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From Jonathan Ellis <jbel...@gmail.com>
Subject Re: Key cache hit rate issue
Date Thu, 16 Feb 2012 21:20:50 GMT
So, you have roughly 1/6 of your (physical) row keys cached and about
1/4 cache hit rate, which doesn't sound unreasonable to me.  Remember,
each logical key may be spread across multiple physical sstables --
each (key, sstable) pair is one entry in the key cache.

On Thu, Feb 16, 2012 at 1:48 PM, Eran Chinthaka Withana
<eran.chinthaka@gmail.com> wrote:
> Hi Aaron,
>
> Here it is.
>
> Keyspace: XXXX
> Read Count: 1123637972
> Read Latency: 5.757938114343114 ms.
> Write Count: 128201833
> Write Latency: 0.0682576607387509 ms.
> Pending Tasks: 0
> Column Family: YY
> SSTable count: 18
> Space used (live): 103318720685
> Space used (total): 103318720685
> Number of Keys (estimate): 92404992
> Memtable Columns Count: 1425580
> Memtable Data Size: 359655747
> Memtable Switch Count: 2522
> Read Count: 1123637972
> Read Latency: 14.731 ms.
> Write Count: 128201833
> Write Latency: NaN ms.
> Pending Tasks: 0
> Bloom Filter False Postives: 1488
> Bloom Filter False Ratio: 0.00000
> Bloom Filter Space Used: 331522920
> Key cache capacity: 16637958
> Key cache size: 16637958
> Key cache hit rate: 0.2708333333333333
> Row cache: disabled
> Compacted row minimum size: 51
> Compacted row maximum size: 6866
> Compacted row mean size: 2560
>
> Thanks,
> Eran Chinthaka Withana
>
>
>
> On Thu, Feb 16, 2012 at 12:30 AM, aaron morton <aaron@thelastpickle.com>
> wrote:
>>
>> Its in the order of 261 to 8000 and the ratio is 0.00. But i guess 8000 is
>> bit high. Is there a way to fix/improve it?
>>
>> Sorry I don't understand what you mean. But if the ratio is 0.0 all is
>> good.
>>
>> Could you include the full output from cfstats for the CF you are looking
>> at ?
>>
>> Cheers
>>
>> -----------------
>> Aaron Morton
>> Freelance Developer
>> @aaronmorton
>> http://www.thelastpickle.com
>>
>> On 15/02/2012, at 1:00 PM, Eran Chinthaka Withana wrote:
>>
>> Its in the order of 261 to 8000 and the ratio is 0.00. But i guess 8000 is
>> bit high. Is there a way to fix/improve it?
>>
>> Thanks,
>> Eran Chinthaka Withana
>>
>>
>> On Tue, Feb 14, 2012 at 3:42 PM, aaron morton <aaron@thelastpickle.com>
>> wrote:
>>>
>>> Out of interest what does cfstats say about the bloom filter stats ? A
>>> high false positive could lead to a low key cache hit rate.
>>>
>>> Also, is there a way to warm start the key cache, meaning pre-load the
>>> amount of keys I set as keys_cached?
>>>
>>> See key_cache_save_period when creating the CF.
>>>
>>> Cheers
>>>
>>>
>>> -----------------
>>> Aaron Morton
>>> Freelance Developer
>>> @aaronmorton
>>> http://www.thelastpickle.com
>>>
>>> On 15/02/2012, at 5:54 AM, Eran Chinthaka Withana wrote:
>>>
>>> Hi,
>>>
>>> I'm using Cassandra 1.0.7 and I've set the keys_cached to about 80%
>>> (using the numerical values). This is visible in cfstats too. But I'm
>>> getting less than 20% (or sometimes even 0%) key cache hit rate. Well, the
>>> data access pattern is not the issue here as I know they are retrieving the
>>> same row multiple times. I'm using hector client with dynamic load balancing
>>> policy with consistency ONE for both reads and writes. Any ideas on how to
>>> find the issue and fix this?
>>>
>>> Here is what I see on cfstats.
>>>
>>> Key cache capacity: 16637958
>>> Key cache size: 16637958
>>> Key cache hit rate: 0.045454545454545456
>>>
>>> Also, is there a way to warm start the key cache, meaning pre-load the
>>> amount of keys I set as keys_cached?
>>>
>>> Thanks,
>>> Eran
>>>
>>>
>>
>>
>



-- 
Jonathan Ellis
Project Chair, Apache Cassandra
co-founder of DataStax, the source for professional Cassandra support
http://www.datastax.com

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