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 <ts...@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.nu...@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.nu...@groupalia.com<nombre.apell...@groupalia.com> [image: >> Twitter] Twitter <http://twitter.com/#%21/groupaliaes> [image: >> Twitter] Facebook <https://www.facebook.com/GroupaliaEspana> [image: >> Twitter] Linkedin <http://www.linkedin.com/company/groupalia> >> > >
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