A Method for Fast Tape Backups and Restores

All tape drives have a buffer memory in which incoming data are stored. When a write command This paper describes a method for fast backups is accepted, as much data as possible is transferred and restores on tape drives. By using a tight into the buffer and the (slower) physical writing synchronization between disk and tape accesses, on the tape starts. When the last byte of data from the total throughput can be accelerated by a the host computer has been written into the buffer factor ranging from 30 to 100 per cent. The (not necessarily on the tape), the drive can accept method imposes no special requirements on an further commands. operating system; only a timer with a resolution A fundamental feature of tape units is repobetter than one fiftieth of a second is needed. sitioning. When there is nothing more to be written Further, it can be used with virtually all types to the tape, tape motion does not stop immediately of streaming tapes and backup data formats. after recording the last byte of data. Further, when key words: Tape backup Tape streamer Tape restore a new write command is given, the drive cannot accelerate the tape in zero time. To overcome this problem and to minimize the inter-data gaps, the INTRODUCTION tape repositions: it changes direction, rewinds behind the end of the data just written and stops, Tape drives have been used for storing large amounts of data since the first days of computing, so that when it starts again, it can reach its working speed and resume writing at the end of the last particularly for backup or archiving purposes. Although other storage devices such as optical data. In this way the tape does not move continuously disks have become available, tapes will continue to be used for a long time, mainly because of the forward during the backup process, but in a series of back-and-forth motions with interleaving repolow cost of both the tape medium and the tape drive itself. sitioning. This repositioning lasts typically from 0·5 to 1·5 s, a time which is almost completely In spite of its obvious role in a computer system the backup process has been only marginally wasted, as the host computer must wait for the tape to reposition, in order to transfer more data covered in the computer literature, most articles discussing optimum disaster-proof archiving strato it. In traditional backup methods this repositioning tegies and not the backup process itself. Very little is known about the impact of various data formats takes place after almost every write command to the tape. The usual method for minimizing the on the speed or effectiveness of tape archiving and restoring. Although daily tape backups are strongly unwanted effect of repositioning is to write as much data as possible with every write command. recommended, backing-up itself has always been regarded as a slow and inelegant operation, whose This has been a common opinion, but the performance gains in the method proposed here are details are better left to the system operator. Even tape-drive manufacturers spare no more than a few achieved by deliberately using small-sized writes.