AWPSYS@RITVAX.BITNET (07/23/87)
>There was a item in this months DEC Professional Magazine that should be of >interest to VMS system managers and operators. >Using the following VMS/BACKUP switches can HALVE your cpu and elapsed time. >I tried this on a one of my VAX 750's Massbus disks (approx. 65 Mb) to a CDC >92185 tape drive at 6250 bpi, and cut the full disk backup elapsed time from >35 minutes to 17 minutes (on an unloaded system) and cputime used from 10 >minutes to 5 minutes. > BACKUP/IMAGE/NOCRC/BUFFERS:5/BLOCKSIZE:16384 dra2: msa0:dra2.bck Note!!!! As of the Spring 1987 DECUS symposium VMS development stated for the record that they STILL did not recommend turning off CRC checking in BACKUP. The reason they gave is that even though the hardware CRC can ensure that tape is properly written, magnetic tape media has a nasty way of dropping bits over time and BACKUP may not know that it is reading BAD data on restore or BACKUP will get a tape read error but may not be able to apply correction. This is VERY important to backup because CRC failure triggers BACKUP group XOR recovery mechanism that allow backup to recover from its redunancy group. So if you are STILL going to trust the hardware alone then I would suggest that save tape as well by specifying no redundancy groups with the /GROUP=0 qualifier. CRC computing becomes the critical bottleneck on slow VAX systems (less than a 780 in speed or the MicroVAX systems where the CRC instruction is emulated). On the faster CPUs, other factors become the critical bottleneck in backup performance. What I have found as MOST important in BACKUP performance is the structure of a disk. Disks with lots of small and fragmented files belonging to many different users tend to perform very poorly in backup. BACKUP is slowed down substantially by having to constantly seek between the index file and the actual file data. Playing with backup qualifiers until you are blue in the face will not affect BACKUP times on these drives. The only thing that can help here is a faster spindle or volume shadowing. (As an aside, the RABBIT-5 product (which we tested but did not buy) DOES give you a big gain here by "looking ahead" and opening up many different files at once and caching the file headers) In conclusion, I feel that the suggestions made by the DEC professional are of no use to sites running higher capacity VAX Cpus. In a typical Cluster environment backups are run at night when CPU cyles abound. Disabling CRCs gains pratically nothing and puts older save sets at greater risk. Andrew W. Potter Rochester Institute of Technology