Use regional PDs for high availability services

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You can use regional persistent disks (PDs) to build high availability (HA) services. When deciding to use regional persistent disks, compare different options for increasing service availability and the cost, performance, and resiliency for different service architectures.

Regional persistent disk is a storage option that provides synchronous replication of data between two zones in a region. Regional persistent disks can be a good building block to use when you implement HA services in Compute Engine.

The benefit of regional persistent disks is that in the event of a zonal outage, where your virtual machine (VM) instance might become unavailable, you can usually force attach a regional persistent disk to a VM instance in a secondary zone in the same region. To perform this task, you must either start another VM instance in the same zone as the regional persistent disk that you are force attaching, or maintain a hot standby VM instance in that zone. A hot standby is a running VM instance that is identical to the one you are using. The two instances have the same data.

The force-attach operation executes in less than one minute. The total recovery time objective (RTO) depends not only on the storage failover (the force attachment of the regional persistent disk), but also on other factors, including the following:

  • Whether you must first create a secondary VM instance
  • The length of time the underlying file system detects a hot-attached disk
  • The recovery time of the corresponding applications

Regional persistent disk favors workload availability, which means there are tradeoffs for data protection in the unlikely event that both disk replicas are unavailable at the same time. For more information, see Regional persistent disk failover.

Building high availability services with regional persistent disks

Design considerations

Before you start designing a HA service, understand the characteristics of the application, the file system, and the operating system. These characteristics are the basis for the design and can rule out various approaches. For example, if an application does not support application-level replication, some corresponding design options are not applicable.

Similarly, if the application, the file system, or the operating system are not crash tolerant, then using regional persistent disks or even zonal persistent disk snapshots might not be an option. Crash tolerance is defined as the ability to recover from an abrupt termination without losing or corrupting data that was already committed to a persistent disk prior to the crash.

Consider the following when designing for high availability:

  • The effect of using regional persistent disks or other solutions on the application and disk write performance.
  • The service recovery time objective. Understand how quickly your service must recover from a zonal outage and the SLA requirements.
  • The cost to build a resilient and reliable service architecture.

In terms of cost, the following options are for synchronous and asynchronous application replication:

  • Use two instances of the database and VM. In this case the following items determine the total cost:

    • VM instance costs
    • Persistent disk costs
    • Costs of maintaining application replication

  • Use a single VM with regional persistent disks. To achieve high availability with a regional persistent disk, use the same VM instance and persistent disk components, but also include a regional persistent disk. Regional persistent disks are double the cost per byte compared to zonal persistent disks because they are replicated in two zones.

    However, using regional persistent disks might reduce your maintenance cost because the data is automatically replicated to two replicas without the requirement of maintaining application replication.

  • Do not start the second VM until failover is required. You can reduce host costs even more by starting the back-up VM only on demand during failover rather than maintaining the VM as a hot standby.

Compare cost, performance, and resiliency

The following table highlights the trade-offs in cost, performance, and resiliency for the different service architectures.

HA service
architecture		 Zonal persistent disk
snapshots		 Application level
synchronous
 Application level
asynchronous		 HA solution
using regional
persistent disk
Protects against application, VM, zone failure1
Mitigation against application corruption (Example: application crash-intolerance) 2 2
Cost $ $$

  • Two instances of the database or VM running, application replication setup and maintenance, cross zone networking.
 $$

  • Two instances of the database or VM running, application replication setup and maintenance, cross zone networking.
 $1.5x - $$

  • The costs are the same as application replication if you use a hot standby. You can achieve a lower cost by spinning up a back-up VM on demand during failover. There is no charge for cross zone networking between regional persistent disk replicas.
Application performance

  • No effect

  • Trade-off on application performance with synchronous replication

  • No effect

  • No effect for most applications
Suited for application with low RPO requirement (Very low tolerance for data loss)

  • Data loss depending on when the snapshot was taken

  • No data loss3

  • Data loss because replication is asynchronous

  • No data loss
Storage recovery time from disaster4
  • 0 (minutes)
  • 0 (seconds)
  • 0 (seconds)
  • 0 (seconds)—to force attach the disk to a standby VM instance

1 Using regional persistent disks or snapshots is not sufficient to protect from and mitigate against failures and corruptions. Your application, file system, and possibly other software components must be crash consistent or use some sort of quiescing.

