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RE: [EXTERNAL] Availability issues for write/update/read workloads (up to 100s downtime) in case of a Cassandra node failure


The VMs’ memory (4 GB) seems pretty small for Cassandra. What heap size are you using? Which garbage collector? Are you seeing long GC times on the nodes? The basic rule of thumb is to give the Cassandra heap 50% of the RAM on the host. 2 GB isn’t very much.

 

Also, I wouldn’t set the replication factor to 5 (the number of nodes). If RF is always equal to the number of nodes, you can’t really scale beyond the size of the disk on any one node (all data is on each node). A replication factor of 3 would be more like a typical production set-up.

 

 

Sean Durity

 

From: Daniel Seybold <daniel.seybold@xxxxxxxxxx>
Sent: Friday, November 09, 2018 5:49 AM
To: user@xxxxxxxxxxxxxxxxxxxx
Subject: [EXTERNAL] Availability issues for write/update/read workloads (up to 100s downtime) in case of a Cassandra node failure

 

Hi Apache Cassandra experts,

we are running a set of availability evaluations under a write/read/update workloads with Apache Cassandra and experience some unexpected results, i.e.  0 ops/s over a period up to 100s.

In order to provide a clear picture find below the details of (1) the setup and (2) the evaluation workflow

1. Setup:

Cassandra version: 3.11.2
Cluster size: 5 nodes
Replication Factor: 5
Each nodes runs in the same private OpenStack based cloud, within the same availability zone and uses the private network.
Each nodes runs as OS Ubuntu 16.04 server and has 2 cores, 4GB RAM and 50GB disk.

Workload:
Yahoo Cloud Serving Benchmark 0.12
W1: 100% write
W2: 100% read
W3: 100% update

2. Evaluation Workflow:
 
1. allocate 5 VMs & deploy DBMS cluster
2. start a YCSB worklod (only one of W1-3) which runs up to 30 minutes
3. wait for 200s
4. trigger the selection of a  random node in the cluster and delete the VM without stopping  Cassandra before
5. analyze throughput time series over the evaluation

3. (Unexpected) Results

We expected to see a (slight) drop in the throughput as soon as the VM was deleted.
But the throughput results show that the there are periods of ~10s - 150s (not deterministic) where no operations are executed (all metrics are collected on client side)
Yet, there are no timeout exceptions on client side and also the logs on cluster side do not show anything that explains this behaviour.

I attached a series of plots which show the throughput and the downtimes over the evaluation runs. 

Do you have any explanations for this behaviour or recommendations how to reduce the  potential "downtime" ?

Thanks in advance for any help and recommendations,

Cheers,
Daniel



-- 
M.Sc. Daniel Seybold
 
Universität Ulm
Institut Organisation und Management
von Informationssystemen (OMI)
Albert-Einstein-Allee 43
89081 Ulm
Phone: +49 (0)731 50-28 799



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