Some comments on FC-TAPE v 1.06b

Bob Snively Bob.Snively at Ebay.Sun.COM
Fri Jul 10 15:00:18 PDT 1998


* From the T10 (formerly SCSI) Reflector (t10 at symbios.com), posted by:
* Bob Snively <Bob.Snively at Ebay.Sun.COM>
*

Dennis,  

Command chaining will not accelerate this function, since completion
information must travel from the tape device to the command source
between each command as part of the protocol.  For this reason, we
have created command queueing, which allows a queue of commands to
be transmitted to the tape drive, then the tape drive can request
the flow of data to begin and can overlap continuous writing and
requesting over a period of many commands, something not allowed by
command chaining.  Ordering must be maintained in the delivery of the
commands, but aside from that there are no other restrictions.

Incidentally, another acceleration possibility is the overlapped transmission
of files to a remote host which performs the actual tape manipulations as
a local function.  Various combinations of these solutions can also be used.

Bob

> > # 37 & #48
> >c)	This recommendation will flush out any real (as opposed to
> >  	hypothetical) usage of it, allowing us to directly address
> >   	the question in the profile.
> 
> Here is a real, non-hypothetical use for command linking/chaining - remote
> electronic data vaulting.  We do it all the time.  Pick out some secure
> facility, perhaps 100's or even 1000's of miles from the client's main site,
> put in some large tape libraries with perhaps ten's of tape transports, and
> connect the two sites with telecommunication links.  What happens in this
> extended channel environment is that I/O channel propagation delay and
> latency (when combined and measured) jumps from under 1 microsecond, to
> perhaps 4 - 100 milliseconds (depending upon the circuit miles, transmission
> facilities, and premise equipment).  Unless one is employing equipment
> highly optimized to exploit this environment, the tape back-ups will grind
> to a crawl.
> 
> It is our intent to deploy open systems tape subsystems in exactly this same
> manner - perhaps by inserting some sort of neutral network transport to
> carry the FC frames between geographically dispersed fabrics (national,
> trans-national, transcontinental)(that is a whole other project... | ; - })
> .  In this case, it would be highly advantageous to chain some
> commands/operations to avoid the affect of latency.  Hence, you may wish to
> chain multiple read operations, or write operations, or perhaps send the
> sense or status command along with certain read/write op. This is feasible
> if the nature of the I/O request and the results are clearly understood.
> True, in a local configuration, unless one is performing massive amounts of
> back-up operations, the effects of chaining are present but not that
> obvious, and the business value may be difficult to measure.  However, the
> larger the infrastructure, and the more geographically disperse it becomes,
> the benefits become measurable and significant.
> 
> I urge careful consideration before elimination of command linking from this
> profile.


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