idallen@watcgl.waterloo.edu (10/20/89)
From: "Ian! D. Allen [CGL]" <idallen>
I'm trying to specify a DS5810 system with about 3.6 Gb of disk.
I see that the VAXBI KDB50 controller advertises a sustained throughput
of only 1.2 Mbyte/s. This is slower than the published transfer rate
from an RA70 (1.4Mb/s) or an RA90 (2.8Mb/s). Does this mean
- that even with only one disk on a KDB50 controller, we won't see
the full performance of an RA70 or RA90 disk?
- that putting more than one disk on a controller is silly if we
anticipate much disk use, since even with only one disk the
controller can't keep up?
If one buys a bunch of RA70 disks (SA70 modules), does the availability
of many seek arms make up for the fact that the RA70 disks are a bit
slower in both seek and transfer rate than the RA90 disks? Will the
slowness of the controller hide the speed advantage of the RA90's, making
the RA70's multiple seek arms a much better idea?
Should I be considering an "HSC40 Intelligent I/O Server" if I really
want to get the most out of my disks? What do the figures mentioned
in regard to the throughput of this thing mean relative to KDB50
throughput, and can Ultrix 3.x really handle one of these effectively?alan@shodha.dec.com ( Alan's Home for Wayward Notes File.) (10/21/89)
In article <12002@watcgl.waterloo.edu>, idallen@watcgl.waterloo.edu writes: > From: "Ian! D. Allen [CGL]" <idallen> > > I'm trying to specify a DS5810 system with about 3.6 Gb of disk. > > I see that the VAXBI KDB50 controller advertises a sustained throughput > of only 1.2 Mbyte/s. This is slower than the published transfer rate > from an RA70 (1.4Mb/s) or an RA90 (2.8Mb/s). The newer KDB50s have a different microcode that should make it a little faster than that. It's still not as fast the RA90 though. > Does this mean > > - that even with only one disk on a KDB50 controller, we won't see > the full performance of an RA70 or RA90 disk? That's right. On the other hand, the best file system I/O I've seen on an Ultrix system using KDB50s and RA90s is about 850 KB/sec for one disk. When I added the 2nd disk the total went to around 900 KB/sec. At the moment I only have two disks on the KDB50s. This was on a VAX 8800. You might see better on a DECsystem, but I suspect the system call overhead will keep you from seeing the spiral I/O rate of the disk. > > - that putting more than one disk on a controller is silly if we > anticipate much disk use, since even with only one disk the > controller can't keep up? That will depend on the application. If you expect to have many disks all running as fast as possible at the same time, then you will need many controllers. If you can describe the throughput and bandwidth requirements, I'll have a better idea of what you'll need. > > If one buys a bunch of RA70 disks (SA70 modules), does the availability > of many seek arms make up for the fact that the RA70 disks are a bit > slower in both seek and transfer rate than the RA90 disks? Will the > slowness of the controller hide the speed advantage of the RA90's, making > the RA70's multiple seek arms a much better idea? Again it depends on the application and how you distribute the data. Ultrix doesn't have multi-volume or striping support. If you want the application to take advantage of the multiple arms of many RA70s, then you have to write the appliation and layout the data to do that. With the right software many RA70s distributed among multiple controllers will probably be faster than equilalent capacity of RA90s. > > Should I be considering an "HSC40 Intelligent I/O Server" if I really > want to get the most out of my disks? What do the figures mentioned > in regard to the throughput of this thing mean relative to KDB50 > throughput, and can Ultrix 3.x really handle one of these effectively? It depends on what you're looking at in the I/O load. If bandwidth is the concern the HSC is currently limited by the CI adapters. Two KDB50s are capable of using more bandwidth than a CI adapter. When you lots of I/O requests with many outstanding requests to each drive then the HSC is better at optimizing throughput. One advantage of the HSC is that it supports the faster tape drives (TA79 and eventually TA90). This might be a concern when you start having to backup many gigabytes of data. Another thing to consider is the number of disks needed to get the 3.6 GB of space you mentioned at the beginning. That requires 14 RA70s or 3 RA90s. 14 RA70s will require at least four KDB50s or an HSC70 with at least four disk controllers. The HSC40 is limited to 12 ports and if you put a tape controller on it you're limited to 8 disks. If you go with many disks then you may want to divide the load between local controllers and an HSC. * * * The right solution really depends on the I/O load that you expect. If the most common I/O operation is going be sequentially reading a many megabyte file then getting the most bandwidth needs to be the goal. If the common operation is going lots of relatively small I/O operations scattered all over the disk then throughput and fast seeks will be the goal. -- Alan Rollow alan@nabeth.enet.dec.com