PowerFlex: SDC unmap called with result larger than buffer blkScsiCmd_UnmapWithSize

SDC for VMware is installed in the Kernel, and its logging will appear in vmkernel.log. In UNMAP operations, the following warning might be shown during an UNMAP operation:

 Unmap called with a result larger than buffer.

For more information about UNMAP and Reclamation, see the following KBs:

PowerFlex: Reclaiming VMFS deleted blocks on Thin Provisioned Volumes

Reclaiming VMFS deleted blocks on Thin Provisioned LUNs (External Link)

Log details

vmkernel.log

2022-10-02T04:15:26.972Z cpu82:2377729)WARNING: PowerFlex blkScsiCmd_UnmapWithSize:391 :Error: Unmap called with result larger than buffer: Opcode UNMAP; command 0x0x45d9738c2b88; Buffer size: 24, Result size 34

• WARNING - The event identification The actual event is at a warning level. The "Error" mentioned inside the context is an internal PowerFlex usage to internally identify an unexpected event. It does not mean the event itself is at an error level.
• PowerFlex blkScsiCmd_UnmapWithSize - This message comes from the PowerFlex SDC when handling a SCSI UNMAP command. UNMAP is part of the SCSI protocol for space reclamation (similar to TRIM in SSDs), allowing the host to inform the storage system that certain blocks are no longer in use.

• PowerFlex internal error details:

  • Opcode UNMAP → Confirms the command type.

  • Buffer size: 24, Result size: 34 → The response from the UNMAP operation was larger than the allocated buffer, from the storage.

    This is a protocol-level mismatch between what the host expects and what the storage system returned.

Impact

• This is typically non-fatal and does not indicate any issue with the data on the host or storage side.

• It often occurs when the host (e.g., VMware ESXi or Linux kernel) sends an UNMAP command and the PowerFlex SDC driver encounters a response that exceeds the expected size.

• The operation may retry in some scenarios, but the warning is logged for visibility only.

Conclusion

This warning message is related to the SCSI UNMAP command, which is used to reclaim space on thin-provisioned storage arrays. When blocks of data are deleted within a virtual machine, the hypervisor can send an UNMAP command to the storage array to let it know that the blocks are no longer in use and can be reclaimed.

The message "Unmap called with result larger than buffer" suggests that the size of the data being unmapped (the "result size") is larger than the buffer allocated for the operation (the "buffer size").

In PowerFlex, if the number of bytes that are reported to be reclaimed (Result size) is greater than the buffer given (Buffer size), the result size is truncated to the buffer size (to avoid buffer overflows).

This warning indicates a buffer size mismatch during a SCSI UNMAP operation. It is harmless and cosmetic only.