Internet Engineering Task Force (IETF) C. Hellwig, Ed.
Request for Comments: 9561
Category: Standards Track C. Lever
ISSN: 2070-1721 Oracle
S. Faibish
Opendrives.com
D. Black
Dell Technologies
March 2024
Using the Parallel NFS (pNFS) SCSI Layout to Access Non-Volatile Memory
Express (NVMe) Storage Devices
Abstract
This document specifies how to use the Parallel Network File System
(pNFS) Small Computer System Interface (SCSI) Layout Type to access
storage devices using the Non-Volatile Memory Express (NVMe) protocol
family.
Status of This Memo
This is an Internet Standards Track document.
This document is a product of the Internet Engineering Task Force
(IETF). It represents the consensus of the IETF community. It has
received public review and has been approved for publication by the
Internet Engineering Steering Group (IESG). Further information on
Internet Standards is available in Section 2 of RFC 7841.
Information about the current status of this document, any errata,
and how to provide feedback on it may be obtained at
https://www.rfc-editor.org/info/rfc9561.
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Table of Contents
1. Introduction
1.1. Requirements Language
1.2. General Definitions
1.3. Numerical Conventions
2. SCSI Layout Mapping to NVMe
2.1. Volume Identification
2.2. Client Fencing
2.2.1. PRs - Key Registration
2.2.2. PRs - MDS Registration and Reservation
2.2.3. Fencing Action
2.2.4. Client Recovery after a Fence Action
2.3. Volatile Write Caches
3. Security Considerations
4. IANA Considerations
5. References
5.1. Normative References
5.2. Informative References
Acknowledgements
Authors' Addresses
1. Introduction
NFSv4.1 [RFC8881] includes a pNFS feature that allows reads and
writes to be performed by means other than directing read and write
operations to the server. Through use of this feature, the server,
in the role of metadata server, is responsible for managing file and
directory metadata while separate means are provided to execute reads
and writes.
These other means of performing file reads and writes are defined by
individual mapping types, which often have their own specifications.
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 Metadata Server (MDS). It is
specified by using concepts from the SCSI protocol family for the
data path to the storage devices.
NVM Express (NVMe), or the Non-Volatile Memory Host Controller
Interface Specification, is a set of specifications to talk to
storage devices over a number of protocols such as PCI Express
(PCIe), Fibre Channel (FC), TCP/IP, or Remote Direct Memory Access
(RDMA) networking. NVMe is currently the predominantly used protocol
to access PCIe Solid State Disks (SSDs), and it is increasingly being
adopted for remote storage access to replace SCSI-based protocols
such as iSCSI.
This document defines how NVMe Namespaces using the NVM Command Set
[NVME-NVM] exported by NVMe Controllers implementing the NVMe Base
specification [NVME-BASE] are to be used as storage devices using the
SCSI Layout Type. The definition is independent of the underlying
transport used by the NVMe Controller and thus supports Controllers
implementing a wide variety of transports, including PCIe, RDMA, TCP,
and FC.
This document does not amend the existing SCSI layout document.
Rather, it defines how NVMe Namespaces can be used within the SCSI
Layout by establishing a mapping of the SCSI constructs used in the
SCSI layout document to corresponding NVMe constructs.
1.1. Requirements Language
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
"OPTIONAL" in this document are to be interpreted as described in
BCP 14 [RFC2119] [RFC8174] when, and only when, they appear in all
capitals, as shown here.
1.2. General Definitions
The following definitions are included to provide context for the
reader.
Client: The "client" is the entity that accesses the NFS server's
resources. The client may be an application that contains the
logic to access the NFS server directly, or it may be part of the
operating system that provides remote file system services for a
set of applications.
Metadata Server (MDS): The Metadata Server (MDS) is the entity
responsible for coordinating client access to a set of file
systems and is identified by a server owner.
1.3. Numerical Conventions
Numerical values defined in the SCSI specifications (e.g., [SPC5])
and the NVMe specifications (e.g., [NVME-BASE]) are represented using
the same conventions as those specifications wherein a 'b' suffix
denotes a binary (base 2) number (e.g., 110b = 6 decimal) and an 'h'
suffix denotes a hexadecimal (base 16) number (e.g., 1ch = 28
decimal).
