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From Mike <mthero...@yahoo.com>
Subject Re: Column Family migration/tombstones
Date Sun, 06 Jan 2013 22:31:36 GMT
Thanks Aaron, I appreciate it.

It is my understanding, major compactions are not recommended because it 
will essentially create one massive SSTable that will not compact with 
any new SSTables for some time.  I can see how this might be a 
performance concern in the general case, because any read operation 
would always require multiple disk reads across multiple SSTables.  In 
addition, information in the new table will not be purged due to 
subsequent tombstones until that table can be compacted.  This might 
then require regular major compactions to be able to clear that data.  
Are there other performance considerations  that I need to keep in mind?

However, this might not be as  much of an issue in our usecase.

It just so happens, the data in this column family is changed very 
infrequently, except for deletes (as of recently, and will now occur 
over time).  In these case, I don't believe having data spread across 
the SSTables will be an issue, as either the data will have a tombstone 
(which causes cassandra to stop looking at other SSTables), or that data 
will be in one SSTable.  So I do not believe I/O will end up being an 
issue here.

What may be an issue is cleaning out old data in the SSTable that will 
exist after a major compaction.  However, this might not require major 
compactions to happen nearly as frequently as I've seen recommended 
(once every gc_grace period), or at all.   With the new design, data 
will be deleted from this table after a number of days.  Deletes again 
the remaining data after a major compaction might not get processed 
until the next major compaction, but any deletes against new data should 
be deleted normally through minor compactions.  In addition, the 
remaining data after we are complete the migration should be fairly 
small (about 500,000 skinny rows per node, including replicas).

Any other thoughts on this?
-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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