krowitz@mit-kermit.UUCP (David Krowitz) (11/21/86)
There are a lot of optical disks out there right now if what you want is a WORM (Write Once, Read Many ... ie. non-erasable) drive. Most are 12-inch platters, 1 Gbyte per side. Optimem, OSI, Alcatel Thomson, and Hitachi have all sent me literature. None of them seems to sell a package including a drive and a multibus (or VME bus) controller. Matching a drive to a controller is the hard part of this business. A few companies offer packages meant to hook up to an IBM PC (and maybe a DN3000?). These usually are smaller drives (5 1/4 inch or CD-ROMs) meant for document storage and retrieval, and they have much slower data transfer rates than the larger 12-inch drives. If you think that a 6250 BPI tape drive (at 500 Kbytes/sec) is a slow way to backup a disk, try a small optical disk at 150 Kbytes/sec! You should also note that the time it takes to backup a disk is frequently limited by the speed of the computer and disk drive. Our DSP80 (not an 80-A) has two 500Mb disks and a Kennedy 9400 tape drive (45 inch/sec at 6250, 75 inch/sec at 1600 or 800 BPI). It takes 5 or 6 tapes to backup the disks when they are full, at an average of 1 hour per tape. However, the tape drive is idle anywhere from 30% to 50% of the time because it is waiting for WBAK to get the files off of the disk! The backup runs much faster when it gets to a directory which has a few large (1 to 5 Mb) files and *much* slower when it gets to a directory which has lots of small (1 to 10 block) files. The bottom line is that the time it takes to run a backup is' limited by the time it takes your DSP to thrash its way throw the file system. The only real advantage of an optical disk is that you don't have to change tapes once every hour or so ... which means that you can leave the backup running overnight. On the other hand ... 6 magtapes cost us about $90, and since we do weekly backups we can recycle each group of 6 tapes every 1 to 2 months. The last time I priced the cost of 12-inch WORM disks they were $300 per platter and they can not be recyled. Doing weekly backups on a WORM drive is *expensive*. From what I've heard, it's not clear whether the WORM platters are more reliable (ie. last longer in the vault) and more secure (ie. someone can't erase them deliberately) than magtape. Both media can be erased by simply overwriting the existing data. If you can afford to wait for awhile I hear that a couple of the WORM drive manufacturers (Optimem comes to mind) will have erasable optical disks available to OEM's for evaluation by the 1st quarter of 1987. -- David Krowitz