PolarDB offers two storage classes to suit different business needs: PSL4 and PSL5. They differ in price, data reliability, and performance. This topic explains these differences to help you choose the right storage class for your workload.
Comparison and guidance
At a glance
| Dimension | PSL4 (PolarStore Level 4) | PSL5 (PolarStore Level 5) |
|---|---|---|
| Cost | Lower price per GB (approximately 34% less than PSL5) | Higher price per GB |
| Data durability | 99.9999999% (nine 9s) | 99.99999999% (ten 9s) |
| Performance | Same maximum storage capacity and I/O bandwidth as PSL5; lower IOPS limits | Higher IOPS limits |
| Technology | Uses Alibaba's proprietary smart-SSD technology to compress and decompress data at the physical SSD disk layer | Default storage class for PolarDB clusters purchased before June 7, 2022 |
| Best for | Cost-sensitive workloads that require high cost-effectiveness | Mission-critical systems that require the highest performance and reliability |
How to choose
Choose PSL5 if your database serves as a core system and you need the highest levels of performance, reliability, and availability. Typical scenarios include finance, e-commerce, government services, and medium-to-large Internet businesses.
Choose PSL4 if your priority is reducing storage costs while maintaining acceptable performance. PSL4 uses Alibaba's proprietary smart-SSD technology, which compresses and decompresses data at the physical SSD disk layer. This lowers the storage price per unit of data while keeping the performance impact under control.
Storage class conversion rules
Some product series support storage class upgrades, which means that PSL4 storage can be upgraded to PSL5 storage.
Downgrading the storage class is not supported. You cannot downgrade PSL5 storage to PSL4 storage.
To switch from PSL5 storage to PSL4 storage, you can purchase a new cluster and migrate the data from the original cluster to the new cluster using a migration tool such as DTS or the major version upgrade feature.
Pricing
The following tables list storage prices by billing method, storage class, and deployment mode. Multi-zone deployment modes (Dual-zone and Three-zone) share the same price within each storage class and region.
For PSL4 storage, a 4-core CPU is allocated for data compression for each TB of data. The cost of this CPU is included in the storage fee, and no extra charges are incurred.
Subscription
| Storage class | Deployment mode | The Chinese mainland (USD/GB/month) | China (Hong Kong) and other regions (USD/GB/month) |
|---|---|---|---|
| PSL4 | Dual-zone deployment (hot standby storage cluster enabled) | 0.2308 | 0.2569 |
| PSL4 | Dual-zone deployment (hot standby storage and compute clusters enabled) | 0.2308 | 0.2569 |
| PSL4 | Three-zone deployment (hot standby storage cluster and logger node enabled) | 0.2308 | 0.2569 |
| PSL4 | Single zone (hot standby storage cluster disabled) | 0.1154 | 0.1285 |
| PSL5 | Dual-zone deployment (hot standby storage cluster enabled) | 0.3492 | 0.3892 |
| PSL5 | Dual-zone deployment (hot standby storage and compute clusters enabled) | 0.3492 | 0.3892 |
| PSL5 | Three-zone deployment (hot standby storage cluster and logger node enabled) | 0.3492 | 0.3892 |
| PSL5 | Single zone (hot standby storage cluster disabled) | 0.1746 | 0.1946 |
Pay-as-you-go
| Storage class | Deployment mode | The Chinese mainland (USD/GB/hour) | China (Hong Kong) and other regions (USD/GB/hour) |
|---|---|---|---|
| PSL4 | Dual-zone deployment (hot standby storage cluster enabled) | 0.000486 | 0.000542 |
| PSL4 | Dual-zone deployment (hot standby storage and compute clusters enabled) | 0.000486 | 0.000542 |
| PSL4 | Three-zone deployment (hot standby storage cluster and logger node enabled) | 0.000486 | 0.000542 |
| PSL4 | Single zone (hot standby storage cluster disabled) | 0.000243 | 0.000271 |
| PSL5 | Dual-zone deployment (hot standby storage cluster enabled) | 0.000748 | 0.000834 |
| PSL5 | Dual-zone deployment (hot standby storage and compute clusters enabled) | 0.000748 | 0.000834 |
| PSL5 | Three-zone deployment (hot standby storage cluster and logger node enabled) | 0.000748 | 0.000834 |
| PSL5 | Single zone (hot standby storage cluster disabled) | 0.000374 | 0.000417 |
Serverless
| Storage class | Deployment mode | The Chinese mainland (USD per GB-hour) | China (Hong Kong) and other regions (USD per GB-hour) |
|---|---|---|---|
| PSL4 | Dual-zone deployment (hot standby storage cluster enabled) | 0.000486 | 0.000486 |
| PSL4 | Dual-zone deployment (hot standby storage and compute clusters enabled) | 0.000486 | 0.000486 |
| PSL4 | Three-zone deployment (hot standby storage cluster and logger node enabled) | 0.000486 | 0.000486 |
| PSL4 | Single zone (hot standby storage cluster disabled) | 0.000243 | 0.000243 |
| PSL5 | Dual-zone deployment (hot standby storage cluster enabled) | 0.000748 | 0.000748 |
| PSL5 | Dual-zone deployment (hot standby storage and compute clusters enabled) | 0.000748 | 0.000748 |
| PSL5 | Three-zone deployment (hot standby storage cluster and logger node enabled) | 0.000748 | 0.000748 |
| PSL5 | Single zone (hot standby storage cluster disabled) | 0.000374 | 0.000374 |
Data reliability
PSL5 provides a higher level of data durability than PSL4. If data durability is your top concern, choose PSL5.
| Storage class | Data durability |
|---|---|
| PSL5 | 99.99999999% (ten 9s) |
| PSL4 | 99.9999999% (nine 9s) |
Performance
Storage capacity and bandwidth
For the same compute node specifications, PSL4 and PSL5 have the same maximum storage capacity and I/O bandwidth limit. However, their input/output operations per second (IOPS) limits are different. For more information, see Enterprise Edition compute node specifications and Standard Edition compute node specifications.
Performance comparison
A test was run on a Dedicated cluster with 8 cores and 64 GB of memory. The backend storage options tested were Enterprise Edition PSL5, Enterprise Edition PSL4, and Standard Edition PL1 disks. The test measured the maximum queries per second (QPS) for several scenarios in the IOBOUND payload pattern. The data volume was approximately 800 GB to 1 TB. The following figure shows the test results.
Upgrade the storage class
If PSL4 storage no longer meets your business requirements, you can manually upgrade it to PSL5 storage in the PolarDB console.
Prerequisites
Your PolarDB for MySQL cluster must meet the following requirements:
Billing method: Subscription and pay-as-you-go
NoteServerless clusters do not support storage class upgrades.
Edition: Enterprise Edition
Product series: Cluster Edition
Storage class: PSL4
Procedure
After you upgrade the storage class, the related fees are calculated based on the billing method for the cluster's storage. For more information, see Billing of configuration changes.
The storage class cannot be downgraded from PSL5 to PSL4. Carefully evaluate your requirements before you perform the upgrade.
Log on to the PolarDB console. In the navigation pane on the left, click Cluster List. Select the region of the target cluster and click its ID to open the Cluster Details page.
On the Cluster Details page, in the Database Distributed Storage section, click Upgrade Storage Class.
