This guide introduces best practices for tuning Riak cluster performance in the Amazon Web Services (AWS) Elastic Compute Cloud (EC2) environment.
The following guide is supplementary. Be sure to check out Improving Performance for general performance and tuning recommendations before continuing with this guide.
EC2 instances are available as predefined types which encapsulate a fixed amount of computing resources. For Riak, the most important of these resources are Disk I/O, RAM, and Network I/O, followed by CPU cores. With this in mind, Riak users have reported success with large, extra large, and cluster compute instance types for use as cluster nodes in the AWS EC2 environment.
The most commonly used instance types for Riak cluster nodes are
m class (General Purpose), such as
m4.xlarge. In cases where 10-gigabit Ethernet networking is desired, the Cluster Compute class of EC2 instances, such as
cc2.8xlarge can be used.
Amazon also offers a High I/O Quadruple Extra Large instance
hi1.4xlarge) that is backed by solid state drives (SSD) and features
very high I/O performance.
EBS-Optimized EC2 instances, which provide between 500 Megabits per second and 1,000 Megabits per second of throughput with Provisioned IOPS EBS volumes are also available, and recommended for use with Provisioned IOPS EBS volumes.
Riak’s primary bottleneck will be disk and network I/O, meaning that in most cases, standard EBS will incur too much latency and iowait. Riak’s I/O pattern tends to operate on small blobs from many places on the disk, whereas EBS is best at bulk reads and writes. The negative effects of this pattern can be mitigated by adding RAID over multiple volumes, using Provisioned IOPS, and/or choosing the Bitcask backend if secondary indexes are not needed for the application.
In any case, proper benchmarking and tuning are needed to achieve the desired performance.
Most successful AWS cluster deployments use more EC2 instances than they would the same number of physical nodes to compensate for the performance variability caused by shared, virtualized resources. Plan to have more EC2 instance based nodes than physical server nodes when estimating cluster size with respect to node count.
NTP is configured by default on Amazon EC2 Linux instances. Please refer to the Set the Time for an Instance section of the EC2 documentation for steps on verifying if NTP is working properly. If NTP is not working properly, significant clock drift can occur.
Mounts and Scheduler
On EBS volumes, the deadline scheduler should be used. To check the scheduler in use for block device xvdf, for example, use the following command:
To set the scheduler to deadline, use the following command:
echo deadline > /sys/block/xvdf/queue/scheduler
More information on the disk scheduler is available in Improving Performance.
Virtual Memory Subsystem
EBS volumes have considerably less bandwidth than hardware disks. To avoid saturating EBS bandwidth and inducing IO latency spikes, it is recommended to tune the Linux virtual memory subsystem to flush smaller amounts of data more often. To do so, please see Linux system performance tuning.
When a failure occurs, collect as much information as possible. Check
monitoring systems, back up log and configuration files if they are
available, including system logs like
syslog. Make sure
that the other nodes in the Riak cluster are still operating normally
and are not affected by a wider problem like an AWS service outage. Try
to determine the cause of the problem from the data you have collected.
If you are a licensed Riak Enterprise Edition user and the failure comes from Riak or is not immediately obvious, you may open a ticket on the Riak Client Services help desk or contact the 24 / 7 emergency line.
Have your collected data ready when contacting Riak Client Services. A Client Services Engineer (CSE) might request log files, configuration files, or other information.
Many failures either do not entail data loss or have minimal loss that can be repaired automatically, without intervention. Outage of a single node does not necessarily cause data loss, as other replicas of every key are available elsewhere in the cluster. Once the node is detected as down, other nodes in the cluster will take over its responsibilities temporarily and transmit the updated data to it when it eventually returns to service (also called hinted handoff).
The more severe data loss scenarios usually relate to hardware failure (in the case of AWS, service failure or instance termination). In the cases where data is lost, several options are available for restoring the data:
- Restore from backup. A daily backup of Riak nodes can be helpful. The data in this backup may be stale depending on the time at which the node failed, but can be used to partially restore data from lost EBS volumes. If running in a RAID configuration, rebuilding the array may also be possible.
- Restore from Multi-Datacenter Replication. If replication is enabled
between two or more clusters, the missing data will gradually be
restored via realtime replication and fullsync replication. A
fullsync operation can also be triggered manually via the
- Restore using intra-cluster repair. Riak versions 1.2 and greater include a “repair” feature which will restore lost partitions with data from other replicas. This currently has to be invoked manually using the Riak console and should be performed with guidance from a Riak CSE.
Once data has been restored, normal operations should continue. If multiple nodes completely lose their data, consultation and assistance from Riak is strongly recommended.
Using a tool such as Riak Bench, you can generate load that simulates application operations by constructing and communicating approximately-compatible data payloads with the Riak cluster directly.
Benchmarking is critical to determining the appropriate EC2 instance types, and strongly recommended. More information is available on benchmarking Riak clusters with Riak Bench.
Besides running Riak Bench, we also advise that you load test Riak with your own tests to ensure that load imparted by MapReduce queries, full-text queries, and index queries are within the expected range.
Simulating Upgrades, Scaling, and Failure states
In addition to simply measuring performance, it is also important to measure how performance degrades when the cluster is not in steady-state. While under a simulation of live load, the following states might be simulated:
- Stop one or more nodes normally and restart them after a few moments (simulates rolling upgrade).
- Join two or more nodes to the cluster.
- Leave nodes from the cluster (after step #2).
- Hard-kill the Riak
kill -9) and then restart it.
- Hard-reboot a node’s instance using the AWS console and then restart it.
- Hard-stop and destroy a node’s instance and build a new one from backup.
- Via networking, e.g. firewall, partition one or more nodes from the rest of the cluster and then restore the original configuration.
Sometimes, Riak will exit when it runs out of available RAM. While this does not necessarily cause data loss, it may indicate that the cluster needs to be scaled out. While the Riak node is out, other nodes may also be at risk if free capacity is low on the rest of the cluster, so monitor carefully.
Replacing the EC2 instance type with one that has greater RAM capacity may temporarily alleviate the problem, but out of memory (OOM) tends to be an indication that the cluster is underprovisioned.
Software bugs (memory leaks) could also be a cause of OOM, so we recommend Riak Enterprise Edition users to contact Riak Client Services if this problem occurs.
Dealing with IP addresses
EC2 instances that are not provisioned inside a VPC can change the following attributes after a restart:
- Private IP address
- Public IP address
- Private DNS
- Public DNS
Because these parameters play a role in a Riak instance’s node name, ensure that you follow the steps outlined in the Node Name Changed section to replace it.
To avoid this inconvenience, you can deploy Riak inside a VPC. Instances inside the VPC do not change their private IP address on restart. In addition you get the following benefits:
- Access control lists can be defined at multiple levels
- The instance is not automatically open to the internet
- Amazon VPC is free
Choice of Storage
EC2 instances support ephemeral and EBS storage. Ephemeral is local to the instance, generally performs better, but disappears when instances go down.
On the other hand, EBS is effectively network attached storage that persists after instances go down. Along with EBS you can optionally enable Provisioned IOPS (PIOPS) provide more stable performance.
For more information on EC2 storage options, please see their documentation.