Relay-Version: version B 2.10 5/3/83; site utzoo.UUCP Path: utzoo!mnetor!seismo!rutgers!sri-spam!ames!ucbcad!ucbvax!GRIN2.BITNET!MCGUIRE From: MCGUIRE@GRIN2.BITNET Newsgroups: comp.os.vms Subject: RE: 6250 tape backup Message-ID: <8706031520.AA24226@ucbvax.Berkeley.EDU> Date: Tue, 2-Jun-87 20:42:00 EDT Article-I.D.: ucbvax.8706031520.AA24226 Posted: Tue Jun 2 20:42:00 1987 Date-Received: Sat, 6-Jun-87 21:03:16 EDT Sender: daemon@ucbvax.BERKELEY.EDU Distribution: world Organization: The ARPA Internet Lines: 53 > Date: Mon, 1 Jun 87 09:30 CDT > From: DNEIMAN%CARLETON.EDU@RELAY.CS.NET > Subject: 6250 tape backup > > We have a 3-vax cluster (two 750's and a 780), all running VMS 4.5, > connected to a single HSC-50 (v300), and 5 RA-81's. Currently, we have two > TU-80 tape drives with which we do our regular backup procedures. > > We are considering the purchase of a 6250 bpi tape drive, which will > replace either or both of our current tape drives. My questions: > > Is is possible from the above information to determine whether and how much > the 6250 tape drive will speed up backup (Is the bottle neck tape > throughput, disk i/o, or cpu?). > > More info: This drive may or may not be placed on the HSC; it may go on > one of the other machines. Is a 750 a reasonable place for a 6250 bpi > drive? How about a 780? What are the performance tradeoffs? > > Currently, backup is done on both tape drives simultaneously, performing > incremental backups on different disks. Backup is usually done in the > evening, when the administrative machines are less loaded. We may purchase > a sixth RA-80, which will be backed up identically as the others. Will the > use of a single 6250 bpi tape drive make backup slower overall? How much > will it compensate for the simultaneous use (on different cpu's) of two > 1600 bpi drives? I can give you some educated guesses. If you are computing checksums as part of your backup (i.e. you are using BACKUP/CRC, the default), and performing the backup from an 11/750, then the CPU is a bottleneck. That is, increasing the CPU to an 11/780 will improve performance. This comes from my experience using a TA78 (which is a 6250 BPI streaming tape drive on the HSC-50) from an 11/750, and comparing the performance of that combination with the performance when an 8600 replaces the 11/750. When we do incremental backups, the process is mostly CPU- and disk-bound. We are only tape-bound when the 8600 is doing an image backup of a disk, and then not 100% of the time. Therefore, I imagine that two tape drives would be faster than one because you would be doing CPU and disk for two backup operations in parallel. However, if your 6250 tape drive is significantly faster than your 1600s, especially in the rewind phase if you tend to fill 2400' tapes, then the 6250 drive might actually be faster. I recommend that you get drive timings from the manufacturer, and contact people who are running the same equipment in a similar environment to find out how long an incremental backup really takes. Ed