When running a database on a Japanese cloud server, you might encounter a system that initially runs smoothly but gradually becomes sluggish over time, even occasionally triggering storage space shortage warnings. Behind this, an often overlooked but crucial factor is the management of the Undo tablespace. The Undo tablespace is a special area in the database used to store "old data," recording the state of the data before modification. When you update or delete a record, the original data doesn't immediately disappear but is moved to the Undo tablespace. This design has two core purposes: first, to ensure transaction rollback, allowing the database to accurately recover data if the operation is canceled; and second, to support consistent reads, ensuring that other users see a consistent view of the data before the transaction is committed. However, if this space is not reclaimed in a timely and effective manner, it will continuously expand, eventually filling the disk and causing database operations to stop completely, directly impacting the continuity of overseas business.

To understand how to optimize it, you first need to understand how the Undo tablespace is filled and released. Its lifecycle is closely tied to database transactions. When you start a transaction and modify data, the database generates a corresponding Undo record in the Undo tablespace. These records are images of the data's "past life." After a transaction is committed, these Undo records are not immediately deleted because they may still be used by other ongoing query sessions to maintain a consistent view of the data. The space they occupy is only marked as "reusable" when it's determined that no active queries require these old data images.

"Reclaiming" here mainly falls into two categories. One is internal space reuse: when new Undo data needs to be written, the database will prioritize using the space marked as "expired." The other is space shrinkage: when the Undo tablespace file abnormally expands due to a single large transaction, its physical file size will not automatically shrink even after the transaction ends. In this case, it's necessary to manually or through strategies reclaim this idle, excessive space and return it to the operating system.

In the specific environment of a Japanese cloud server, managing the Undo tablespace presents some additional challenges. Firstly, there is relatively high network latency. If the database is deployed overseas and you are managing it from within China, any interactive monitoring or emergency handling will experience latency, requiring us to rely more heavily on automated monitoring and alerting. Secondly, there are performance and cost considerations for cloud disks. High-performance SSD storage in the cloud is expensive, and allowing the Undo tablespace to grow uncontrollably will result in significant resource waste. At the same time, frequent automatic expansion operations may also cause temporary I/O performance fluctuations.

Mainstream Database Reclamation Strategies and Optimization Configurations

Different database management systems have different focuses in their Undo tablespace management mechanisms.

MySQL/InnoDB's Undo space management is relatively automated. Its core reclamation thread is the `purge` thread, which is responsible for cleaning up Undo logs that are no longer needed by committed transactions in the background. Two key parameters control its behavior: `innodb_max_undo_log_size` defines the maximum size of a single Undo tablespace file; exceeding this limit may trigger shrinkage; and `innodb_undo_log_truncate`, when enabled, allows automatic truncation and reclamation of space when the Undo tablespace exceeds the threshold. For MySQL 8.0, the Undo log is separated from the system tablespace by default, using an independent Undo tablespace for clearer management. The optimization recommendation is: On cloud servers, ensure that `innodb_undo_tablespaces` is set reasonably (usually 2-4), and enable `innodb_undo_log_truncate`, while also setting a reasonable `innodb_max_undo_log_size` based on disk size.

Oracle databases offer more granular undo management. It uses the `UNDO_RETENTION` parameter to specify how long (in seconds) the system should retain committed undo information to accommodate long-running queries. However, setting an excessively large `UNDO_RETENTION` is a common cause of undo tablespace overload. Oracle's automatic tablespace expansion feature will automatically grow when space is insufficient, but it will not automatically shrink. Therefore, the optimization focus is on: First, reasonably assess and set `UNDO_RETENTION`, avoiding blindly pursuing excessively large values; second, regularly monitor the `V$UNDOSTAT` view and check for "pending query" errors; finally, you can manually reclaim space by periodically executing `ALTER TABLESPACE UNDOTBS1 SHRINK`, or use Oracle's automatic shrinking function.

Implementing optimization on overseas servers requires more than just adjusting database parameters; it necessitates an operational and maintenance approach tailored to the characteristics of the cloud platform.

First, backup and planning are essential before implementation. Before making any significant parameter modifications or space reclamation operations (especially shrinking operations), ensure you have performed a complete backup of the database and related configuration files. Assess the current peak usage of the Undo tablespace and set an initial size accordingly, avoiding an excessively small initial size that leads to frequent expansions.

Second, establish a proactive monitoring and alerting system. Don't wait for disk alerts to take action. Utilize cloud monitoring services to set alerts for the usage rate of the cloud disk hosting the database (e.g., exceeding 80%). Internally, regularly query the usage rate of the Undo tablespace within the database. For Oracle, monitor `V$UNDOSTAT`; for MySQL, check metrics such as `trx_rseg_history_len` in `INFORMATION_SCHEMA.INNODB_METRICS`. Integrate these monitoring metrics with the cloud platform's alerting system to achieve proactive notifications across time zones.

Third, develop and test routine maintenance procedures. Include checking Undo tablespace usage in your daily or weekly maintenance checklist. For databases requiring manual reclamation, perform shrinkage operations during off-peak hours (note the time difference between you and overseas servers). Simultaneously, optimize the application layer to avoid designing and running extremely large, long-uncommitted transactions, which are the most direct cause of rapid Undo space expansion. Consider breaking large transactions into multiple smaller transaction batches.

In conclusion, effectively managing Undo tablespaces, especially on remote cloud servers in Japan, is a crucial preventative measure. It requires us to deeply understand its working principles, configure it specifically according to the characteristics of the database used, and ultimately transform potential risks from "emergency failures" into "predictable and manageable daily maintenance projects" through automated monitoring and standardized processes. Only in this way can we ensure that the database supporting overseas operations has a reliable foundation in terms of performance and stability.