RE: Status summary on multiple connections

[JoeBre@exabyte.com]

Title: RE: Status summary on multiple connections

Much good discussion on SCSI device level flow control snipped....

> From: Robert Snively <rsnively@Brocade.COM>
> To: "'David Robinson'" <David.Robinson@EBay.Sun.COM>, Robert Snively
> <rsnively@Brocade.COM>
> Subject: RE: Status summary on multiple connections
> Date: Thu, 28 Sep 2000 09:36:50 -0700

...

> are again available.  Note that there is a possibility that commands
> that are inflight and have ordering constraints may be accepted out of
> order, a question that has caused lots of agonizing, but is apparently
> reasonably well managed by most file systems today by the selective
> use of ordering only for blocking boundaries of a particular logical
> stream of commands.

I am left with the following impression as to what was indicated here:
- In general, command ordering is not relevant
- If the initiator filesystem detects an ordering dependency, it will wait until outstanding commands are complete before issuing the dependant command.

This may be a reasonable means of operation for the disk world. It is woefully inadequate for the tape world, as follows:

- In general, command ordering is crucial - out of order command processing will lead to data corruption.
- This would require the initiator backup application to block on completion of every single write command of a backup operation before issuing the next command.

If this blocking were performed, both the throughput and capacity of a tape device/media would be negatively impacted by an order of magnitude or more. This would occur even assuming an instantaneous transport.

Joe Breher
Exabyte Corp