PolarDB for MySQL provides two storage types: PolarStore Level 4 (PSL4) and PolarStore Level 5 (PSL5). This topic compares the two storage types to assist your selection.
Overview
| Storage type | Benefit | Scenario |
|---|---|---|
| PSL5 | This is the storage type supported by historical versions of PolarDB and the default storage type for PolarDB clusters purchased before June 7, 2022. It delivers higher performance, reliability and availability. This storage type is also called PSL5. | Databases are the key component of business systems that require high reliability, such as finance, e-commerce, public service sectors, and medium- or large-scale Internet services. |
| PSL4 | As a new storage type for PolarDB for MySQL, PSL4 uses the Smart-SSD technology developed by Alibaba Cloud to compress and decompress data at the physical SSD disk level. This can minimize the storage price of data while keep a high disk performance. | Cost-effectiveness is prioritized. |
Note The storage type of existing clusters cannot be changed to PSL4. To change the storage type, we recommend that you purchase a new cluster, configure the desired storage type, and then migrate data from the original cluster to the new cluster by using an external migration tool (such as DTS) or the one-click upgrade feature of PolarDB for MySQL. For more information, see Major version upgrades.
Pricing
| Storage type | Regions in the Chinese mainland | Regions outside the Chinese mainland | ||
|---|---|---|---|---|
| Pay-as-you-go | Subscription | Pay-as-you-go | Subscription | |
| PSL5 | USD 0.00077 per GB-hour | USD 0.365 per GB-month | USD 0.00085 per GB-hour | USD 0.403 per GB-month |
| PSL4 | USD 0.0005 per GB-hour | USD 0.238 per GB-month | USD 0.000552 per GB-hour | USD 0.263 per GB-month |
Data reliability
| Storage type | Data durability |
|---|---|
| PSL5 | 99.99999999% |
| PSL4 | 99.9999999% |
Maximum IOPS and capacity
For the same node specifications, PSL4 provides half the maximum IOPS of PSL5 and the same maximum capacity as PSL5. For more information, see Specifications of compute nodes.
Performance
The test on the maximum QPS of a dedicated cluster that uses the specifications of 8 cores and 32 GB of memory shows that the performance difference between PSL4 and PSL5 in IOBOUND load modes is generally within 10%. 
- The performance of PSL4 is slightly lower than that of PSL5 in oltp_insert, oltp_point_select, oltp_read_only, and oltp_read_write load modes.
- The performance of PSL4 is slightly higher than that of PSL5 in oltp_update_index, oltp_update_non_index, and oltp_write_only load modes.