PolarDB:Overview

Data backup

• Storage type: Enterprise Edition PolarStore Level 4 (PSL4) or PSL 5. Data backups are categorized into two levels based on their storage location: level-1 backup and level-2 backup.

  Data backup feature	Enabled by default	Retention period	Description	How to view backup size
  Level-1 backup	Yes	3 to 14 days	The level-1 backup feature creates snapshots by using the Redirect-on-Write (ROW) method. The snapshots are directly stored in the distributed storage system of the PolarDB cluster. The level-1 backup feature does not duplicate data. When a data block is modified, the system saves the original version of the data block to a snapshot and redirects the writes to a new data block. In this case, you can back up data within a few seconds regardless of the size of your database storage. PolarDB clusters use multi-threaded parallel processing and other technical innovations to enhance data backup and restoration and can restore data from a backup set to a new cluster within 10 minutes. Note By default, the level-1 backup feature is enabled, and you cannot disable this feature. Level-1 backups can be retained for up to 30 days. If you want to use this feature, contact us for admission to the whitelist.	The physical size of level-1 backups is shown in the following figure. Note Total Physical Size of Level-1 Backups (Snapshots) of a PolarDB cluster: the total physical storage space occupied by all level-1 backups (callout 1 in the figure). The data of a PolarDB cluster and multiple level-1 backups (snapshots) use the same physical data blocks. You are charged based on data blocks. For more information about data backup, see FAQ.
  Level-2 backup	No	30 to 7,300 days To permanently retain level-2 backups, enable the Permanently Retain Backups before Cluster Deletion feature.	The level-2 backup feature compresses and then stores level-1 backups in on-premises storage. Level-2 backup is slower compared with level-1 backup. However, level-2 backup is more cost-effective than level-1 backup. If you enable this feature, expired level-1 backups are transferred to on-premises storage and stored as level-2 backups. The backups are transferred at a rate of approximately 150 MB/s. You can implement level-2 backup across different zones in the same region or across different regions. For more information, see Intra-region and cross-region backups. Note If a level-1 backup expires before the previous level-1 backup is transferred to a level-2 backup, the level-1 backup is deleted and is not transferred to a level-2 backup. For example, a PolarDB cluster creates a level-1 backup at 01:00 every day and retains the backup for 24 hours. If the PolarDB cluster creates Level-1 Backup A at 01:00 on January 1 and creates Level-1 Backup B at 01:00 on January 2. Level-1 Backup A expires at 01:00 on January 2 and starts to be transferred to a level-2 backup. However, Level-1 Backup A stores a large amount of data, and the transfer task is not complete by 01:00 on January 3. In this case, Level-1 Backup B is deleted after it expires at 01:00 on January 3 and is not transferred to a level-2 backup.	The following figure shows the total size of level-2 backups. The total size of level-2 backups is the sum of the data sizes of all level-2 backups.

Physical log backup

• Benefits

  The log backup feature allows you to create backups by uploading real-time redo logs to Object Storage Service (OSS) in parallel. The feature is enabled by default, and log backups are retained for 3 to 7,300 days. You can permanently save log backups by enabling the Permanently Retain Backups before Cluster Deletion feature.

  Note

  By default, the log backup feature is enabled, and you cannot disable this feature.

  Based on a full backup set and the log backups generated after the backup set is created, you can perform point-in-time recovery for a PolarDB cluster. This prevents data loss and ensures data security. If you perform PITR, you must consider the amount of time that is required to query redo logs, which is affected by the amount and workload of your business data. In most cases, redo logs are queried at a rate of 1 GB every 20 seconds to 180 seconds. The total restoration duration is the sum of the time required to restore backup sets and the time required to query redo logs.

• View backup size

  The following figure shows the total size of log backups.

  

Intra-region backup and cross-region backup

• Differences

  Backup type	Description	Enabled by default	Scenario	Benefits
  Intra-region backup	Backup files are stored across different zones in the same region.	Yes Note By default, this storage method is enabled when you enable level-2 backup.	Long-term archiving	Allows you to reduce costs by dumping data at a low backup frequency.
  Cross-region backup feature	Backup files are stored across regions.	Disabled. You must manually enable cross-region backup.	Geo-redundancy and MLPS level 3	Allows you to reduce recovery point objective (RPO). This backup storage method is suitable for private network environments that are encrypted and highly secure. You can reduce costs by dumping data at a low backup frequency.

  Note

  Low-frequency level-2 backup: The backup frequency of level-2 backup is lower than that of level-1 backup.

• Alibaba Cloud regions in which the cross-region backup feature is supported

  Source region	Destination region
  China (Hangzhou), China (Shanghai), China (Qingdao), China (Beijing), China (Shenzhen), and China (Hong Kong).	China (Hangzhou), China (Shanghai), China (Qingdao), China (Beijing), China (Shenzhen), and China (Hong Kong). Note Cross-region backup files are replicated to a region that is different from the source region.
  US (Virginia)	US (Silicon Valley)

Pricing

You can use the backup and restoration feature free of charge. However, backup files consume storage space. For information about the billing rules for the backup storage space, see Billing rules for backup storage that exceeds the free quota.