This invention relates to an improvement in a data backup system, and more particularly to a backup system suited to a database.
A database backup system that uses two computer systems has recently been in wide use. The database backup system creates data identical to that in one computer system, and saves the data to another computer system. A backup source computer system is called an active system (or primary site), and a backup target computer system is called a standby system (or secondary site). Data saved in the standby system is called backup data.
A log transfer system has been known as a database backup system based on a redundant system (see Cristos A. Polyzois, Hector Garcia-Molina, “Evaluation of remote backup algorithms for transaction processing systems”, ACM transactions on Database Systems, Vol. 19, No. 3, published in September, 1994, p. 423 to p. 449).
The log transfer system transfers a data update record from the active system to the standby system, and creates backup data at the standby system. According to this log transfer system, data of the active system and the backup data of the standby system are made identical to each other at the stage of starting the backup. After the start of the backup, the data update record of the active system is transferred to the standby system.
The standby system updates the backup data based on the transferred data update record, and creates latest backup data.
In general, a computer system includes an external storage system. Data of the computer system is stored in the external storage system. The external storage system stores the data of the computer system, including a magnetic disk drive such as a hard disk drive or a disk array. There has been known an external storage system that has a copy function (remote copy) of copying data between two devices (see JP 2004-78746 A). This copy function transfers data via a network, and copies data from one external storage system to the other external storage system. By using the copy function, when data is added to an external storage system of the copy source, the same data is added to an external storage system of the copy target.
Backup by the log transfer system is implemented by using the copy function of the external storage system. The mechanism is as follows. When the backup is started, the data of the active system and the backup data of the standby system are made identical to each other. The copy function is used to perform a data update record, and any data update record added to the external storage system of the active system is copied to the external storage system of the standby system. The standby system creates latest backup data based on the copied data update record.
In the backup, a DB cache is set on a server memory of the standby system, the data is updated by its page unit, and the external storage system is accessed via the DB cache. This DB cache is used as follows when there is a write command in the external storage system.
(i) If a page of an update target has been read in the DB cache, this page is reused. If not read in the DB cache, a relevant page is read from the external storage system into the DB cache.
(ii) The page is updated.
(iii) The updated page is stored in the DB cache, thereby enabling its reuse.
(iv) When the data backup is finished, the page on the DB cache is written in the external storage system.
Thus, the number of I/O times in the external storage system is reduced by reusing the page on the DB cache. For technologies applicable to such a DB cache, see JP 5-225074 A and JP 2000-267936 A.
The capacity of the DB cache is limited because it is set on the memory of the server. When the capacity for reading pages becomes insufficient, it may become necessary to write those pages of the DB cache in the external storage system. However, the pages written from the DB cache into the external storage system cannot be reused by the DB cache. Thus, there has been a method known as a least recently used (LRU) system for selecting and writing a page of least reusability in an external storage system: a paging system for paging out a page with the longest unused time. According to this LRU system, a page of a longest passage of time after last reading/writing is determined to be lowest in reusability, and written in the external storage system.