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From aaron morton <aa...@thelastpickle.com>
Subject Re: Column Family migration/tombstones
Date Sun, 06 Jan 2013 20:49:30 GMT
> 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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