SSDs with NVMe Zoned Namespace (ZNS) Support
The NVM Express (NVMe) Zoned Namespace (ZNS) Command Set introduce a new division of functionality between host software and the device controller. A NVMe device that implements ZNS support exposes its capacity into zones, where each zone can be read in any order but must be written sequentially.
The NVM Express (NVMe) organization released as part of the NVMe 2.0 specifications the NVMe Zoned Namespace Command Set specification. The latest revision of this specification available is 1.1. The specification defines a command interface that applies to all NVMe defined command transport. This command set is independent of the storage media technology used by the device and applies equally to flash-based solid state drives (SSDs) or SMR hard disks.
The most common type of NVMe devices with Zoned Namespace support available today are flash-based SSDs. For this type of device, the ZNS interface characteristics allow improving internal data placement and thus leads to higher performance through higher write throughput, improved QoS (lower access latencies) and increased capacity.
See ZNS: Avoiding the Flash-Based Block Interface Tax for Flash-Based SSDs for a deep dive on SSDs with Zoned Namespace support. The article was published at USENIX ATC 2021.
Overview
The ZNS specifications follows the Zoned Storage Model. This standards-based architecture, which takes a unified approach to storage that enables both Shingled Magnetic Recording (SMR) in HDDs and SSDs with Zoned Namespace support to share a unified software stack.
Specifically for SSDs with Zoned Namespace support, the zone abstraction allows the host aligning its writes to the sequential write required properties of flash-based SSDs, and thereby optimizes data placement onto the SSD's media. Note that the management of media reliability continues to be the sole responsibility of the SSD and is managed the same way as conventional SSDs.
The ZNS Zoned Storage Model
The ZNS Command Set specification builds upon the host-managed zoned storage model which was first introduced for SMR hard-disks with the SCSI ZBC (Zoned Block Command) standard and the ATA ZAC (Zoned ATA Commands) standard. A compatible zone state machine was defined, and a similar set of zone management commands was defined.
These similarities simplify the implementation of the host storage stack and applications for simultaneously supporting both host-managed SMR hard-disks and SSDs with Zoned Namespace support.
Zone types
ZBC and ZAC SMR hard-disks can optionally expose a number of conventional zones which accept random write operations. The ZNS specification does not define this optional set of random write zones, as NVMe supports multiple namespace, and therefore can expose a separate namespace that supports conventional I/O accesses.
The ZNS Command Set specification mandates that all zones of a zoned namespace must have the sequential-write-required type.
Zone Capacity and Zone Size
The ZNS Command Set specification introduced the concept of a zone capacity. This concept is not defined in the ZBC and ZAC standards.
Similar to ZBC and ZAC standards, ZNS defines the zone size as the total number of logical blocks within a zone. A zone capacity is an additional per-zone attribute that indicates the number of usable logical blocks within each zone, starting from the first logical block of each zone. A zone capacity is always smaller or equal to the zone size.
This new attribute was introduced to allow for the zone size to remain a power of two number of logical blocks (facilitating logical block to zone number conversions) while allowing optimized mapping of a zone storage capacity to the underlying media characteristics. For instance, in the case a flash based device, a zone capacity can be aligned to the size of flash erase blocks without requiring that the device implements a power-of-two sized erased block.
As the logical block addresses between the zone capacity and the end of the zone are not mapped to any physical storage blocks, write accesses to these blocks will result in an error. Therefore, reading in this area is handled in the same way as when reading unwritted blocks.
A zone with a zone capacity smaller than the zone size will be transitioned to a full condition when the number of written blocks equals the zone capacity.
The total namespace capacity reported by a controller is always equal to the total number of logical blocks defined by the zones. In other words, this reported capacity includes unusable logical blocks of zones with a zone capacity lower than the zone size. The usable capacity of the namespace is equal to the sum of all zones capacities. This usable capacity is always smaller than the reported namespace capacity if the namespace contains zones with a zone capacity lower than the zone size.
Zone Resources Limits
The ZNS specification allows a ZNS controller to report a limit on the total number of zones that can be simultaneously in the implicit open or explicit open state (open zones limit). This potential limit on the maximum number of open zones is similarly defined in the ZBC, and ZAC standards.
However, unlike the ZBC and ZAC standards, the ZNS specification defines an additional limit on the number of active zones, that is, zones that have the implicit open, explicit open or closed state. A SSD with Zoned Namespace support may impose a limit on the maximum number of zones that can be active. This limit is always equal or larger than the limit on the maximum number of open zones.
Zone Append
The NVMe specifications allow a device controller to execute commands present in the several submission queues available in any order. This has implications for the host IO stack, namely, even if the host submits write commands directed at a zone sequentially, the commands may be reordered before they are processed and violate the sequential write requirement, resulting in errors. Host software can avoid such error by limiting the number of write commands outstanding per zone to one. This can potentially result in poor performance, especially for workloads issuing mostly small write operations.
To avoid this problem, the ZNS specification introduced the new Zone Append command. Support for this command is defined as optional in the ZNS specification. However, Linux support for zoned block devices requires that a ZNS device supports the zone append command.
Presentations
The following OCP 2019 Global Summit presentation covers the motivation for SSDs with Zoned Namespace support, the journey, and a general overview of the interface.
The following SNIA SDC presentations illustrate how SSDs with Zoned Namespace support can be used with real-world applications.
