T10/97-155R1.TXT More Discussion of Tapes in PLDA rev 1
Doug Hagerman, Digital Equipment, 508-841-2145, Flames to /dev/null 27-Mar-1997 0933
hagerman at starch.enet.dec.com
Thu Mar 27 06:26:18 PST 1997
* From the SCSI Reflector (scsi at symbios.com), posted by:
* "Doug Hagerman, Digital Equipment, 508-841-2145, Flames to /dev/null 27-Mar-1997 0933" <hagerman at starch.ENET.dec.com>
"More Discussion of Tapes in PLDA" T10/97-155r1
Here is another attempt at putting together a "what to do about
tapes on Fibre Channel" proposal. This version is modified based
on discussion that took place during the SSC meeting during the
March 1997 T10 week.
I had wanted to keep the introductory material brief, but it became
clear at the meeting that most of the confusion is in that material,
so it's longer in this case.
Note that there are really two configurations where this topic
i. Native Fibre Channel tape drives.
ii. Native Fibre Channel subsystem controllers (e.g. RAID controllers)
that have the capability of having a tape drive behind them. This
drive could be a regular SCSI tape drive. Fibre Channel needs to have
a tape-oriented protocol on the FC connection to the host
even if no native Fibre Channel tape drive is ever built.
2. Overview of the Fibre Channel Tape Problem
Tape devices have different performance requirements than disks.
The special characteristics of tapes in an FC-AL environment are
summarized as follows:
a. If a tape command or data transfer fails on the interconnect, the
recovery requires more than simply the reissuance of the command.
The operating system driver software must manage the position of
the media by issuing a sequence of repositioning commands in addition
to reissuing the failed I/O command. This code is in SCSI tape
drivers now, but the mechanical process required to complete the
recovery may be time consuming.
Note the distinction between "the application" (user's FORTRAN
program) and "the driver" (operating system device driver). The user's
program is not supposed to worry about repositioning after an
interconnect data error.
Also note that the driver has two parts: "the class driver" (knows
about tapes, not interconnects) and "the port driver" (knows about
interconnects, not tapes). A goal is to keep these 100% distinct.
b. Using the SCSI command timeout to detect errors is generally unacceptable
because the timeout value must be set to a large number (e.g. 10 minutes)
to enable normal tape device operation. The timeout method may be
acceptable if the error rate at the physical level is low enough
so that the timeout is only excercised once or twice a day.
c. When devices are swapped on an FC-AL loop the loop signal is
disrupted. It may not be possible to predict when this will occur,
but in some environments many devices may be swapped in a day.
d. The FC-AL loop may under normal conditions experience fairly
frequent random bit errors. A normal parallel SCSI bus experiences
errors at an extremely low rate--weeks may pass between parity errors.
It is not known how frequently bit errors will occur on a normally
operationg FC-AL loop. Worst-case calculations indicate that
hardware complying with the standards may deliver an error bit
every 10 seconds.
One may argue what the delivered error rate will be. However, in
order to minimize risk at the system level, the PLDA profile must
protect against the worst case. The following is based on
A secondary goal is to avoid the introduction of Class 2 as a special
case for tapes. This is particularly important in the case of
subsystem controllers that must support both disk and tape device
models. How is the driver to know whether to send a given
INQUIRY command using Class 2 or Class 3? Must the driver handle
INQUIRY commands differently from READ or WRITE commands?
The best place to fix the tape problem is at the FCP level as
described in PLDA. FC-PH and SCSI are long-established, and changes
to SCSI driver software or FC-PH hardware are not desireable.
Furthermore, it has already been agreed by the owner of FCP that FCP
could be changed if a need can be demonstrated. Small changes to FCP
and PLDA cause the minimum amount of disturbance to the status quo.
3. Reliable Tape Transfers to Be Constrained in Size
My previous contention was: It is widely agreed (not universally) that
ALL tape transfers may be classified as one of:
a. Transfers where data integrity is required, and where a maximum
of 64kBytes will be transferred in any SCSI I/O command, or
b. Transfers where bulk data is being moved and a data error should be
ignored, and where the maximum transfer size may be greater than 64kB.
This contention was rejected by the committee. Therefore any solution
must handle the case of very long transfers done by a single SCSI
4. Overview of Proposed Solution
During the meeting the original proposal was modified so as to
add, for READs, what amounts to an FCP-level acknowledgement for
every sequence. This can be though of as an "FCP ACK 1".
(In FC terminology, ACK 1 is "acknowledge receipt of one frame".
ACK 0 is "acknowledge receipt of all frames of a sequence".
ACK n is "acknowledge receipt of n frames".)
A new FCP information unit FCP_CONF is needed to send this
acknowledgement or confirmation. This allows the initiator to request
retransmission of data if a transfer fails, and does not involve the
user's application program in the retransmission.
Under this proposal, transfers would look like this:
WRITE: Transfer of "n" DATA sequences. Each DATA below is one sequence.
The target tells the host how much data it can
accept before another FCP_XFR_RDY will be needed
Say it's two sequences in this example
DATA a ----------> This DATA sequence transferred successfully
DATA b ----------> This DATA sequence transferred successfully
DATA c ---------->
DATA d -----X..... Error occurs at "X"
Error is detected by target using sequence count
All further frames are ignored
Target waits RA_TOV to age any pending frames
With offset set back to "c"
DATA c ---------->
DATA d ---------->
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