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From Josh Elser <>
Subject Re: Fwd: why compaction failure on one table brings other tables offline, how to recover
Date Fri, 08 Apr 2016 14:52:05 GMT

Billie Rinaldi wrote:
> *From:* Jayesh Patel
> *Sent:* Thursday, April 07, 2016 4:36 PM
> *To:* ' <>'
> < <>>
> *Subject:* RE: why compaction failure on one table brings other tables
> offline, how to recover____
> __ __
> I have a 3 node Accumulo 1.7 cluster with a few small tables (few MB in
> size at most).____
> __ __
> I had one of those table fail minc because I had configured a
> SummingCombiner with FIXEDLEN but had smaller values:____
> MinC failed (trying to convert to long, but byte array isn't long
> enough, wanted 8 found 1) to create
> hdfs://instance-accumulo:8020/accumulo/tables/1/default_tablet/F0002bcs.rf_tmp
> retrying ...____
> __ __
> I have learned since to set the ‘lossy’ parameter to true to avoid this.
> *Why is the default value for it false* if it can cause catastrophic
> failure that you’ll read about ahead.____

I'm pretty sure I told you this on StackOverflow, but if you're not 
writing 8-byte long values, don't used FIXEDLEN. Use VARLEN instead.

> However, this brought other the tablets for other tables offline without
> any apparent errors or warnings. *Can someone please explain why?*____

Can you provide logs? We are not wizards :)

> In order to recover from this, I did a ‘droptable’ from the shell on the
> affected tables, but they all got stuck in the ‘DELETING’ state.  I was
> able to finally delete them using zkcli ‘rmr’ command. *Is there a
> better way?____*

Again, not sure why they would have gotten stuck in the deleting phase 
without more logs/context (nor how far along in the deletion process 
they got). It's possible that there were still entries in the 
accumulo.metadata table.

> I’m assuming there is a more proper way because when I created the
> tables again (with the same name), they went back to having a single
> offline tablet right away. *Is this because there are “traces” of the
> old table left behind that affect the new table even though the new
> table has a different table id?*  I ended up wiping out hdfs and
> recreating the accumulo instance. ____

Accumulo uses monotonically increasing IDs to identify tables. The 
human-readable names are only there for your benefit. Creating a table 
with the same name would not cause a problem. It sounds like you got the 
metadata table in a bad state or have tabletservers in a bad state (if 
you haven't restarted them).

> It seems that a small bug, writing 1 byte value instead of 8 bytes,
> caused us to dump the whole accumulo instance.  Luckily the data wasn’t
> that important, but this whole episode makes us wonder why doing things
> the right way (assuming there is a right way) wasn’t obvious or if
> Accumulo is just very fragile.____

Causing Accumulo to be unable to flush data from memory to disk in a 
minor compaction is a very bad idea. One that we cannot automatically 
recover from because of the combiner configuration you set.

If you can provide logs and stack traces from the Accumulo services, we 
can try to help you further. This is not normal. If you don't believe 
me, take a look at the distributed tests we run each release where we 
write hundreds of gigabytes of data across many servers while randomly 
killing Accumulo processes.

> Please ask away any questions/clarification you might have. We’ll
> appreciate any input you might have so we make educated decisions about
> using Accumulo going forward.____
> __ __
> Thank you,____
> Jayesh____

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