Natarajan wrote:

> I am running a cassandra 0.8.6 cluster. I started a clean test setup and run my tests
for a while. Later when I run cfstats and cfhistograms ( both ran at the same time )
> the values for Read/Write latency doesn't match.   As per  cfstats  the latency for read
and write are 5.086 and 0.018 ms respectively. However per cfhistogram output the 
> latency doesn't look correct. Attached are the output.. Can someone explain how to correlate
the data?
> 
> Thanks
> Ramesh
> 
> 
> cfstats
> Column Family: uid
>                 SSTable count: 10
>                 Space used (live): 7453864915
>                 Space used (total): 7453864915
>                 Number of Keys (estimate): 2669184
>                 Memtable Columns Count: 6864
>                 Memtable Data Size: 9254197
>                 Memtable Switch Count: 1037
>                 Read Count: 353627031
>                 Read Latency: 5.086 ms.
>                 Write Count: 325803780
>                 Write Latency: 0.018 ms.
>                 Pending Tasks: 0
>                 Key cache capacity: 2000000
>                 Key cache size: 2000000
>                 Key cache hit rate: 0.8106968059650433
>                 Row cache: disabled
>                 Compacted row minimum size: 104
>                 Compacted row maximum size: 11864
>                 Compacted row mean size: 2629
> 
> 
> cfhistograms for uid
> MSA/uid histograms
> Offset      SSTables     Write Latency      Read Latency          Row Size      Column
Count
> 1            4148086                 9            198896                 0          
      0
> 2            8680130                 2            993805                 0          
      0
> 3           17138720                 2           2487034                 0          
      0
> 4           29039539                 3           4712246                 0          
      0
> 5           41925392                20           7805708                 0          
      0
> 6           52669945               126          11641747                 0          
      0
> 7           57474130               457          15812298                 0          
      0
> 8           53613212              1034          19846340                 0          
      0
> 10          67641689              5463          48797478                 0          
      0
> 12          19456795             15703          52875124                 0          
      0
> 14           1841556             35196          47573455                 0          
      0
> 17              3095            102787          51065577                 0          
      0
> 20                 0            145706          27439942                 0          
      0
> 24                 0            196614          14573201                 0          
      0
> 29                 0            237579           4983641                 0          
      0
> 35                 0            489150           2167481                 0          
      0
> 42                 0           1234257           2613908                 0          
      0
> 50                 0           2623421           2887838                 0          
      0
> 60                 0           5991578           1767507                 0          
      0
> 72                 0          13091537           1187687                 0          
      0
> 86                 0          23362939           1001303                 0          
      0
> 103                0          34216661            966773                 0          
      0
> 124                0          39232688            580505              3790          
   3790
> 149                0          33531717            411380             63631          
  63631
> 179                0          28741050            297513             11083          
  11083
> 215                0          26604624            211311             82446          
  82446
> 258                0          23621426            152905             16650          
  16650
> 310                0          21389177            115147             77831          
  77831
> 372                0          17854763             89825             77962          
  77962
> 446                0          12658366             73162             63647          
  63647
> 535                0           8777140             62366             55015          
  55015
> 642                0           6714295             58156             89230          
  89230
> 770                0           5919024             52929             50298          
  50298
> 924                0           5793271             49989             45053          
  45053
> 1109               0           5793698             45865             41470          
  41470
> 1331               0           5451214             42312             87700          
  87700
> 1597                           4720406             32027            220189          
 220189
> 1916                           3745143             27804            313070          
 313070
> 2299                           2864750             27902            188374          
 188374
> 2759                           2123842             19042             51096          
  51096
> 3311                           1560835             17596              4718          
   4718
> 3973                           1188835             11658              4045          
   4045
> 4768                            976833              9965             33284          
  33284
> 5722                            911089              7722            192321          
 192321
> 6866                            923357              5895            428685          
 428685
> 8239                            957669              4667            217879          
 217879
> 9887                            866131              3357              9478          
   9478
> 11864                           708365              2506                11          
     11
> 14237                           630587              1847                 0          
      0
> 17084                           557397              1397                 0          
      0
> 20501                           499436              1049                 0          
      0
> 24601                           457665               779                 0          
      0
> 29521                           435142               597                 0          
      0
> 35425                           415220               470                 0          
      0
> 42510                           393854               375                 0          
      0
> 51012                           372677               206                 0          
      0
> 61214                           352421               152                 0          
      0
> 73457                           326874               107                 0          
      0
> 88148                           304298                85                 0          
      0
> 105778                          285494               115                 0          
      0
> 126934                          275249               163                 0          
      0
> 152321                          258549               125                 0          
      0
> 182785                          242338                89                 0          
      0
> 219342                          235146                48                 0          
      0
> 263210                          242166                28                 0          
      0
> 315852                          241692                22                 0          
      0
> 379022                          175100                10                 0          
      0
> 454826                          131247                 6                 0          
      0
> 545791                          161308                 9                 0          
      0
> 654949                          317944                 4                 0          
      0
> 785939                          554445                 1                 0          
      0
> 943127                          230545                 0                 0          
      0
> 1131752                          63083                 0                 0          
      0
> 1358102                          36235                 0                 0          
      0
> 1629722                          41269                 0                 0          
      0
> 1955666                          10652                 0                 0          
      0
> 2346799                           3113                 0                 0          
      0
> 2816159                           1016                 1                 0          
      0
> 3379391                            148                 0                 0          
      0
> 4055269                             18                 0                 0          
      0
> 4866323                              9                 0                 0          
      0
> 5839588                              0                 0                 0          
      0
> 7007506                              0                 0                 0          
      0
> 8409007                              0                 0                 0          
      0
> 10090808                             0                 0                 0          
      0
> 12108970                             0                 0                 0          
      0
> 14530764                             0                 0                 0          
      0
> 17436917                             0                 0                 0          
      0
> 20924300                             0                 0                 0          
      0
> 25109160                             0                 0                 0          
      0
> 


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