robm@ataraxia.Berkeley.EDU (Rob McNicholas) (05/11/91)
Hi all, I have a DECstation 3100 (24M memory, ULTRIX V4.1 (Rev. 55), one storage expansion box) with the following SCSI configuration: SCSI id device order in chain -------- ------ -------------- 0 rz57 1 ? (internal to storage expansion) 1 rz55 3 2 rz57 1 ? (internal to storage expansion) 3 UNUSED UNUSED 4 TZK08 (exabyte 8mm) 2 5 TK50 4 I was wondering if the order the devices are connected together affects their performance. I seem to remember reading somewhere that slower devices, like tape drives, should be either earlier or later in the chain, but I can't remeber the details or where I might have read it. Reading through the operators manual that came with my machine revealed no relevant information. Anyone have any tips? Thanks, -Rob -- Rob McNicholas Computer Systems Support Group, U.C. Berkeley robm@janus.berkeley.edu ....!ucbvax!janus!robm Home: 415/339-1514 Work: 415/642-8633
ajc@thendara.pa.dec.com (AJ Casamento) (05/15/91)
Rob McNicholas writes:
I have a DECstation 3100 (24M memory, ULTRIX V4.1 (Rev. 55), one
storage expansion box) with the following SCSI configuration:
SCSI id device order in chain
------- ------ --------------
0 rz57 1 ? (internal to storage expansion)
1 rz55 3
2 rz57 1 ? (internal to storage expansion)
3 UNUSED UNUSED
4 TZK08 (exabyte 8mm) 2
5 TK50 4
I was wondering if the order the devices are connected together
affects their performance.
There are a few items to be aware of:
* SCSI ID indicates priority, with the highest number receiving the
highest priority.
* Within the 6M limit of electrical cable equivalent, the devices
should be impacted by their SCSI ID and not by the order from
the system SCSI port.
* SCSI Host ID - On the DECstation 2100/3100 systems this will be a
default of 6 (the ID 7 was reserved for a Realtime CPU slot). On
the DECsystem 5000 series (1XX or 2XX) it will default to ID 7).
* In the first RISC/Ultrix SCSI driver implementations the following
recommendations for ID assignments was made (I haven't seen it
contradicted since)
7 Realtime CPU
6 CPU
5 TK50
4 RRD40
3 TZK08
2 RZ23
1 RZ55
0 RZ56
Obviously, there isn't a Realtime CPU so that address is useable.
The idea is to give the slower SCSI devices (except for CPU) a
higher SCSI ID number. In the case of the TK50 this is supposed
to be a real help. The RRD40 is somewhat faster. The position of
the 8mm is arguable. Although, I prefer to keep the tape media
at a higher priority. Most of the disk drives should play well
regardless of how you mix them. I will tell you honestly that I
remember some flakey things happening when a disk drive was given
a higher priority than the CPU, but I don't know if that's still
true.
I hope that this information helps.
Thanx,
AJ
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