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From "Nikolai Grigoriev (JIRA)" <>
Subject [jira] [Commented] (CASSANDRA-7949) LCS compaction low performance, many pending compactions, nodes are almost idle
Date Mon, 24 Nov 2014 15:07:13 GMT


Nikolai Grigoriev commented on CASSANDRA-7949:

I have recently realized that there may be relatively cheap (operationally and development-wise)
workaround for that limitation. It would also partially address the problem with bootstrapping
new node. The root cause of all this is a large amount of data in a single CF on a single
node when using LCS for that CF. The performance of a single compaction task running on a
single thread is limited anyway. One of the obvious ways to break this limitation is to shard
the data across multiple "clones" of that CF at the application level. Something as dumb as
row key hash mod X and add this suffix to the CF name. In my case looks like having X=4 would
be more than enough to solve the problem.

> LCS compaction low performance, many pending compactions, nodes are almost idle
> -------------------------------------------------------------------------------
>                 Key: CASSANDRA-7949
>                 URL:
>             Project: Cassandra
>          Issue Type: Bug
>          Components: Core
>         Environment: DSE 4.5.1-1, Cassandra 2.0.8
>            Reporter: Nikolai Grigoriev
>         Attachments: iostats.txt, nodetool_compactionstats.txt, nodetool_tpstats.txt,
pending compactions 2day.png, system.log.gz, vmstat.txt
> I've been evaluating new cluster of 15 nodes (32 core, 6x800Gb SSD disks + 2x600Gb SAS,
128Gb RAM, OEL 6.5) and I've built a simulator that creates the load similar to the load in
our future product. Before running the simulator I had to pre-generate enough data. This was
done using Java code and DataStax Java driver. To avoid going deep into details, two tables
have been generated. Each table currently has about 55M rows and between few dozens and few
thousands of columns in each row.
> This data generation process was generating massive amount of non-overlapping data. Thus,
the activity was write-only and highly parallel. This is not the type of the traffic that
the system will have ultimately to deal with, it will be mix of reads and updates to the existing
data in the future. This is just to explain the choice of LCS, not mentioning the expensive
SSD disk space.
> At some point while generating the data I have noticed that the compactions started to
pile up. I knew that I was overloading the cluster but I still wanted the genration test to
complete. I was expecting to give the cluster enough time to finish the pending compactions
and get ready for real traffic.
> However, after the storm of write requests have been stopped I have noticed that the
number of pending compactions remained constant (and even climbed up a little bit) on all
nodes. After trying to tune some parameters (like setting the compaction bandwidth cap to
0) I have noticed a strange pattern: the nodes were compacting one of the CFs in a single
stream using virtually no CPU and no disk I/O. This process was taking hours. After that it
would be followed by a short burst of few dozens of compactions running in parallel (CPU at
2000%, some disk I/O - up to 10-20%) and then getting stuck again for many hours doing one
compaction at time. So it looks like this:
> # nodetool compactionstats
> pending tasks: 3351
>           compaction type        keyspace           table       completed           total
     unit  progress
>                Compaction      myks     table_list1     66499295588   1910515889913 
   bytes     3.48%
> Active compaction remaining time :        n/a
> # df -h
> ...
> /dev/sdb        1.5T  637G  854G  43% /cassandra-data/disk1
> /dev/sdc        1.5T  425G  1.1T  29% /cassandra-data/disk2
> /dev/sdd        1.5T  429G  1.1T  29% /cassandra-data/disk3
> # find . -name **table_list1**Data** | grep -v snapshot | wc -l
> 1310
> Among these files I see:
> 1043 files of 161Mb (my sstable size is 160Mb)
> 9 large files - 3 between 1 and 2Gb, 3 of 5-8Gb, 55Gb, 70Gb and 370Gb
> 263 files of various sized - between few dozens of Kb and 160Mb
> I've been running the heavy load for about 1,5days and it's been close to 3 days after
that and the number of pending compactions does not go down.
> I have applied one of the not-so-obvious recommendations to disable multithreaded compactions
and that seems to be helping a bit - I see some nodes started to have fewer pending compactions.
About half of the cluster, in fact. But even there I see they are sitting idle most of the
time lazily compacting in one stream with CPU at ~140% and occasionally doing the bursts of
compaction work for few minutes.
> I am wondering if this is really a bug or something in the LCS logic that would manifest
itself only in such an edge case scenario where I have loaded lots of unique data quickly.
> By the way, I see this pattern only for one of two tables - the one that has about 4
times more data than another (space-wise, number of rows is the same). Looks like all these
pending compactions are really only for that larger table.
> I'll be attaching the relevant logs shortly.

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