[rescue] Sun 711

[James Lockwood]

On Fri, 3 May 2002, George Adkins wrote:

> Ahh, but my point was, since in this case, we are talking about a system
> thwch needs only to perform *reads* at the quoted spec, (it'sn not a
> read/write thing, it's just display of data) then it's *not* illusory, it's
> what the customer wants.  He's not going to complain if it can only write at
> 150MB/sec, as long as it can read 300MB/sec...

It is illusory.  You can only "read" 300MB/sec in your example if half of
it is already cached in RAM (from a previous read).

If your read access is bursty, you may see higher appearant read speeds.
For example, you read for 1sec and get 150MB, then do something else for
1sec, then read for 1sec and get 300MB, etc.  In this case you were really
performing readahead for the 1sec when you were otherwise crunching
numbers, but your true I/O never exceeded 150MB/s.  You will never
sustain[1] a read speed higher than the data rate to the platters.

-James [TANSTAAFL]

[1] For sufficiently high values of sustain.