SCSI-2 PERFORMANCE VS. SCSI-1 AND SERIAL LINK
ITEM: RTA000030630
QUESTION:
Please explain performance improvements of SCSI-2 vs. SCSI-1. I have
read that the SCSI-2 is a 10MB/sec transfer (right?). Has the technology
changed such that more than 1 SCSI device can utilize that SCSI Bus
concurrently? If not, what is the advantage over SCSI-1, given that
none of our SCSI Disk drives will currently transfer data faster than
4MB/sec? Has the 6M bus length limitation been changed with SCSI-2?
I have seen foils indicating the serial link adapter is a "8MB/sec"
link, but according to an item I just read, it seems to indicate
this adapter actually supports up to 28MB/sec transfers because more
than one serial disk can be active at one time. Is this true? How much
"faster" is the serial link subsystem vs. SCSI-2 (i.e. - Maximum data
transfer, CPU utilization, MC Bus I/O efficiency). The serial link disks
are considerably more expensive than SCSI but they may be
worth it if actual performance is that much better.
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A: It is correct that the peak SCSI-2 bus transfer rate is 10MB/sec.
However, the peak SCSI-1 bus transfer rate has been revised to 5MB/sec.
This is documented in the System Overview (GC23-2406-01).
For either SCSI-1 or SCSI-2, only one device can utilize the bus at
one time. This does not imply that SCSI-2 disks cannot achieve
the 10 MB/sec transfer rate. All SCSI-2 disks, except the 7204-001,
can achieve 10 MB/sec bus transfer rate (this is documented on pages
1-58 and 1-59 of the above mentioned System Overview). I believe you
have mistaken the media transfer rate (which is approximately 3 MB/
sec) with the bus transfer rate. Media transfer rate is the speed
that data is read from the disk media to the disk buffer, mostly
during the time when they have no control of the bus.
Currently the IBM SCSI-2 devices have the following length limitation:
3 meters with non-supported cables, 3.75 meters with IBM supported
cables, and 4.5 meters for twin-tailing. The limit for SCSI-1 is 6
meters.
As for the serial link disk drives, you are correct that it is
possible to achieve 28MB/sec transfer rate per adapter due to the
concurrent access capability. For your information, below are some
benchmarks comparing SCSI-1, SCSI-2 and Serial Attached Drives:
The data is in units of kbytes/sec @ % CPU utilization. All reads
and writes are 4k and use the JFS file system. The filesystem has
read ahead and write behind algorithms that optimize sequential I/O.
The random I/O's are only over 16MB of the disk and would be slower
if they used an average seek. The multiple disk cases report the
aggregate throughput. These tests were run on AIX 3.2.3. The
performance of the SCSI-1 disks on a SCSI-2 adapter are about the
same as on a SCSI-1 adapter. There is little difference in the
performance of the 1.0 and 1.2GB SCSI-2 disks.
The serial disk numbers have not changed since AIX 3.2.0. The tests
were run on a Model 530H.
SCSI-1 SCSI-1 SCSI-2 SCSI-2 Serial
1 disk, 1 adapter --------- --------- --------- --------- ---------
1.2GB 1.37GB 1.2GB 1.37GB 857MB
read sequential 2169 @ 21 2667 @ 24 2180 @ 23 3123 @ 33 2291 @ 23
read random 292 @ 6 299 @ 6 285 @ 6 288 @ 6 358 @ 7
write sequential 1464 @ 22 2189 @ 32 2156 @ 34 2357 @ 37 2396 @ 28
write random 362 @ 7 491 @ 10 405 @ 9 549 @ 13 560 @ 12
2 disks, 2 adapters ------- --------- --------- --------- ---------
read sequential 4323 @ 42 5307 @ 48 4317 @ 43 6223 @ 59 -
read random 579 @ 13 594 @ 14 567 @ 13 569 @ 13 -
write sequential 2912 @ 43 3978 @ 57 4252 @ 67 4633 @ 70 -
write random 696 @ 15 915 @ 19 770 @ 18 1018 @ 24 -
2 disks, 1 adapter -------- --------- --------- --------- ---------
read sequential 3178 @ 29 2977 @ 27 4308 @ 45 3470 @ 38 4560 @ 48
read random 566 @ 13 576 @ 13 560 @ 13 553 @ 14 709 @ 15
write sequential 2895 @ 43 2986 @ 43 4238 @ 66 3595 @ 56 4515 @ 65
write random 680 @ 14 853 @ 18 761 @ 18 939 @ 23 1053 @ 21
3 disks, 3 adapters ------- --------- --------- --------- ---------
read sequential 6367 @ 61 7915 @ 78 6432 @ 71 8405 @ 91 -
read random 859 @ 20 885 @ 20 846 @ 20 848 @ 19 -
write sequential 3971 @ 58 5019 @ 76 5864 @ 93 5847 @ 91 -
write random 982 @ 21 1272 @ 27 1098 @ 27 1409 @ 34 -
3 disks, 1 adapter -------- --------- --------- --------- ---------
read sequential 3228 @ 29 3107 @ 28 5672 @ 56 3494 @ 36 6829 @ 72
read random 818 @ 18 820 @ 19 817 @ 18 782 @ 18 1052 @ 22
write sequential 2995 @ 43 3003 @ 43 5020 @ 76 3454 @ 55 6176 @ 85
write random 928 @ 20 1109 @ 24 1051 @ 25 1188 @ 29 1445 @ 30
4 disks, 1 adapter -------- --------- --------- --------- ---------
read sequential - - 5875 @ 58 - -
read random - - 1041 @ 23 - -
write sequential - - 4829 @ 76 - -
write random - - 1256 @ 30 - -
6 disks, 3 adapters ------- --------- --------- --------- ---------
read sequential 8709 @ 94 - 8562 @ 97 - -
read random 1649 @ 39 - 1634 @ 38 - -
write sequential 5708 @ 91 - 6021 @ 97 - -
write random 1701 @ 39 - 1878 @ 48 - -
========================================================================
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This item was created from library item Q625797 CDPPJ
Additional search words:
CDPPJ CHAIN CONNECT DISKS HARDWARE IX LINK LINKAGE MAY93 MEASURE
PERFORMANCE RISC RISCOHW RISCSYSTEM SCSI SERIAL SERIALIZE SERIES
TUNE VS 6000
WWQA: ITEM: RTA000030630 ITEM: RTA000030630
Dated: 04/1996 Category: RISCOHW
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