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From aaron morton <aa...@thelastpickle.com>
Subject Re: Column Family migration/tombstones
Date Mon, 07 Jan 2013 20:38:14 GMT
> are there two rows being tracked by bloomfilters
Yes. 
Bloom filters are just for the SSTables. 

> or does Cassandra possibly do something more efficient?
Bloom Filters are a space efficient data structure. 
You can reduce their size by adjusting the bloom_filter_fp_chance

> are bloomfilters accounting for 2,000,000 rows, or 1,000,000?

2,000,000

>  As we delete rows, will our bloomfilters grow?
Yes. 
Because deletions are just an insert that creates a row which must be persisted to an SSTable.


If you are concerned about bloom filter size stealing too much memory increase the max JVM
heap to get through your maintenance task. 

Cheers

-----------------
Aaron Morton
Freelance Cassandra Developer
New Zealand

@aaronmorton
http://www.thelastpickle.com

On 8/01/2013, at 1:35 AM, Mike <mtheroux2@yahoo.com> wrote:

> Thanks,
> 
> Another related question.  In the situation described below, where we have a row and
a tombstone across more than one SSTable, and it would take a very long time for these SSTables
to be compacted, are there two rows being tracked by bloomfilters (since there is a bloom
filter per SSTable), or does Cassandra possibly do something more efficient?
> 
> To extend the example, if I delete a 1,000,000 rows, and that SSTable containing 1,000,000
tombstones is not compacted with the other SSTables containing those rows, are bloomfilters
accounting for 2,000,000 rows, or 1,000,000?
> 
> This is more related to the current activities of deletion, as opposed to a major compaction
(although the question is applicable to both).  As we delete rows, will our bloomfilters grow?
> 
> -Mike
> 
> On 1/6/2013 3:49 PM, aaron morton wrote:
>>> When these rows are deleted, tombstones will be created and stored in more recent
sstables.  Upon compaction of sstables, and after gc_grace_period, I presume cassandra will
have removed all traces of that row from disk.
>> Yes.
>> When using Size Tiered compaction (the default) tombstones are purged when all fragments
of a row are included in a compaction. So if you have rows which are written to for A Very
Long Time(™) it can take a while for everything to get purged.
>> 
>> In the normal case though it's not a concern.
>> 
>>> However, after deleting such a large amount of information, there is no guarantee
that Cassandra will compact these two tables together, causing the data to be deleted (right?).
 Therefore, even after gc_grace_period, a large amount of space may still be used.
>> In the normal case this is not really an issue.
>> 
>> In your case things sound a little non normal. If you will have only a few hundred
MB's, or a few GB's, of data level in the CF I would consider running a major compaction on
it.
>> 
>> Major compaction will work on all SSTables and create one big SSTable, this will
ensure all deleted data is deleted. We normally caution agains this as the one new file is
often very big and will not get compacted for a while. However if you are deleting lots-o-data
it may work. (There is also an anti compaction script around that may be of use.)
>> 
>> Another alternative is to compact some of the older sstables with newer ones via
User Defined Compaction with JMX.
>> 
>> 
>>> Is there a way, other than a major compaction, to clean up all this old data?
 I assume a nodetool scrub will cleanup old tombstones only if that row is not in another
sstable?
>> I don't think scrub (or upgradesstables) remove tombstones.
>> 
>>> Do tombstones take up bloomfilter space after gc_grace_period?
>> Any row, regardless of the liveness of the columns, takes up bloom filter space (in
-Filter.db).
>> Once the row is removed it will no longer take up space.
>> 
>> Cheers
>> 
>> -----------------
>> Aaron Morton
>> Freelance Cassandra Developer
>> New Zealand
>> 
>> @aaronmorton
>> http://www.thelastpickle.com
>> 
>> On 6/01/2013, at 6:44 AM, Mike <mtheroux2@yahoo.com> wrote:
>> 
>>> A couple more questions.
>>> 
>>> When these rows are deleted, tombstones will be created and stored in more recent
sstables.  Upon compaction of sstables, and after gc_grace_period, I presume cassandra will
have removed all traces of that row from disk.
>>> 
>>> However, after deleting such a large amount of information, there is no guarantee
that Cassandra will compact these two tables together, causing the data to be deleted (right?).
 Therefore, even after gc_grace_period, a large amount of space may still be used.
>>> 
>>> Is there a way, other than a major compaction, to clean up all this old data?
 I assume a nodetool scrub will cleanup old tombstones only if that row is not in another
sstable?
>>> 
>>> Do tombstones take up bloomfilter space after gc_grace_period?
>>> 
>>> -Mike
>>> 
>>> On 1/2/2013 6:41 PM, aaron morton wrote:
>>>>> 1) As one can imagine, the index and bloom filter for this column family
is large.  Am I correct to assume that bloom filter and index space will not be reduced until
after gc_grace_period?
>>>> Yes.
>>>> 
>>>>> 2) If I would manually run repair across a cluster, is there a process
I can use to safely remove these tombstones before gc_grace period to free this memory sooner?
>>>> There is nothing to specifically purge tombstones.
>>>> 
>>>> You can temporarily reduce the gc_grace_seconds and then trigger compaction.
Either by reducing the min_compaction_threshold to 2 and doing a flush. Or by kicking of a
user defined compaction using the JMX interface.
>>>> 
>>>>> 3) Any words of warning when undergoing this?
>>>> Make sure you have a good breakfast.
>>>> (It's more general advice than Cassandra specific.)
>>>> 
>>>> 
>>>> Cheers
>>>> 
>>>> -----------------
>>>> Aaron Morton
>>>> Freelance Cassandra Developer
>>>> New Zealand
>>>> 
>>>> @aaronmorton
>>>> http://www.thelastpickle.com
>>>> 
>>>> On 30/12/2012, at 8:51 AM, Mike <mtheroux2@yahoo.com> wrote:
>>>> 
>>>>> Hello,
>>>>> 
>>>>> We are undergoing a change to our internal datamodel that will result
in the eventual deletion of over a hundred million rows from a Cassandra column family.  From
what I understand, this will result in the generation of tombstones, which will be cleaned
up during compaction, after gc_grace_period time (default: 10 days).
>>>>> 
>>>>> A couple of questions:
>>>>> 
>>>>> 1) As one can imagine, the index and bloom filter for this column family
is large.  Am I correct to assume that bloom filter and index space will not be reduced until
after gc_grace_period?
>>>>> 
>>>>> 2) If I would manually run repair across a cluster, is there a process
I can use to safely remove these tombstones before gc_grace period to free this memory sooner?
>>>>> 
>>>>> 3) Any words of warning when undergoing this?
>>>>> 
>>>>> We are running Cassandra 1.1.2 on a 6 node cluster and a Replication
Factor of 3.  We use LOCAL_QUORM consistency for all operations.
>>>>> 
>>>>> Thanks!
>>>>> -Mike
> 


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