wbc@dartvax.dartmouth.edu (12/14/88)
Well, it seems that SCSI disks are the preferred choice for speed and reliability. I am planning to go with a Xylogics 753, which can controll our existing Fujitsi Eagles as well as a new gigabyte drive (Fujitsu or Hitachi) Hopefully, overlapped seeks should gain something too. A lot of time is spent seeking on the swap disks. Here are some reports on why smd disks are better: We have a3/160 with 3 3/50(60) clients, OS 3.4. long ago we decided to go to SMD despite the extra expense for the following reason: The scsi bus seems to be able to talk to one device at a time. IF the tape drive is exicuting a seek or even a rewind the scsi disk is inaccessable until the tape is done. THis is sort of rediculous on a networked multiuser system. We have only one SMD disk so I cannot confirm that the 753 can do multiple seeks, but I have heard this is so. ********** Berkeley-based systems like sun make intelligent use of the semantics of a disk drive. This is the Fast File System in the documentation. As an example, there is an attempt to put all of the files in a directory on the same cylinder group, allowing files to be accessed without moving the head to a new cylinder. Also, blocks are accumulated so that all of the blocks in one cylinder are hopefully written at once, without moving the disk heads. The SCSC disks, however, hide all of the information from the kernal, so no intelligent layout policy can be achieved. The general run of thumb is that a local SCSI disk is the same performance as a remote SMD disk. If that ration is 1:4, then a remote SCSI disk will perform 4 times slower than the remote SMD disk. ********** I talked to several people when I was configuring a Sun 4/260 system that we will order soon, and everyone said that there is a big difference in speed with the SMD disks faster. I am planning to order CDC Sabre V's (1 GB). I had considered EDSI disks as being cheaper. They also said that the CDC Sabres are more reliable than disks like Maxtors. ********** I'm not sure I follow your logic here. If it is performance you want, a large disk is the last alternative. While it is true that they back more bits per inch (circumferentially) and hence achieve higher transfer rates per rotation, they also have more tracks, and a greater distance to seek. Performance for me also includes reliability. So, you are better off getting several mid-sized disks and putting them on separate controllers. You get higher reliability, better utilization (as you can be seeking on one unit while reading on another), and about the same bandwidth. Some disks (typically the CDC units) have options for two controllers to talk to them -- so you can have one CPU run "standby" in case the other ("hot") CPU goes down. Just be sure not to have both CPUs using the disk at the same time or it will be corrupted. ********** With 18 clients I would heartily recommend sticking to SMD drives. They (and their controllers) are *faster* for overall system throughput. The 753 is an excellent idea, being much better than the 451 in my experience. Seek time is important, but its not the only factor. ********** For anything performance intensive, go with SMDE over SCSI or ESDI. The transfer rates are higher, and seek times will be shorter. Reliability is likely to be better, too, which may justify the cost differential by itself. However, I am getting some ESDI disks that I will use for low performace applications, like mail, news and on-line source code. These drives will be in one of those RT-135s that IBM is pushing right now, but the same logic would apply to SCSI disks (which are usually ESDI disks with a SCSI interface attached). But, for NFS those big Fujitsus look like the way to go to me. I've got one of their 8" 690 MB drives now and am very pleased with it. An 892 MB on a 753 should be dynamite. ********** Here at Washington University we have been very dissapointed with the performance of Sun's SCSI disks. Although we haven't tried other sources for SCSI drives, I think your are going to be better off with an SMD. While you're at it pick up a fast controller like the Ciprico Rimfire. ********** Wayne Cripps wbc@maxlin.DARTMOUTH.EDU wbc@maxlin.dartvax.uucp Box 6188 Dartmouth College, Hanover N.H. 03755 (603) 646-3198
aad@uunet.uu.net (Anthony A. Datri) (12/23/88)
maxlin!wbc@dartvax.dartmouth.edu writes: >We have a3/160 with 3 3/50(60) clients, OS 3.4. long ago we decided to go >to SMD despite the extra expense for the following reason: The scsi bus >seems to be able to talk to one device at a time. As I remember, Sun didn't implement disconnect/reconnect on the scsi bus, but I thought this was fixed in 3.4.