SEQUENTIAL WRITE PROBLEM

[GFRAZIER at AUSVM6.VNET.IBM.COM]

* From the SCSI Reflector, posted by:
* GFRAZIER at AUSVM6.VNET.IBM.COM
*
PROBLEM
Many existing applications put time stamps on each end of log records so
thay they may recover after a system crash during the log write. If the
timestamps at each end of the log record are equal during recovery, then
they treat the record as valid.
Note that this behavior depends on the sectors being written to the disk
media in order. If a drive happens to write the last half of the data
before the first half (for latency minimization), then both beginning and
ending stamps can be correct while the center data has not been written.
This obviously results in an invalid log being recovered on restart.

SOLUTIONS
SSA has addressed this problem by adding a bit in it's frame header to
require sequential writes to the media, but unfortunately it cannot be used
for FCP or parallel SCSI. In order to provide a common method for all
physical media, a very slight revision to the EMDP bit in the
Disconnect/Reconnect mode page could be used.
The only modification would be to make the following change to the wording
of the EDMP bit definition: (Please see SPC, rev 8, p.101)

     "If the EMDP bit is zero, the target shall not reorder the SCSI data
      transfer OR THE DATA TRANSFER TO THE MEDIA."
          (Capitals indicate the change.)

Of course many other possible solutions to the problem exist, but I do not
believe this would be incompatible with existing SCSI devices, and it
results in the minimum impact to the standard.

Giles Frazier
IBM Austin
gfrazier at ausvm6.vnet.ibm.com