SMC FAQ

Q: Can I replace HiperSockets with SMC-D?
A: No - SMC-D supplements HiperSockets, just like SMC-R supplements traditional TCP connections. The major reasons for that are:

1. SMC supports TCP only - not UDP
2. The initial setup of each connection always requires traditional connectivity. And even if both peers support SMC-D and/or SMC-R, there is always the possibility that a connection using SMC-D/R cannot be established, and traditional connectivity will be used instead.

Q: What tuning parameters exist for SMC-D and SMC-R?
A: The following tuning parameters exist:

1. SMC-R and SMC-D: There is an option to increase the buffer sizes used by SMC. Bigger sizes are usually beneficial for streaming workloads, but might also benefit high frequency request-response workloads.
   
   The current Linux SMC implementation has a maximum buffer size of 512KBytes (SMC-R) or 1MBytes (SMC-D). It can either be set using smc_run (see options -r and -t), or derived from the tcp sysctls as follows:
   Use the following commands to switch to the maximum buffer sizes:
     # receive buffers (aka RMBEs)
     sysctl -w net.ipv4.tcp_rmem="4096 1048576 6291456"
     # send buffers
     sysctl -w net.ipv4.tcp_wmem="4096 1048576 4194304"
   Note: The significant value is the default value in the middle.
2. SMC-R only: The MTU size for RoCE traffic is derived from the respective port's MTU. SMC will use the highest available MTU supported by both peers.

Q: Does the peer have to use the same PNET ID as our host?
A: No. PNET IDs help to locate a suitable (R)DMA device for a given NIC within a host. The peer can use totally different PNET IDs (as long as the right devices are grouped).

Q: Attaching a PCI device (ISM or RoCE Express) in z/VM results in the following error message:
  HCPPCI1168E PCI function 00000650 cannot be attached to
  XYZ because a function with the same UID is already attached
  to that guest.
A: The respective IOCDS entries for the PCI devices have no UIDs specified, hence all PCI devices default to the same UID. By default, z/VM validates this, and prevents attaching two PCI devices with identical UIDs.
To resolve, either update IOCDS with the correct UIDs, or turn off the check in z/VM using CP command
  SET IO_OPT UID OFF
However, note that UIDs are required for stable device IDs across reboots.

Q: Does SMC support live guest relocation (z/VM) aka live guest migration (KVM)?
A: Neither SMC-R nor SMC-D support live guest relocation/migration at this point in time.