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From aaron morton <aa...@thelastpickle.com>
Subject Re: How to reduce the Read Latency.
Date Sun, 22 May 2011 10:15:20 GMT
It's hard to say the latency is to high without knowing how many columns and how many bytes
you are asking for. It's also handy to know what the query looks like, i.e. is  it a slice
or a get by name, and the CF level 

Latency reported at the CF or KS level are for local read / write operations. Do all nodes
in the cluster show similar values or just this one? If you are looking for it the o.a.c.db.StorageProxy
MBean has the latency trackers for the cluster wide requests. 

If you are looking for a quick speed bump consider enabling the row cache. It looks like you
have a small hot set, they key cache has only 611 entries and gets a 90%+ hit rate and the
average row is 144KB. You will need more memory though.  

If you want to dig deeper into it:
- make sure the app making the correct query and not wasting effort
- watch TP stats to see if the server is keeping up with other things
- check if the IO system is overloaded  (google is your friend) 

I'd start with the data model and the query.

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

On 20 May 2011, at 20:32, Dikang Gu wrote:

> I use the default consistency level in the hector client, so it should be QUORUM.
> 
> -- 
> Dikang Gu
> 0086 - 18611140205
> On Friday, May 20, 2011 at 4:25 PM, Jeffrey Kesselman wrote:
> 
>> What consistency are you asking for?
>> 
>> On Fri, May 20, 2011 at 7:42 AM, Dikang Gu <dikang85@gmail.com> wrote:
>>> Hi All,
>>> I'm running three cassandra 0.7.4 nodes in a cluster, and I give 2G memory
>>> to each node.
>>> Now, I get the cfstats here:
>>> Keyspace: UserMap
>>> Read Count: 38411
>>> Read Latency: 123.54214613001484 ms.
>>> Write Count: 44155
>>> Write Latency: 0.02341093873853471 ms.
>>> Pending Tasks: 0
>>> Column Family: Map
>>> SSTable count: 3
>>> Space used (live): 32704387
>>> Space used (total): 32704387
>>> Memtable Columns Count: 49
>>> Memtable Data Size: 3348
>>> Memtable Switch Count: 56
>>> Read Count: 38411
>>> Read Latency: 123.542 ms.
>>> Write Count: 44155
>>> Write Latency: 0.023 ms.
>>> Pending Tasks: 0
>>> Key cache capacity: 200000
>>> Key cache size: 611
>>> Key cache hit rate: 0.9294361241314483
>>> Row cache: disabled
>>> Compacted row minimum size: 125
>>> Compacted row maximum size: 17436917
>>> Compacted row mean size: 147647
>>> You can find that the Read Latency is really high here, so what can I do to
>>> reduce the latency?  Give more memory to the three nodes? Any other options?
>>> Thanks.
>>> --
>>> Dikang Gu
>>> 0086 - 18611140205
>> 
>> 
>> 
>> -- 
>> It's always darkest just before you are eaten by a grue.
> 


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