joe@dayton.UUCP (Joseph P. Larson) (01/23/90)
Dayton's is considering purchasing Amiga 2000s or 500s and adding mega-storage for some in-store capabilities. The systems purchased would directly affect the customer. Thus, reliability of the 2000 or 500 is important. However, reliability of the disk is an absolute. While an occassional system crash can be tolerated, loss of data can not. (Another proposed use for an Amiga involves less disk space, but lots of math.... We're actually starting to *use* these things here.) For these reasons, I'm nervous. Disk space on our more traditional machines (such as our NCR Towers) is substantially more expensive than on an Amiga. (Several thousand vs. several hundred.) This suggests to me that NCR is making an awful lot more on disk drives than they have the right to or that they are making them substantially better. I'm more willing to believe the drives cost more because they are worth more. Thus, I need to know -- if we had, say, 40 Amiga 2000s with good SCSI (Quantum?) SCSI drives on them, are we going to experience more than one disk failure a year? (That's about the number we get with our Towers.) How about failed 2000s? These machines are always going to be left on, and the disk can expect to do a fair amount of work, especially at night when we download data from our mainframe. Comments from those In The Know? Does Commodore keep mean-time-between- failure numbers? How about the disk drive manufacturers? Reply to me and I'll summarize. (Let's keep down the net clutter.) -Joe -- UUCP: rutgers!dayton!joe (Picts 1-16 are DHDSC - Joe Larson/MIS 1060 ATT : (612) 375-3537 now ready.) 700 on the Mall, Mpls, Mn. 55402
eachus@aries.mitre.org (Robert I. Eachus) (01/23/90)
If you truely mean that reliabiltiy of the disk is absolute, you
are going to have to have some scheme for fault tolerance. From
experience, cheap disks or expensive disks, they can all be killed by
a major power failure. (Surge protectors and such are nice, but when
the whole city goes out, power lines do very strange things, and these
up and down surges often cause head crashes.) Thus the best strategy
for protecting data is either 1.) Write everything to two hard disks.
or 2.) Keep the days transactions in battery backed RAM (and on hard
disk) and back that up using floppies or some other media.
Approach number two is very inexpensive on the Amiga. Several
manufacturers of RAM boards sell a battery backup option, and the
Amiga kernel requires no modification to support it. With careful
system design, you can have a fault tolerant Amiga application for
less than $1000 extra in hardware costs, and no modifications to any
commercial applications you may be running (assuming they allow
multitasking). All you need is a background task which shadows the
key files,or you can modify the device driver for the hard disk to
write to two places.
Having said all that, the choice should be obvious. You can
either rely on high-cost high-reliability disk drives (there are very
reliable SCSI disks available in the $4000 range), and do regular
backups (and hope they are recent enough when something happens) or
set up your system to assume disks will fail, and keep a few spare
disks in stock. If you plan to do good backups (or should I say if
you will do good backups), then even one failure per month should be
no problem.
Some quick relability calculations: 24*365 = 8760 hours/year.
Above 40000 hours guarenteed MBTF, disk drives get VERY expensive.
Forty machines at 40000 hours MBTF is eight to nine failures per year.
Recommendation: buy 25000 hour MBTF drives, and set things up so that
a disk failure results in less than an hour of lost time. (If that
means external drives so be it.) BB-RAM is worth the price whatever
you do.
Another alternative is to buy Ethernet cards (or perhaps use DNet)
and maintain all records on a single central fileserver. Hard disks
in the Amigas will be unnecessary, and your only problem will be
making damn sure the file system on the fileserver is reliable. When
the inevitable crash occurs three or five years down the road should
not be the first time you try to restore things from backups. My
experience with the Ameristar card can be summed up as I'm not going
to tear my hard disk out of the Amiga, but if it fails I may not
replace it. (Acutally I'll probably replace it with a Bernoulli or
some such.) The convenience of having to do one backup instead of 40
may make this the best approach for you.
--
Robert I. Eachus
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