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The Evolving Role of Disk and Tape
Disk drives were originally designed to support random I/O operations. They have been and continue to be the right storage device for online applications that require fast access.
The explosion of internet commerce has required some applications to be available 24 hours a day. Other applications that support online customers have very limited backup windows. These new availability requirements leave little, if any time, to bring down applications for backup processing. This has led to a new role for disk in the data center; disk is now a target for backing up high priority application volumes and files.
Unlike disk, tape is a sequential device. It is well suited for reading and writing large streams of data. Large data streams include feeds from satellites, video surveillance, or broadcast news clips, for example. Backup applications, on the other hand, cannot always continuously stream data to tape drives. The performance of backup applications can be limited by the amount of bandwidth that is available between the source and target devices. Tape drives operate at maximum performance when they continuously write (or read) data. Backup applications typically write data to target devices in small blocks. In situations with limited bandwidth, the blocks do not follow each other quickly enough. Tape drives are forced to write the first block of data, then stop and re-position the tape while waiting for the next block to arrive. This write-stop-reposition-write again motion causes tape drives to operate at less than optimal performance, which can elongate the backup process. Applications with constrained backup windows and limited bandwidth can benefit from disk-based backups, which do not suffer from the same degraded performance when data arrives in small bursts. Disk-based backups may not be necessary for all applications. Applications that have larger backup windows can continue to benefit from backing up data to lower cost tape. Tape continues to maintain its role as a backup target for applications that do not have con-strained backup windows or stringent Recovery Time Objectives (RTO).
Does this mean that all backups should be saved on disk? Absolutely not! Most restores occur within the first few days after a backup has been completed. The likelihood that you will need to recover data is reduced the more time that has elapsed since you did the backup. Therefore, it may not make economical sense to store all copies of backups on disk because not only are you adding cost but also because you only need to recover from disk for a short period of time. Older disk-based copies of backups can be migrated to tape, freeing up disk space to store more backups that are current.
Writing backups to disk (disk-to-disk), then migrating older backups to tape (disk-to-disk-to-tape) blend the speed of restoration from disk with the economy of tape.
What Tape Does Well
Tape has always been easily transportable. Tape cartridges can be shipped offsite in boxes, transported by truck or hand carried to remote locations for secure offsite storage1. If a disaster occurs in the data center, the offsite tapes can be retrieved to restore applications.
Recently, some vendors have developed re-movable disk drives. Data is written to a disk device that can later be ejected and transported to other location. Current tape cartridge capacities exceed the capacities of removable disk drives. However, we expect the capacities of these removable devices to increase in the future. For now, we continue to view tape as the most commonly used transportable media. Tape continues to maintain its role as a low cost media for offsite storage.
Archival Storage
Tape, by its very nature, works at its optimal performance when reading and writing large streams of data. Some examples of large data streams include the following.
- Images from surveillance cameras that record activities at ATM machines, in public buildings, on city streets, and toll booths;
- Broadcast news events; and
- Digitized medical images, such as MRIs
Broadcast companies must retain news clips for many years. Today’s news story can be part of tomorrow’s review of a politician’s career. Government agencies and security organizations may need to review surveillance tapes days, weeks, or months after an event. Many of these large images must be kept for a long time.
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