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From Brian Tarbox <tar...@cabotresearch.com>
Subject Re: Alternate "major compaction"
Date Thu, 11 Jul 2013 13:31:33 GMT
Perhaps I should already know this but why is running a major compaction
considered so bad?  We're running 1.1.6.

Thanks.


On Thu, Jul 11, 2013 at 7:51 AM, Takenori Sato <tsato@cloudian.com> wrote:

> Hi,
>
> I think it is a common headache for users running a large Cassandra
> cluster in production.
>
>
> Running a major compaction is not the only cause, but more. For example, I
> see two typical scenario.
>
> 1. backup use case
> 2. active wide row
>
> In the case of 1, say, one data is removed a year later. This means,
> tombstone on the row is 1 year away from the original row. To remove an
> expired row entirely, a compaction set has to include all the rows. So,
> when do the original, 1 year old row, and the tombstoned row are included
> in a compaction set? It is likely to take one year.
>
> In the case of 2, such an active wide row exists in most of sstable files.
> And it typically contains many expired columns. But none of them wouldn't
> be removed entirely because a compaction set practically do not include all
> the row fragments.
>
>
> Btw, there is a very convenient MBean API is available. It is
> CompactionManager's forceUserDefinedCompaction. You can invoke a minor
> compaction on a file set you define. So the question is how to find an
> optimal set of sstable files.
>
> Then, I wrote a tool to check garbage, and print outs some useful
> information to find such an optimal set.
>
> Here's a simple log output.
>
> # /opt/cassandra/bin/checksstablegarbage -e /cassandra_data/UserData/Test5_BLOB-hc-4-Data.db
> [Keyspace, ColumnFamily, gcGraceSeconds(gcBefore)] = [UserData, Test5_BLOB, 300(1373504071)]
> ===================================================================================
> ROW_KEY, TOTAL_SIZE, COMPACTED_SIZE, TOMBSTONED, EXPIRED, REMAINNING_SSTABLE_FILES
> ===================================================================================
> hello5/100.txt.1373502926003, 40, 40, YES, YES, Test5_BLOB-hc-3-Data.db
> -----------------------------------------------------------------------------------
> TOTAL, 40, 40
> ===================================================================================
>
> REMAINNING_SSTABLE_FILES means any other sstable files that contain the
> respective row. So, the following is an optimal set.
>
> # /opt/cassandra/bin/checksstablegarbage -e /cassandra_data/UserData/Test5_BLOB-hc-4-Data.db
/cassandra_data/UserData/Test5_BLOB-hc-3-Data.db
> [Keyspace, ColumnFamily, gcGraceSeconds(gcBefore)] = [UserData, Test5_BLOB, 300(1373504131)]
> ===================================================================================
> ROW_KEY, TOTAL_SIZE, COMPACTED_SIZE, TOMBSTONED, EXPIRED, REMAINNING_SSTABLE_FILES
> ===================================================================================
> hello5/100.txt.1373502926003, 223, 0, YES, YES
> -----------------------------------------------------------------------------------
> TOTAL, 223, 0
> ===================================================================================
>
> This tool relies on SSTableReader and an aggregation iterator as Cassandra
> does in compaction. I was considering to share this with the community. So
> let me know if anyone is interested.
>
> Ah, note that it is based on 1.0.7. So I will need to check and update for
> newer versions.
>
> Thanks,
> Takenori
>
>
> On Thu, Jul 11, 2013 at 6:46 PM, Tomàs Núnez <tomas.nunez@groupalia.com>wrote:
>
>> Hi
>>
>> About a year ago, we did a major compaction in our cassandra cluster (a
>> n00b mistake, I know), and since then we've had huge sstables that never
>> get compacted, and we were condemned to repeat the major compaction process
>> every once in a while (we are using SizeTieredCompaction strategy, and
>> we've not avaluated yet LeveledCompaction, because it has its downsides,
>> and we've had no time to test all of them in our environment).
>>
>> I was trying to find a way to solve this situation (that is, do something
>> like a major compaction that writes small sstables, not huge as major
>> compaction does), and I couldn't find it in the documentation. I tried
>> cleanup and scrub/upgradesstables, but they don't do that (as documentation
>> states). Then I tried deleting all data in a node and then bootstrapping it
>> (or "nodetool rebuild"-ing it), hoping that this way the sstables would get
>> cleaned from deleted records and updates. But the deleted node just copied
>> the sstables from another node as they were, cleaning nothing.
>>
>> So I tried a new approach: I switched the sstable compaction strategy
>> (SizeTiered to Leveled), forcing the sstables to be rewritten from scratch,
>> and then switching it back (Leveled to SizeTiered). It took a while (but so
>> do the major compaction process) and it worked, I have smaller sstables,
>> and I've regained a lot of disk space.
>>
>> I'm happy with the results, but it doesn't seem a orthodox way of
>> "cleaning" the sstables. What do you think, is it something wrong or crazy?
>> Is there a different way to achieve the same thing?
>>
>> Let's put an example:
>> Suppose you have a write-only columnfamily (no updates and no deletes, so
>> no need for LeveledCompaction, because SizeTiered works perfectly and
>> requires less I/O) and you mistakenly run a major compaction on it. After a
>> few months you need more space and you delete half the data, and you find
>> out that you're not freeing half the disk space, because most of those
>> records were in the "major compacted" sstables. How can you free the disk
>> space? Waiting will do you no good, because the huge sstable won't get
>> compacted anytime soon. You can run another major compaction, but that
>> would just postpone the real problem. Then you can switch compaction
>> strategy and switch it back, as I just did. Is there any other way?
>>
>> --
>> [image: Groupalia] <http://es.groupalia.com/>
>> www.groupalia.com <http://es.groupalia.com/> Tomàs Núñez IT-Sysprod Tel.
+
>> 34 93 159 31 00  Fax. + 34 93 396 18 52 Llull, 95-97, 2º planta, 08005
>> BarcelonaSkype: tomas.nunez.groupalia tomas.nunez@groupalia.com<nombre.apellido@groupalia.com>
[image:
>> Twitter] Twitter <http://twitter.com/#%21/groupaliaes>    [image:
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>>
>
>

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