As NVMe SSDs make inroads into the enterprise market, the question remains: What's in store for SAS drives?
While SAS drives don't provide the performance enhancements and latency reduction of NVMe SSDs, they do have a future.
SAS is part of the ongoing SCSI development effort led by the T10 Technical Committee, now the SCSI Technical Committee, of the InterNational Committee for Information Technology Standards and backed by more than 50 companies.
Performance and scalability
SAS hard disks are designed with performance and scalability in mind, even though single-disk performance is substantially lower than that of an NVMe disk. A SAS disk's performance varies based on its rotational speed -- 7,200 rpm, 10K rpm or 15K rpm -- but a high-performance SAS drive can deliver speeds of up to 12 Gbps. SAS SSDs also offer 12 Gbps of throughput.
The SAS architecture enables the creation of large storage arrays with numerous SAS drives. A single storage array may include bays for dozens of SAS drives. However, hardware constraints may prevent a SAS array from achieving a higher level of performance. A PCIe 5.0 x16 controller, for example, can transfer 64 GBps. While this throughput rate far exceeds that of a single SAS drive, the disks within a large storage array could collectively exceed the controller's maximum throughput, thereby causing the controller to become a storage performance bottleneck.
SAS drives are available in 2.5-inch and 3.5-inch form factors. Despite these limited options, SAS is flexible. SAS disks are available as SSDs or HDDs, with multiple rotational speeds supported.
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SAS drives are also available in various capacities and can fit in a variety of storage architectures. For example, SAS storage is sometimes organized into various tiers with lower-capacity SSDs as a hot storage tier or a caching tier and larger-capacity but slower disks for the cool storage tier. An organization might also create an archive storage tier with extremely high-capacity but relatively slow SAS disks.
SAS manageability and serviceability
SAS arrays are designed to be easily serviceable with most arrays supporting hot-swappable disks. Other components, such as power supplies, may also be hot-swappable. Storage vendors usually offer their own proprietary management software that enables customers to configure, provision, monitor and maintain their storage arrays.
Prior to the development of the NVMe spec, PCIe SSDs were based on proprietary technology. NVM Express Inc. was formed to develop an open industry standard to take advantage of the high-performance, low-latency capabilities of solid-state technology.
Performance and scalability
NVMe disk performance varies based on architecture and use. High-performance, enterprise-grade NVMe SSDs can achieve write speeds of 5-6 GBps and read speeds of up to 7 GBps. These disks can deliver random read/write rates in excess of 500,000 IOPS. NVMe disks can achieve these speeds because they are designed to use the PCIe bus, which provides multiple lanes for data transmission.
While there are controllers that can support large numbers of NVMe disks, the cost per gigabyte tends to be considerably higher when compared to SAS storage. As such, NVMe may not be the best choice for large data sets.
NVMe SSDs come in a variety of form factors, reflecting the ubiquitous nature of the PCIe interface. The three main types are the following:
- AIC/CEM (Add-In Card/Card Electromechanical) is the general-purpose form factor for PCIe in different sizes and heights that most SSDs use.
- M.2, developed for mobile devices and laptops, is also used in some desktops and data centers and by hyperscalers.
- U.2 is the 2.5-inch form factor that makes up the majority of NVMe SSDs today.
With PCIe Gen 4 SSDs, NVMe is starting to transition to EDSFF (Enterprise and Data Center SSD Form Factor) purpose-built storage form factors for data centers. E1 is designed for 1U, with versions being developed for high-capacity drives, scalable performance and mainstream compute use. E3, in various lengths and heights, is optimized for 2U compute and storage.
Manageability and serviceability
NVMe Management Interface (NVMe-MI) was built upon the SCSI Enclosure Services management capabilities. NVMe-MI provides in-band and out-of-band management of NVMe devices and one management console that supports multiple OSes.
SAS vs. NVMe conclusion
Both SAS and NVMe are capable enterprise storage technologies.
SAS is the older of the two architectures and also tends to be the cheaper option. Conversely, NVMe is the better choice for users who need the highest possible throughput rates.