2. SCSI Layout Mapping to NVMe
The SCSI layout definition [RFC8154] references only a few SCSI-
specific concepts directly. This document provides a mapping from
these SCSI concepts to NVM Express concepts that are used when using
the pNFS SCSI layout with NVMe namespaces.
2.1. Volume Identification
The pNFS SCSI layout uses the Device Identification Vital Product
Data (VPD) page (page code 83h) from [SPC5] to identify the devices
used by a layout. Implementations that use NVMe namespaces as
storage devices map NVMe namespace identifiers to a subset of the
identifiers that the Device Identification VPD page supports for SCSI
logical units.
To be used as storage devices for the pNFS SCSI layout, NVMe
namespaces MUST support either the IEEE Extended Unique Identifier
(EUI64) or Namespace Globally Unique Identifier (NGUID) value
reported in a Namespace Identification Descriptor, the I/O Command
Set Independent Identify Namespace data structure, and the Identify
Namespace data structure, NVM Command Set [NVME-BASE]. If available,
use of the NGUID value is preferred as it is the larger identifier.
| Note: The PS_DESIGNATOR_T10 and PS_DESIGNATOR_NAME have no
| equivalent in NVMe and cannot be used to identify NVMe storage
| devices.
The pnfs_scsi_base_volume_info4 structure for an NVMe namespace SHALL
be constructed as follows:
1. The "sbv_code_set" field SHALL be set to PS_CODE_SET_BINARY.
2. The "pnfs_scsi_designator_type" field SHALL be set to
PS_DESIGNATOR_EUI64.
3. The "sbv_designator" field SHALL contain either the NGUID or the
EUI64 identifier for the namespace. If both NGUID and EUI64
identifiers are available, then the NGUID identifier SHOULD be
used as it is the larger identifier.
RFC 8154 [RFC8154] specifies the "sbv_designator" field as an XDR
variable length opaque<>. opaque<> (refer to Section 4.10 of RFC 4506
[RFC4506]). The length of that XDR opaque<> value (part of its XDR
representation) indicates which NVMe identifier is present. That
length MUST be 16 octets for an NVMe NGUID identifier and MUST be 8
octets for an NVMe EUI64 identifier. All other lengths MUST NOT be
used with an NVMe namespace.
2.2. Client Fencing
The SCSI layout uses Persistent Reservations (PRs) to provide client
fencing. For this to be achieved, 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.
The following subsections provide a full mapping of the required
PERSISTENT RESERVE IN and PERSISTENT RESERVE OUT SCSI commands [SPC5]
to NVMe commands that MUST be used when using NVMe namespaces as
storage devices for the pNFS SCSI layout.
2.2.1. PRs - Key Registration
On NVMe namespaces, reservation keys are registered using the
Reservation Register command (refer to Section 7.3 of [NVME-BASE])
with the Reservation Register Action (RREGA) field set to 000b (i.e.,
Register Reservation Key) and supplying the reservation key in the
New Reservation Key (NRKEY) field.
Reservation keys are unregistered using the Reservation Register
command with the Reservation Register Action (RREGA) field set to
001b (i.e., Unregister Reservation Key) and supplying the reservation
key in the Current Reservation Key (CRKEY) field.
One important difference between SCSI Persistent Reservations and
NVMe Reservations is that NVMe reservation keys always apply to all
controllers used by a host (as indicated by the NVMe Host
Identifier). This behavior is analogous to setting the ALL_TG_PT bit
when registering a SCSI Reservation Key, and it is always supported
by NVMe Reservations, unlike the ALL_TG_PT for which SCSI support is
inconsistent and cannot be relied upon. Registering a reservation
key with a namespace creates an association between a host and a
namespace. A host that is a registrant of a namespace may use any
controller with which that host is associated (i.e., that has the
same Host Identifier, refer to Section 5.27.1.25 of [NVME-BASE]) to
access that namespace as a registrant.
2.2.2. PRs - MDS Registration and Reservation
Before returning a PNFS_SCSI_VOLUME_BASE volume to the client, the
MDS needs to prepare the volume for fencing using PRs. This is done
by registering the reservation generated for the MDS with the device
(see Section 2.2.1) followed by a Reservation Acquire command (refer
to Section 7.2 of [NVME-BASE]) with the Reservation Acquire Action
(RACQA) field set to 000b (i.e., Acquire) and the Reservation Type
(RTYPE) field set to 4h (i.e., Exclusive Access - Registrants Only
Reservation).
