Cassandra compaction tasks stuck

Question

I'm running Datastax Enterprise in a cluster consisting of 3 nodes. They are all running under the same hardware: 2 Core Intel Xeon 2.2 Ghz, 7 GB RAM, 4 TB Raid-0

This should be enough for running a cluster with a light load, storing less than 1 GB of data.

Most of the time, everything is just fine but it appears that sometimes the running tasks related to the Repair Service in OpsCenter sometimes get stuck; this causes an instability in that node and an increase in load.

However, if the node is restarted, the stuck tasks don't show up and the load is at normal levels again.

Because of the fact that we don't have much data in our cluster we're using the min_repair_time parameter defined in opscenterd.conf to delay the repair service so that it doesn't complete too often.

It really seems a little bit weird that the tasks that says that are marked as "Complete" and are showing a progress of 100% don't go away, and yes, we've waited hours for them to go away but they won't; the only way that we've found to solve this is to restart the nodes.

Edit:

Here's the output from nodetool compactionstats

Edit 2:

I'm running under Datastax Enterprise v. 4.6.0 with Cassandra v. 2.0.11.83

Edit 3:

This is output from dstat on a node that behaving normally

This is output from dstat on a node with stucked compaction

Edit 4:

Output from iostat on node with stucked compaction, see the high "iowait"

Answer 1

azure storage

Azure divides disk resources among storage accounts under an individual user account. There can be many storage accounts in an individual user account.

For the purposes of running DSE [or cassandra], it is important to note that a single storage account should not should not be shared between more than two nodes if DSE [or cassandra] is configured like the examples in the scripts in this document. This document configures each node to have 16 disks. Each disk has a limit of 500 IOPS. This yields 8000 IOPS when configured in RAID-0. So, two nodes will hit 16,000 IOPS and three would exceed the limit.

See details here

Answer 2

So, this has been an issue that have been under investigation for a long time now and we've found a solution, however, we aren't sure what the underlaying problem that were causing the issues were but we got a clue even tho that, nothing can be confirmed.

Basically what we did was setting up a RAID-0 also known as Striping consisting of four disks, each at 1 TB of size. We should have seen somewhere 4x one disks IOPS when using the Stripe, but we didn't, so something was clearly wrong with the setup of the RAID.

We used multiple utilities to confirm that the CPU were waiting for the IO to respond most of the time when we said to ourselves that the node was "stucked". Clearly something with the IO and most probably our RAID-setup was causing this. We tried a few differences within MDADM-settings etc, but didn't manage to solve the problems using the RAID-setup.

We started investigating Azure Premium Storage (which still is in preview). This enables attaching disks to VMs whose underlaying physical storage actually are SSDs. So we said, well, SSDs => more IOPS, so let us give this a try. We did not setup any RAID using the SSDs. We are only using one single SSD-disk per VM.

We've been running the Cluster for almost 3 days now and we've stress tested it a lot but haven't been able to reproduce the issues.

I guess we didn't came down to the real cause but the conclusion is that some of the following must have been the underlaying cause for our problems.

• Too slow disks (writes > IOPS)
• RAID was setup incorrectly which caused the disks to function non-normally

These two problems go hand-in-hand and most likely is that we basically just was setting up the disks in the wrong way. However, SSDs = more power to the people, so we will definitely continue using SSDs.

If someone experience the same problems that we had on Azure with RAID-0 on large disks, don't hesitate to add to here.