DNEIMAN@carleton.EDU.UUCP (06/01/87)
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?
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Replies to me _may_ be summarized, they _will_ be appreciated.
Thank you,
Dave Neiman Csnet: DNeiman@carleton.edu
Carleton College uucp: ...{decvax|ihnp4}!stolaf!ccnfld!dneiman
Northfield, MN 55057-4040 (When in operation)
(507) 663-0545
Disclaimer: "It all started when he hit me back..."JMS@ARIZMIS.BITNET (You do their work, and they shall have good ...) (06/03/87)
DNEIMAN%CARLETON.EDU@RELAY.CS.NET asks: >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? At the University of Arizona we have just converted from dual TU77s to dual TU78s. The driving CPU is a 780; the disks are RA81s on an HSC50. Our observation is that the 780 CPU is a bottleneck for using BACKUP in the canonical fashion (with ECC). We have not seen any real significant speed increase in backups when moving from TU77s to TU78s. Our backups were done as two-drive backups, and that made things a LOT faster -- while one drive was rewinding, the other drive was spinning -- than normal, one-drive backups. We never tried running dual backups on two CPUs at the same time. I suspect that the HSC50 would have no trouble handling such a load. The TU78s reduce operations time significantly, and make the tape library a lot easier to deal with, but don't seem to run much faster due to the CPU bottleneck. I can't say for sure, but I would guess that you would find a TU78 on a VAX 750 to run at about the same speed as your TU80s. We don't use the TU80s for BACKUP, except in emergency, but when we have, they seem to run about 20 - 30% slower than the TU77s. Here are some representative times: Four RA81s -- BACKUP incremental on 3, full on 1: BACKUP/BUF=65534/BLOCK=5 with CRC TU77s: 2:30, 2:30, 2:15, 1:55, 2:00, 2:35, 1:55, 2:05, 2:25 TU78s: 1:45, 2:30, 2:15, 2:10, 3:00, 2:10 We will install an RH on an 8600 and put the drives on that next week; I'll send out results if there is a significant improvement. +-------------------------------+ | Joel M Snyder | BITNET: jms@arizmis.BITNET | Univ of Arizona Dep't of MIS | Internet: jms@mrsvax.mis.arizona.edu | Tucson, Arizona 85721 | Pseudo-PhoneNET: (602) 621-2748 +-------------------------------+ ICBM: 32 13 N / 110 58 W (I have gotten into trouble too many times to put any faith in disclaimers) "There's nothing here that an overdose of Seconal won't cure."
MCGUIRE@GRIN2.BITNET (06/03/87)
> 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 <MCGUIRE@GRIN2.BITNET>
BOLTHOUSE%MCOPN1@eg.ti.COM.UUCP (06/03/87)
>From: DNEIMAN%carleton.edu@RELAY.CS.NET > > (Information describing configuration: 2 750s, 1 780, 2 TU80s) > >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?). All of this depends upon the command you're using for backups. If you're using /CRC (the default), your CPUs won't be able to keep up with a TU78, for example. (This according to DEC Storage folks at DECUS, Spring 1986). If you're going to buy a TU81(+), you probably won't be able to get it to stream continuously by using your 750s. You *may* be able to get it to stream using your 780, but I doubt it, especially if you have lots of detached processes running around or users on the system during incremental backups. Also, BACKUP spends a bunch of time searching disks for files to back up. If your disks are fragmented, then this time increases dramatically. Using the /FAST qualifier (and the requisite memory for the BACKUP process' working set!) will ease this. The upshot? For incrementals, you're probably spending most of your time doing disk I/O and waiting for the tape drive in stop/start mode. I don't think getting a TU81+/TA81 will help much there. For /IMAGE backups, you *might* get the tape drive to stream a little. My recommendation is to trade your 2 TU80s for a single TA78 if backup time is an issue. If, on the other hand, you're using cheap labor...maybe the costs of a TA78 are too high. In this case, get a TA81 and drive it off the HSC, and take the performance hit. (The HSC will at least tend to offload the VAX CPU). You can always help yourself by keeping your disks contiguous. And if you don't care about your data, you can use things like /NOCRC, /BLOCK=65000 or RABBIT-5 to speed up your backups. But I take the hit on backup speed because I like knowing my customer's data are safe. David L. Bolthouse Texas Instruments Defense Electronics Information Systems VAX System Support McKinney, TX Ma Bell: 214.952.2059 Internet: bolthouse%mcopn1@ti-eg.com Disclaimer: My company's lawyers say you shouldn't take anything I say seriously. Then again, when was the last time you took what a lawyer says seriously...