2 The replication of some applications does provide mitigation against some application corruptions. For example, MySQL primary application corruption doesn't cause its replica instances to become corrupted as well. Review your application's documentation for details.

3 Data loss means unrecoverable loss of data committed to persistent storage. Any non-committed data is still lost.

4 Failover performance doesn't include file system check and application recovery and load after failover.

Building HA database services using regional persistent disks

This section covers high level concepts for building HA solutions for stateful database services (MySQL, Postgres, etc.) using regional persistent disks and Compute Engine.

If there are broad outages in Google Cloud, for example, if a whole region becomes unavailable, your application might become unavailable. Depending on your needs, consider cross-regional replication techniques for even higher availability.

Database HA configurations typically have at least two VM instances. Preferably these instances are part of one or more managed instance groups:

  • A primary VM instance in the primary zone
  • A standby VM instance in a secondary zone

A primary VM instance has at least two persistent disks: a boot disk, and a regional persistent disk. The regional persistent disk contains database data and any other mutable data that should be preserved to another zone in case of an outage.

A standby VM instance requires a separate boot disk to be able to recover from configuration-related outages, which could result from an operating system upgrade, for example. You cannot force attach a boot disk to another VM during a failover.

The primary and standby VM instances are configured to use a load balancer with the traffic directed to the primary VM based on health check signals. This configuration is also known as a hot standby. The disaster recovery scenario for data outlines other failover configurations, which might be more appropriate for your scenario.

Challenges with database replication

The following table lists some common challenges with setting up and managing application synchronous or semi-synchronous replication (like MySQL) and how they compare to block replication with regional persistent disks.

Challenges Application synchronous
or semi-synchronous replication		 Block replication with
regional persistent disks
Maintaining stable replication between primary and failover replica. There are a number of things that could go wrong and cause an instance to fall out of HA mode:
  1. Misconfiguration of replication parameters such as SSL certificate mismatch or missing ACL on the primary side.
  2. High load on primary instance causing failover replica to be unable to keep up.
  3. Bugs causing replication issues such as application issues, OS misconfiguration, or Docker failure.
  4. Infrastructure failures like CPU contention, frozen VM, or intermediate network interruption.
 Storage failures are handled by regional persistent disks. This happens transparently to the application except for a possible fluctuation in the disk's performance.

There must be user-defined health checks to reveal any application or VM issues and trigger failover.
The end-to-end failover time is longer than desired. The time taken for the failover operation doesn't have an upper bound. Waiting for all transactions to be replayed (step 2 above) could take an arbitrarily long time, depending on the schema and the load on the database. Regional persistent disks provide synchronous replication, so the failover time is bounded by the sum of the following latencies:
  1. Creation of a secondary VM, unless there is already a hot standby VM instance available.
  2. Force attaching a regional persistent disk.
  3. Application initialization.
Split-brain To avoid split-brain, both approaches require provisions to ensure that there is only one primary at a time.

Health checks

The health checks used by the load balancer are implemented by the health check agent. The health check agent serves two purposes:

  1. The health check agent resides within the primary and secondary VMs to monitor the instances and communicate with the load balancer to direct traffic. This works best when configured with instance groups.
  2. The health check agent syncs with the application-specific regional control plane and makes failover decisions based on control plane behavior. The control plane must be in a zone that differs from the instance whose health it is monitoring.

The health check agent itself must be fault tolerant. For example, notice that, in the image that follows, the control plane is separated from the primary instance, which resides in zone us-central1-a, while the standby VM resides in zone us-central1-f.

Health check agent role in the VM.

The health check agent's role in primary and standby VM instances.

Failover

When a failure is detected within a primary VM or database, the application control plane can initiate failover to the standby VM in the secondary zone. During the failover, the regional persistent disk that is synchronously replicated to the secondary zone is force attached to the standby VM by the application control plane, and all traffic is directed to that VM based on health check signals.

Overall failover latency, excluding failure-detection time, is the sum of the following latencies:

  • Zero seconds to force attach a regional persistent disk to a standby VM
  • Time required for application initialization and crash recovery

The Disaster Recovery Building Blocks page covers the current building blocks available on Compute Engine.

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