The present invention relates to data synchronization control in asynchronous remote copy between disk control devices.
In computer systems of recent years, basic business of banks and stock companies has been shifted from centralized management using a large sized computer to a distributed database system (DDBMS) around a client/server system. In such a distributed database system environment, there is adopted an HA (High Availability) cluster configuration for processing data by using a plurality of servers and disk array devices in response to a client request. In such an HA cluster configuration, there is adopted a method of duplicating data between data centers located in remote places by way of precaution against a disaster such as an earthquake. As for duplication, there is typically adopted a method of effecting remote connection of two disk array devices (storage systems) via a public line or a private line, and copying write data to be fed from a host computer to a local disk array device to a remote disk array device.
Methods for effecting duplication between disk array devices are broadly classified into a synchronous system and an asynchronous system.
In the synchronous system, a write request from a host device of local side is first written into a cache of a disk array device of the local side. Subsequently, the disk array device of the local side transfers data written into the cache to a remote disk array device. Upon receiving an acknowledgement signal indicating that write request data has been received by a disk array device of remote side, the disk array device of the local side thereafter returns an acknowledgement about the write request to the host. In other words, the disk array device of the local side makes sure that data has been copied to the disk array device of the remote side, and returns an acknowledgement to the host. This acknowledgement ensures the host that data has been delivered to the remote side. (The term “synchronous” is used in the meaning that a write request from the host and copying onto the disk array device of the remote side are conducted in synchronism.) Since a delay is caused by waiting for an acknowledgement signal from the remote side, this synchronous system is suitable for a relatively short distance (of at most 100 km) causing a short propagation delay of data transmission between the local side and the remote side. However, the synchronous system is not suitable for long distance transfer using, for example, a public line network or the like. By the way, data recorded in the disk array devices of the local side and the remote side are written onto respective physical disks via respective drive control circuits.
On the other hand, the asynchronous system is suited for long distance transfer. As for an acknowledgement (write completion) to a host device of the local side about a write request issued by the host device of the local side, the write completion is returned to the host device at the time point when data of the write request has been written into a cache of the local side. After the acknowledgement to the host device, the data written into the cache is copied (transferred) to a disk array device of the remote side at different timing (in this meaning, this system is asynchronous). In this asynchronous system, an acknowledgement about the above described write request is returned to the host device irrespective of timing of transferring data to the disk array device of the remote side. Therefore, the acknowledgement timing is made earlier as compared with the synchronous system, and the host can shift to the next processing earlier.
By the way, as for the remote copy, it is described in Tokuhyo-Hei-8-509565.