2.2.3. Fencing Action
In case of a non-responding client, the MDS fences the client by
executing a Reservation Acquire command (refer to Section 7.2 of
[NVME-BASE]), with the Reservation Acquire Action (RACQA) field set
to 001b (i.e., Preempt) or 010b (i.e., Preempt and Abort), the
Current Reservation Key (CRKEY) field set to the server's reservation
key, the Preempt Reservation Key (PRKEY) field set to the reservation
key associated with the non-responding client, and the Reservation
Type (RTYPE) field set to 4h (i.e., Exclusive Access - Registrants
Only Reservation). The client can distinguish I/O errors due to
fencing from other errors based on the Reservation Conflict NVMe
status code.
2.2.4. Client Recovery after a Fence Action
If an NVMe command issued by the client to the storage device returns
a non-retryable error (refer to the DNR bit defined in Figure 92 in
[NVME-BASE]), the client MUST commit all layouts that use the storage
device through the MDS, return all outstanding layouts for the
device, forget the device ID, and unregister the reservation key.
2.3. Volatile Write Caches
For NVMe controllers, a volatile write cache is enabled if bit 0 of
the Volatile Write Cache (VWC) field in the Identify Controller data
structure, I/O Command Set Independent (see (refer to Figure 275 in
[NVME-BASE]) is set and the Volatile Write Cache Enable (WCE) bit
(i.e., bit 00) in the Volatile Write Cache Feature (Feature
Identifier 06h) (see (refer to Section 5.27.1.4 of [NVME-BASE]) is set.
If a volatile write cache is enabled on an NVMe namespace used as a
storage device for the pNFS SCSI layout, the pNFS server (MDS) MUST
use the NVMe Flush command to flush the volatile write cache to
stable storage before the LAYOUTCOMMIT operation returns by using the
Flush command (see (refer to Section 7.1 of [NVME-BASE]). The NVMe Flush
command is the equivalent to the SCSI SYNCHRONIZE CACHE commands.
3. Security Considerations
NFSv4 clients access NFSv4 metadata servers using the NFSv4 protocol.
The security considerations generally described in [RFC8881] apply to
a client's interactions with the metadata server. However, NFSv4
clients and servers access NVMe storage devices at a lower layer than
NFSv4. NFSv4 and RPC security are not directly applicable to the I/
Os
I/Os to data servers using NVMe. Refer to Sections 2.4.6 (Extents
Are Permissions) and 4 (Security Considerations) of [RFC8154] for the
security considerations of direct block access to block storage from NFS
clients.
pNFS with an NVMe layout can be used with NVMe transports (e.g., NVMe
over PCIe [NVME-PCIE]) that provide essentially no additional
security functionality. Or, pNFS may be used with storage protocols
such as NVMe over TCP [NVME-TCP] that can provide significant
transport layer security.
It is the responsibility of those administering and deploying pNFS
with an NVMe layout to ensure that appropriate protection is deployed
to that protocol based on the deployment environment as well as the
nature and sensitivity of the data and storage devices involved.
When using IP-based storage protocols such as NVMe over TCP, data
confidentiality and integrity SHOULD be provided for traffic between
pNFS clients and NVMe storage devices by using a secure communication
protocol such as Transport Layer Security (TLS) [RFC8446]. For NVMe
over TCP, TLS SHOULD be used as described in [NVME-TCP] to protect
traffic between pNFS clients and NVMe namespaces used as storage
devices.
A secure communication protocol might not be needed for pNFS with
NVMe layouts in environments where physical and/or logical security
measures (e.g., air gaps, isolated VLANs) provide effective access
control commensurate with the sensitivity and value of the storage
devices and data involved (e.g., public website contents may be
significantly less sensitive than a database containing personal
identifying information, passwords, and other authentication
credentials).
Physical security is a common means for protocols not based on IP.
In environments where the security requirements for the storage
protocol cannot be met, pNFS with an NVMe layout SHOULD NOT be
deployed.
When security is available for the data server storage protocol, it
is generally at a different granularity and with a different notion
of identity than NFSv4 (e.g., NFSv4 controls user access to files,
and NVMe controls initiator access to volumes). As with pNFS with
the block layout type [RFC5663], the pNFS client is responsible for
enforcing appropriate correspondences between these security layers.
In environments where the security requirements are such that client-
side protection from access to storage outside of the layout is not
sufficient, pNFS with a SCSI layout on a NVMe namespace SHOULD NOT be
deployed.
