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From Nate McCall <n...@thelastpickle.com>
Subject Re: Notes and questions from performing a large delete
Date Thu, 05 Dec 2013 19:30:22 GMT
Per the 256mb to 5mb change, check the very last section of this page:
http://www.datastax.com/documentation/cql/3.0/webhelp/cql/cql_reference/alter_table_r.html

"Changing any compaction or compression option erases all previous
compaction or compression settings."

In other words, you have to include the whole 'WITH' clause each time - in
the future just grab the output from 'show schema' and add/modify as
needed.

I did not know this either until it happened to me as well - could probably
stand to be a little bit more front-and-center, IMO.


On Wed, Dec 4, 2013 at 2:59 PM, Josh Dzielak <josh@keen.io> wrote:

> We recently had a little Cassandra party I wanted to share and see if
> anyone has notes to compare. Or can tell us what we did wrong or what we
> could do better. :) Apologies in advance for the length of the narrative
> here.
>
> *Task at hand*: Delete about 50% of the rows in a large column family
> (~8TB) to reclaim some disk. These are rows are used only for intermediate
> storage.
>
> *Sequence of events*:
>
> - Issue the actual deletes. This, obviously, was super-fast.
> - Nothing happens yet, which makes sense. New tombstones are not
> immediately compacted b/c of gc_grace_seconds.
> - Adjust gc_grace_seconds down to 60 from 86400 using ALTER TABLE in CQL.
>
> - Every node started working very hard. We saw disk space start to free
> up. It was exciting.
> - Eventually the compactions finished and we had gotten a ton of disk
> back.
> - However, our SSTables were now 5Mb, not 256Mb as they had always been :(
> - We inspected the schema in CQL/Opscenter etc and sure enough
> sstable_size_in_mb had changed to 5Mb for this CF. Previously all CFs were
> set at 256Mb, and all other CF's still were.
>
> - At 5Mb we had a huge number of SSTables. Our next goal was to get these
> tables back to 256Mb.
> - First step was to update the schema back to 256Mb.
> - Figuring out how to do this in CQL was tricky, because CQL has gone
> through a lot of changes recently and getting the docs for your version is
> hard. Eventually we figured it out - ALTER TABLE events WITH
> compaction={'class':'LeveledCompactionStrategy','sstable_size_in_mb':256};
> - Out of our 12 nodes, 9 acknowledged the update. The others showed the
> old schema still.
> - The remaining 3 would not. There was no extra load was on the systems,
> operational status was very clean. All nodes could see each other.
> - For each of the remaining 3 we tried to update the schema through a
> local cqlsh session. The same ALTER TABLE would just hang forever.
> - We restarted Cassandra on each of the 3 nodes, then did the ALTER TABLE
> again. It worked this time. We finally had schema agreement.
>
> - Starting with just 1 node, we kicked off upgradesstables, hoping it
> would rebuild the 5Mb tables to 256Mb tables.
> - Nothing happened. This was (afaik) because the sstable size change
> doesn't represent a new version of schema for the sstables. So existing
> tables are ignored.
> - We discovered the "-a" option for upgradesstables, which tells it to
> skip the schema check just and just do all the tables anyway.
> - We ran upgradesstables -a and things started happening. After a few
> hours the pending compactions finished.
> - Sadly, this node was now using 3x the disk it previously had. Some
> sstables were now 256Mb, but not all. There were tens of thousands of ~20Mb
> tables.
> - A direct comparison to other nodes owning the same % of the ring showed
> both the same number of sstables and the same ratio of 256Mb+ tables to
> small tables. However, on a 'normal' node the small tables were all 5-6Mb
> and on the fat, upgraded node, all the tables were 20Mb+. This was why the
> fat node was taking up 3x disk overall.
> - I tried to see what was in those 20Mb files relative to the 5Mb ones but
> sstable2json failed against our authenticated keyspace. I filed a bug<https://issues.apache.org/jira/browse/CASSANDRA-6450>
> .
> - Had little choice here. We shut down the fat node, did a manual delete
> of sstables, brought it back up and did a repair. It came back to the right
> size.
>
> *TL;DR* / Our big questions are:
> How could the schema have spontaneously changed from 256Mb
> sstable_size_in_mb to 5Mb?
> How could schema propagation failed such that only 9 of 12 nodes got the
> change even when cluster was healthy? Why did updating schema locally hang
> until restart?
> What could have happened inside of upgradesstables that created the node
> with the same ring % but 3x disk load?
>
> We're on Cassandra 1.2.8, Java 6, Ubuntu 12. Running on SSD's, 12 node
> cluster across 2 DCs. No compression, leveled compaction. Happy to provide
> more details. Thanks in advance for any insights into what happened or any
> best practices we missed during this episode.
>
> Best,
> Josh
>



-- 
-----------------
Nate McCall
Austin, TX
@zznate

Co-Founder & Sr. Technical Consultant
Apache Cassandra Consulting
http://www.thelastpickle.com

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