| Internet-Draft | pNFS SCSI Layout for NVMe | June 2022 |
| Hellwig | Expires 4 December 2022 | [Page] |
This document explains how to use the Parallel Network File System (pNFS) SCSI Layout Type with transports using the NVMe or NVMe over Fabrics protocol.¶
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The pNFS Small Computer System Interface (SCSI) layout [RFC8154] is a layout type that allows NFS clients to directly perform I/O to block storage devices while bypassing the MDS. It is specified by using concepts from the SCSI protocol family for the data path to the storage devices. This documents explains how to access PCI Express, RDMA or Fibre Channel devices using the NVM Express protocol [NVME] using the SCSI layout type by leveraging the SCSI Translation Reference ([NVME-STLR]). This document does not amend the pNFS SCSI layout document in any way, instead of explains how to map the SCSI constructs used in the pNFS SCSI layout document to NVMe concepts using the NVMe SCSI translation reference.¶
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in [RFC2119].¶
The following definitions are provided for the purpose of providing an appropriate context for the reader.¶
The SCSI layout definition [RFC8154] only references few SCSI specific concepts directly. This document uses the NVMe SCSI Translation Reference document ([NVME-STLR]) to provide mappings from these SCSI concepts to NVM Express ([NVME]) concepts that SHOULD be used when using the pNFS SCSI layout with NVMe devices.¶
The NVMe SCSI Translation Reference is used to define the NVMe command and concepts that SHOULD be used to implement the pNFS SCSI layout. Implementations MAY or MAY not use an actual SCSI to NVMe translation layer.¶
The pNFS SCSI layout uses the Device Identification VPD page (page code 0x83) from [SPC4] to identify the devices used by a layout. Section 6.1.4 of [NVME-STLR] lists ways to build SCSI Device Identification descriptors from NVMe Identify data. To be used as storage devices for the pNFS SCSI layout, NVMe devices MUST support either the EUI64 or NGUID value in the Identify Namespace data, as the methods based on the Serial Number for legacy devices might not be suitable for unique addressing needs and thus MUST NOT be used. If possible NVMe devices uses as storage devices for the pNFS SCSI layout SHOULD support the NGUID value as it is the larger identifier.¶
The SCSI layout uses Persistent Reservations to provide client fencing. For this both the MDS and the Clients have to register a key with the storage device, and the MDS has to create a reservation on the storage device. Section 6.7 of [NVME-STLR] contains a full mapping of the required PERSISTENT RESERVE IN and PERSISTENT RESERVE OUT SCSI command to NVMe commands which SHOULD be used when using NVMe devices as storage devices for the pNFS SCSI layout. One important difference between SCSI and NVMe Persistent Reservations is that NVMe reservation keys always apply to all controllers used by a host (as indicated by the NVMe HOSTID). This behavior is somewhat similar to setting the ALL_TG_PT bit when registering a SCSI Reservation key, but actually guaranteed to work reliably.¶
The equivalent of the WCE bit in the Caching Mode Page in [SBC3] is the Write Cache Enable field in the NVMe Get Features command, see Section 6.3.3.2 of [NVME-STLR]. If a write cache is enable on a NVMe device used as a storage device for the pNFS SCSI layout, the MDS must ensure to use the NVMe FLUSH command to flush the volatile write cache.¶
Since no protocol changes are proposed here, no security considerations apply.¶
The document does not require any actions by IANA.¶