As with other block-oriented pNFS layout types, the metadata server
is able to fence off a client's access to the data on an NVMe
namespace used as a storage device. If a metadata server revokes a
layout, the client's access MUST be terminated at the storage devices
via fencing as specified in Section 2.2. The client has a subsequent
opportunity to acquire a new layout.
4. IANA Considerations
This document has no IANA actions.
5. References
5.1. Normative References
[NVME-BASE]
NVM Express, Inc., "NVM Express Base Specification",
Revision 2.0c, October 2022, 2.0d, January 2024, <https://nvmexpress.org/wp-
content/uploads/NVM-Express-Base-Specification-2.0c-
2022.10.04-Ratified.pdf>.
content/uploads/NVM-Express-Base-Specification-2.0d-
2024.01.11-Ratified.pdf>.
[NVME-NVM] NVM Express, Inc., "NVM Express NVM Command Set
Specification,
Specification", Revision 1.0c", October 2022, 1.0d, December 2023,
<https://nvmexpress.org/wp-content/uploads/NVM-Express-
NVM-Command-Set-Specification-1.0c-
2022.10.03-Ratified.pdf>.
NVM-Command-Set-Specification-1.0d-
2023.12.28-Ratified.pdf>.
[NVME-TCP] NVM Express, Inc., "NVM Express TCP Transport
Specification", Revision 1.0c, October 2022, 1.0d, December 2023,
<https://nvmexpress.org/wp-content/uploads/NVM-Express-
TCP-Transport-Specification-1.0c-2022.10.03-Ratified.pdf>.
TCP-Transport-Specification-1.0d-2023.12.27-Ratified.pdf>.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC2119, March 1997,
<https://www.rfc-editor.org/info/rfc2119>.
[RFC4506] Eisler, M., Ed., "XDR: External Data Representation
Standard", STD 67, RFC 4506, DOI 10.17487/RFC4506, May
2006, <https://www.rfc-editor.org/info/rfc4506>.
[RFC5663] Black, D., Fridella, S., and J. Glasgow, "Parallel NFS
(pNFS) Block/Volume Layout", RFC 5663,
DOI 10.17487/RFC5663, January 2010,
<https://www.rfc-editor.org/info/rfc5663>.
[RFC8154] Hellwig, C., "Parallel NFS (pNFS) Small Computer System
Interface (SCSI) Layout", RFC 8154, DOI 10.17487/RFC8154,
May 2017, <https://www.rfc-editor.org/info/rfc8154>.
[RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
May 2017, <https://www.rfc-editor.org/info/rfc8174>.
[RFC8446] Rescorla, E., "The Transport Layer Security (TLS) Protocol
Version 1.3", RFC 8446, DOI 10.17487/RFC8446, August 2018,
<https://www.rfc-editor.org/info/rfc8446>.
[RFC8881] Noveck, D., Ed. and C. Lever, "Network File System (NFS)
Version 4 Minor Version 1 Protocol", RFC 8881,
DOI 10.17487/RFC8881, August 2020,
<https://www.rfc-editor.org/info/rfc8881>.
[SPC5] INCITS Technical Committee T10, "SCSI Primary Commands - 5
(SPC-5)", INCITS 502-2019, 2019.
5.2. Informative References
[NVME-PCIE]
NVM Express, Inc., "NVMe over PCIe Transport
Specification", Revision 1.0c, October 2022, 1.0d, December 2023,
<https://nvmexpress.org/wp-content/uploads/NVM-Express-
PCIe-Transport-Specification-1.0c-
2022.10.03-Ratified.pdf>.
PCIe-Transport-Specification-1.0d-
2023.12.27-Ratified.pdf>.
Acknowledgements
Carsten Bormann converted an earlier RFCXML v2 source for this
document to a markdown source format.
David Noveck provided ample feedback to various drafts of this
document.
Authors' Addresses
Christoph Hellwig (editor)
Email: hch@lst.de
Charles Lever
Oracle Corporation
United States of America
Email: chuck.lever@oracle.com
Sorin Faibish
Opendrives.com
11 Selwyn Road
Newton, MA 02461
United States of America
Phone: +1 617-510-0422
Email: s.faibish@opendrives.com
David L. Black
Dell Technologies
176 South Street
Hopkinton, MA 01748
United States of America
Email: david.black@dell.com