RE: Status summary on multiple connections

[Raghavendra Rao <jp.raghavendra@india.sun.com>]

> From: "Y P Cheng" <ycheng@advansys.com>
> To: "Mark A. Carlson" <mark.carlson@sun.com>, "Randall R. Stewart" 
<randall@stewart.chicago.il.us>
> Cc: "David Robinson" <David.Robinson@ebay.sun.com>, <ips@ece.cmu.edu>

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> commands inflight.  My preference is letting the target accept the N+2 as
> long as the application software does not mind out-of-order execution.  For
> folks who design tape drives, I don't recall if we ever send multiple writes
> without interleaving data. In fact, if we need write a large amount of data
> to a tape drive, instead of multiple commands, we should have one command
> with a very large block count.  With one command at a time, there should not
> be any out-of-order execution problem.  Modern tape drives are doing "lying
> writes", i.e. accepting write data immediately after the command, then
> report command complete before data is written to the media.  By accepting a
> write, receiving data, and report completion sequentially, there is no need
> to accept more than one write command at a time.  On reading from a tape, we
> could send multiple reads to keep the pipeline filled. 
>

Well, on reads, a similar problem could occur, and for both read and write
it is just wrong to execute anything out-of-order for a sequential access
device, which otherwise will cause reading/writing from wrong/un-intended
location; This is simply because you aren't writing/reading from an absolute
location rather from the current location (the position of head) - The read
or write CDB for a sequential access device doesn't contain the block or file
position to write/read.

-JP