1. Technical Field
This application relates to computer storage devices, and more particularly to the field of selectively maintaining and modifying portions of data stored on a computer storage device and corresponding to particular points in time.
2. Description of Related Art
Host processor systems may store and retrieve data using a storage device containing a plurality of host interface units (host adapters), disk drives, and disk interface units (disk adapters). Such storage devices are provided, for example, by EMC Corporation of Hopkinton, Mass. and disclosed in U.S. Pat. No. 5,206,939 to Yanai et al., U.S. Pat. No. 5,778,394 to Galtzur et al., U.S. Pat. No. 5,845,147 to Vishlitzky et al., and U.S. Pat. No. 5,857,208 to Ofek. The host systems access the storage device through a plurality of channels provided therewith. Host systems provide data and access control information through the channels of the storage device and the storage device provides data to the host systems also through the channels. The host systems do not address the disk drives of the storage device directly, but rather, access what appears to the host systems as a plurality of logical volumes. The logical volumes may or may nor correspond to the actual disk drives.
Data backup services may be used to protect against data loss. Such services may be performed periodically (e.g., once or twice a day). When data on the main system is lost, it may be recovered from the backup media.
There are a number of drawbacks to such data backup services, including the fact that it is only possible to recover data that corresponds to data that was saved at a periodic backup. For example, if data is backed up at 9:00 a.m. and 3:00 p.m. daily, then a user is not able to recover data from, say, 11:00 a.m. If a user desires the 11:00 a.m. version of the data, the best he or she can do is obtain a copy of the backed up 9:00 a.m. version of the data and then perform steps to construct the 11:00 a.m. version of the data (e.g., by manually reconstructing the data).
One solution to this problem is to perform backups more frequently. However, increasing the frequency of backups increases the storage requirements for backup data and increases the overhead and complexity of the backup data. Ideally, it is desirable to allow obtaining data from any previous time by having a system with continuous or near continuous backups that does not have the increased storage requirements or complexity associated with increasing the frequency of backups.