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Document ID: SRUL-3DYM9G |
Servers - Performance advantage of Ultra-SCSI vs. SCSI-2
Applicable to: World-Wide
Ultra SCSI only improves the transfer rate of data over the SCSI bus. In most transaction processing environments the data is accessed randomly within a large database. Furthermore, data is read 2K pages. Typically the drives have 8 mSec Seek. Rotational speed is 7200 RPM ==> 120 Rev/Sec ==> 8 mSec/Rev ==> 4 mSec for 1/2 rev.
1/2 revolution is the average latency (since data can be anywhere on the disk and the average location is 1/2 a revolution away from current head location).
The last piece is transfer time:
For 2 Kbytes at 20 MB/Sec we get 0.976 mSec.
And for 2 Kbytes at 40 MB/Sec we get 0.488 mSec.
Thus, for the typical access for F/W SCSI we have:
Seek : 8 mSec
Latency : 4 mSec
Transfer: 0.976 mSec
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Total : 12.976 mSec
And for an access using UltraSCSI we have:
Seek : 8 mSec
Latency : 4 mSec
Transfer: 0.488 msec
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Total 12.488 msec
A whopping difference of 12.976/12.499 = 4%
Of course when the transfer size increases for say file serving or where images are transmitted (e.g. Webserver, multimedia) the block size increases to up to 64KBytes. In this case the transfer time becomes 64 KB / 20 MB/sec for F/W SCSI which yeilds a transfer time of 3 mSec. And the transfer time for 64 KB / 40 MB/sec = 1.5 mSec.
In this case total access time is 15 mSec for F/W SCSI and 13.5 mSec for Ultra.
Improvement here is 15/13.5 = 11%. Good but probably not what most people expect.
On the desktop, where a single application accesses the hard disk (greatly reduced seek operations) the accesses are more sequential in nature. In this case the seek can be reduced to 0. This dramatically changes the performance improvement that one would obtain from UltraSCSI.
e.g.
For 2K operations:
F/W SCSI
Seek : 0 mSec
Latency : 4 mSec
Transfer: 0.976 mSec
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Total : 4.976 mSec
And for an access using UltraSCSI we have:
Seek : 0 mSec
Latency : 4 mSec
Transfer: 0.488 msec
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Total 4.488 msec
A gain of 11%.
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For 64K operations:
F/W SCSI
Seek : 0 mSec
Latency : 4 mSec
Transfer: 3.0 mSec
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Total : 7.0 mSec
And for an access using UltraSCSI we have:
Seek : 0 mSec
Latency : 4 mSec
Transfer: 1.5 msec
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Total 5.5 msec
A gain of 27%.
Servers almost always have a seek and latency associated with each I/O because the server is scheduling I/O for many different users. Each user is accessing unrelated data. Furthemore, servers see more disk fragmentation than desktop environments. In summary, you can see that on a defragmanted desktop system UltraSCSI can provide a very significant boost in performance. I suspect more then a few people will expect this same performance improvment to translate to the server space. Unfortunately this is not usually the case.
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Hint Category |
SCSI | |
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Date Created |
04-02-97 | |
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Last Updated |
19-01-99 | |
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Revision Date |
19-01-2000 | |
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Brand |
IBM PC Server | |
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Product Family |
Servers | |
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Machine Type |
Various | |
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