I'm taking over the content here as well.
Based on the number of transferred bytes per track the calculation for the amount of SMBs (sequential memory buffer) the DS8870 needs to allocate for transferring data over the channel may be wrong.
This wrong calculation leads to a code exception or an I/O hang in the DS8870 which may result in job aborts and IOS071I START PENDING messages.
The issue was introduced in the R7.1 GA bundle 18.104.22.168 and fixed in the bundles identified in the Resolution section below.
R7.1: all Microcode bundles below 22.214.171.124 R7.1 SP3, multiple field occurrences observed, all on bundle 126.96.36.199
Disable zHPF until a service window can be scheduled to update DS8870 firmware to a bundle containing the fix.
A DS8870 microcode build to fix this issue has been made available in Bundle 188.8.131.52 or higher.
184.108.40.206 R7.1 SP3
220.127.116.11 R7.2 GA
For those customers upgrading code from R7.0 to R7.1 and running zHPF the minimum recommended microcode level is 18.104.22.168 (R7.1 SP3)