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From aaron morton <aa...@thelastpickle.com>
Subject Re: Weird problem with empty CF
Date Wed, 05 Oct 2011 08:36:40 GMT
No. 

It's generally only an issue with heavy delete workloads, and it's sometimes possible to design
around it. 

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

On 5/10/2011, at 1:18 PM, Daning wrote:

> Thanks.  Do you have plan to improve this? I think tombstone should be separated with
live data since it serves different purpose, built in separate SSTable or indexed differently.
It is pretty costly to do filtering while reading.
> 
> Daning
> 
> On 10/04/2011 01:34 PM, aaron morton wrote:
>> 
>> I would not get gc_grace seconds to 0, set to to something small. 
>> 
>> gc_grace_seconds or ttl is only the minimum amount of time the column will stay in
the data files. The columns are only purged when compaction runs some time after that timespan
has ended. 
>> 
>> If you are seeing issues where a heavy delete workload is having an noticeably adverse
effect on read performance then you should look at the data model. Consider ways to spread
the write / read / delete workload over multiple rows.
>> 
>> If you cannot get away from it then experiment with reducing the min_compactioon_threshold
of the CF's so that compaction kicks in quicker, and (potentially) tombstones are purged faster.

>> 
>> Chees
>> 
>>  
>> -----------------
>> Aaron Morton
>> Freelance Cassandra Developer
>> @aaronmorton
>> http://www.thelastpickle.com
>> 
>> On 5/10/2011, at 6:03 AM, Daning wrote:
>> 
>>> Thanks Aaron.  How about I set the gc_grace_seconds to 0 or like 2 hours? I like
to clean up tomebstone sooner, I don't care losing some data and all my columns have ttl.

