> On a related theme, I have an Advansys 940UW controller
> connected to a IBM DCHS04U which should allow for 40 MB/sec
> of bandwidth. But /proc/scsi/advansys/0 reports:
> ..
> 0: Transfer Period Factor: 12 (20.8 Mhz), REQ/ACK Offset: 8
????????
Transfer period factor 12 -> period 50 nano-seconds which can be reported
as _exactly_ 20 MHz frequency even if that does not make full sense given
that the period is some minimal delay requirement in SCSI.
> Furthermore with tests using the SCSI READ BUFFER command I
> can measure >30 MB/sec throughput. My guess is that this
> ultra, u2 and u160 is done at a lower level and hidden
> from the messaging level lest older things get upset.
The data throughput does not make sense if you donnot consider the command
overhead. This overhead is not negligible at all when using ultra-2 and
will be an important differentiation point with Ultra-3, IMO.
Given that the average IO chunk is not that large in real applications, it
is important that softwares, controllers and devices result in a very low
command overhead. Bursting max data throughput in data phase is far easier
to achieve and I generally assume devices are quite able to sustain such
bursting throughput.
Example of calculation:
I can get up to 8900 TPS, reading sequentially 2 very fast drives at a
time in Ultra-2 mode using 4KB actual IO chunks (on a 53C896). Result is:
- Total command latency = 1000000/8900 = 112 micro-seconds per
transaction.
- Data phase latency _assuming_ 80 MB/s data phase throughput =
12.5 * 4 = 50 micro-seconds.
Result is that command overhead is less than 112-50 = 62 micro-seconds.
As a rule of thumb, speaking of throughput/bandwidth without speaking of
_latency_, when speaking about transactions based devices is chatting in
the wind.
Gérard.
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