>>> 
>>> If one node is down longer than gc_grace_seconds, and I got tombstone removed,
once the node is up, from my understanding deleted data will be synced back. In this case
my data will be processed twice and it will not be a big deal to me.
>>> 
>>> Thanks,
>>> 
>>> Daning
>>> 
>>> 
>>> On 10/04/2011 01:27 AM, aaron morton wrote:
>>>> 
>>>> Yes that's the slice query skipping past the tombstone columns. 
>>>> 
>>>> Cheers
>>>> 
>>>> -----------------
>>>> Aaron Morton
>>>> Freelance Cassandra Developer
>>>> @aaronmorton
>>>> http://www.thelastpickle.com
>>>> 
>>>> On 4/10/2011, at 4:24 PM, Daning Wang wrote:
>>>> 
>>>>> Lots of SliceQueryFilter in the log, is that handling tombstone?
>>>>> 
>>>>> DEBUG [ReadStage:49] 2011-10-03 20:15:07,942 SliceQueryFilter.java (line
123) collecting 0 of 1: 1317582939743663:true:4@1317582939933000
>>>>> DEBUG [ReadStage:50] 2011-10-03 20:15:07,942 SliceQueryFilter.java (line
123) collecting 0 of 1: 1317573253148778:true:4@1317573253354000
>>>>> DEBUG [ReadStage:43] 2011-10-03 20:15:07,942 SliceQueryFilter.java (line
123) collecting 0 of 1: 1317669552951428:true:4@1317669553018000
>>>>> DEBUG [ReadStage:33] 2011-10-03 20:15:07,942 SliceQueryFilter.java (line
123) collecting 0 of 1: 1317581886709261:true:4@1317581886957000
>>>>> DEBUG [ReadStage:52] 2011-10-03 20:15:07,942 SliceQueryFilter.java (line
123) collecting 0 of 1: 1317568165152246:true:4@1317568165482000
>>>>> DEBUG [ReadStage:36] 2011-10-03 20:15:07,941 SliceQueryFilter.java (line
123) collecting 0 of 1: 1317567265089211:true:4@1317567265405000
>>>>> DEBUG [ReadStage:53] 2011-10-03 20:15:07,941 SliceQueryFilter.java (line
123) collecting 0 of 1: 1317674324843122:true:4@1317674324946000
>>>>> DEBUG [ReadStage:38] 2011-10-03 20:15:07,941 SliceQueryFilter.java (line
123) collecting 0 of 1: 1317571990078721:true:4@1317571990141000
>>>>> DEBUG [ReadStage:57] 2011-10-03 20:15:07,941 SliceQueryFilter.java (line
123) collecting 0 of 1: 1317671855234221:true:4@1317671855239000
>>>>> DEBUG [ReadStage:54] 2011-10-03 20:15:07,941 SliceQueryFilter.java (line
123) collecting 0 of 1: 1317558305262954:true:4@1317558305337000
>>>>> DEBUG [RequestResponseStage:11] 2011-10-03 20:15:07,941 ResponseVerbHandler.java
(line 48) Processing response on a callback from 12347@/10.210.101.104
>>>>> DEBUG [RequestResponseStage:9] 2011-10-03 20:15:07,941 AbstractRowResolver.java
(line 66) Preprocessed data response
>>>>> DEBUG [RequestResponseStage:13] 2011-10-03 20:15:07,941 AbstractRowResolver.java
(line 66) Preprocessed digest response
>>>>> DEBUG [ReadStage:58] 2011-10-03 20:15:07,941 SliceQueryFilter.java (line
123) collecting 0 of 1: 1317581337972739:true:4@1317581338044000
>>>>> DEBUG [ReadStage:64] 2011-10-03 20:15:07,941 SliceQueryFilter.java (line
123) collecting 0 of 1: 1317582656796332:true:4@1317582656970000
>>>>> DEBUG [ReadStage:55] 2011-10-03 20:15:07,941 SliceQueryFilter.java (line
123) collecting 0 of 1: 1317569432886284:true:4@1317569432984000
>>>>> DEBUG [ReadStage:45] 2011-10-03 20:15:07,941 SliceQueryFilter.java (line
123) collecting 0 of 1: 1317572658687019:true:4@1317572658718000
>>>>> DEBUG [ReadStage:47] 2011-10-03 20:15:07,940 SliceQueryFilter.java (line
123) collecting 0 of 1: 1317582281617755:true:4@1317582281717000
>>>>> DEBUG [ReadStage:48] 2011-10-03 20:15:07,940 SliceQueryFilter.java (line
123) collecting 0 of 1: 1317549607869226:true:4@1317549608118000
>>>>> DEBUG [ReadStage:34] 2011-10-03 20:15:07,940 SliceQueryFilter.java (line
123) collecting 0 of 1: 
>>>>> On Thu, Sep 29, 2011 at 2:17 PM, aaron morton <aaron@thelastpickle.com>
wrote:
>>>>> As with any situation involving the un-dead, it really is the number
of Zombies, Mummies or Vampires that is the concern.  
>>>>> 
>>>>> If you delete data there will always be tombstones. If you have a delete
heavy workload there will be more tombstones. This is why implementing a queue with cassandra
is a bad idea.
>>>>> 
>>>>> gc_grace_seconds (and column TTL) are the *minimum* about of time the
tombstones will stay in the data files, there is no maximum. 
>>>>> 
>>>>> Your read performance also depends on the number of SSTables the row
is spread over, see http://thelastpickle.com/2011/04/28/Forces-of-Write-and-Read/
>>>>> 
>>>>> If you really wanted to purge them then yes a repair and then major compaction
would be the way to go. Also consider if it's possible to design the data model around the
problem, e.g. partitioning rows by date. IMHO I would look to make data model changes before
implementing a compaction policy, or consider if cassandra is the right store if you have
a delete heavy workload.
>>>>> 
>>>>> Cheers
>>>>> 
>>>>>  
>>>>> -----------------
>>>>> Aaron Morton
>>>>> Freelance Cassandra Developer
>>>>> @aaronmorton
>>>>> http://www.thelastpickle.com
>>>>> 
>>>>> On 30/09/2011, at 3:27 AM, Daning Wang wrote:
>>>>> 
>>>>>> Jonathan/Aaron,
>>>>>> 
>>>>>> Thank you guy's reply, I will change GCGracePeriod to 1 day to see
what will happen.
>>>>>> 
>>>>>> Is there a way to purge tombstones at anytime? because if tombstones
affect performance, we want them to be purged right away, not after GCGracePeriod. We know
all the nodes are up, and we can do repair first to make sure the consistency before purging.
>>>>>> 
>>>>>> Thanks,
>>>>>> 
>>>>>> Daning
>>>>>> 
>>>>>> 
>>>>>> On Wed, Sep 28, 2011 at 5:22 PM, aaron morton <aaron@thelastpickle.com>
wrote:
>>>>>> if I had to guess I would say it was spending time handling tombstones.
If you see it happen again, and are interested, turn the logging up to DEBUG and look for
messages from something starting with "Slice"
>>>>>> 
>>>>>> Minor (automatic) compaction will, over time, purge the tombstones.
Until then reads must read discard the data deleted by the tombstones. If you perform a big
(i.e. 100k's ) delete this can reduce performance until compaction does it's thing.
>>>>>> 
>>>>>> My second guess would be read repair (or the simple consistency checks
on read) kicking in. That would show up in the "ReadRepairStage" in TPSTATS
>>>>>> 
>>>>>> it may have been neither of those two things, just guesses. If you
have more issues let us know and provide some more info.
>>>>>> 
>>>>>> Cheers
>>>>>> 
>>>>>> 
>>>>>> -----------------
>>>>>> Aaron Morton
>>>>>> Freelance Cassandra Developer
>>>>>> @aaronmorton
>>>>>> http://www.thelastpickle.com
>>>>>> 
>>>>>> On 29/09/2011, at 6:35 AM, Daning wrote:
>>>>>> 
>>>>>> > I have an app polling a few CFs (select first N * from CF),
there were data in CFs but later were deleted so CFs were empty for a long time. I found Cassandra
CPU usage was getting high to 80%, normally it uses less than 30%. I issued the select query
manually and feel the response is slow. I have tried nodetool compact/repair for those CFs
but that does not work. later, I issue 'truncate' for all the CFs and CPU usage gets down
to 1%.
>>>>>> >
>>>>>> > Can somebody explain to me why I need to truncate an empty CF?
and what else I could do to bring the CPU usage down?
>>>>>> >
>>>>>> > I am running 0.8.6.
>>>>>> >
>>>>>> > Thanks,
>>>>>> >
>>>>>> > Daning
>>>>>> >
>>>>>> 
>>>>>> 
>>>>> 
>>>>> 
>>>> 
>>> 
>> 
> 


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