Wideband RF RPA systems are widely used in commercial, aerospace, and defense markets. They digitize and store RF data in I/Q format (typically 16-bit/16-bit), support real-time and non-real-time analysis of the data, and, in some cases, convert it back to RF (playback) to stimulate devices under test (DUTs).
Wideband RF RPA systems are very data-transfer- and storage-intensive. A rule of thumb for data rate requirements for RF RPA systems is 5 bytes/second of I/Q data (16/16) per Hz of bandwidth. So, a single channel (1-CH), 500 MHz bandwidth wideband RF RPA system must sustain 2.5 GB/sec write and read speeds for the duration of the recording or playback session. At a 2.5 GB/sec data rate—which is about 10x the performance of a single SATA III 6gbps HDD—the data storage capacity requirements get very big, very quickly. The 500 MHz bandwidth 1-CH RF RPA system described above would need 2.5 GB/sec x 60 sec/min x 60 min/hour = 9 terabytes (TB) of storage to support just one hour of recording duration. The required data rates and capacities increase linearly with the number of channels and the bandwidth per channel in the system.
RAID is essential for wideband RF RPA
M.2 and NVMe
Growth of NAND Flash-based SSDs
PXIe-based RF RPA and SSD RAID modules
With its intrinsic scalability (to support multiple, wideband RF channels) and compact form factor, PXIe has become a very popular format for wideband RF RPA systems. With the advent of small, high data rate, low-power, low-cost M.2 SSDs that directly supports PCIe interface, it’s now possible to combine a 2, 4 or even 8 x M.2 SSDs with a PCIe switch to produce anThere are three companies making COTS SSD RAID modules today, all of which employ multiple M.2 SSDs and support a PCIe Gen 3 x 8 interface. The National Instruments (NI) PXIe-8267 and the Conduant, Inc. DM-4M.2-3U are COTS, single-slot, PCIe Gen 3 x 8, SSD RAID modules with four (4) M.2 SSDs per module and published sequential write/read benchmark performance in the 5 to 7 GB/sec range. The RADX Trifecta-SSD supports 4 or 8 x 2 TB M.2 SSDs to provide 8 or 16 TB of storage per slot and 6.9 to 7.2 GB/sec sequential write/read (benchmark) performance.
Manufacturer | National Instruments | Conduant Inc. | RADX |
SSD RAID module | PXIe-8267 | DM-4M.2-3U | PXIe-4M.2F-xTB |
M.2 SSDs / module | 4 | 4 | 4/8 |
Capacity | 4 TB | 4/8 TB | 8 TB/16 TB |
Write & read performance | 5/5 GB/sec | 5.8/7 GB/sec | 6.9/7.2 GB/sec |
For more info | https://bit.ly/2T94SDm | https://bit.ly/2xthGLv | https://bit.ly/308BWi2 |
No free lunch
Sadly, there is no such thing as a free lunch in engineering. While SSDs often “feel” like fast HDDs, because of their underlying NAND flash architecture, they are fundamentally different and there are aspects of SSDs that must be actively considered when using them, especially in write-intensive wideband RF RPA applications:
- Endurance: NAND flash SSDs have a finite number of write cycles. It’s usually a high number, but one should be mindful of this feature when selecting an SSD-based storage solution. SSD manufacturers specify the endurance of an SSD in terms of terabytes written (TBW) or drive writes per day (DWPD). For example, a Samsung 970 EVO Plus 2 TB M.2 SSD supports a TBW of 1,200 TB. This means the SSD can be completely overwritten 1,200/2 = 600 times before suffering performance degradation for writes. The TBW scales with the number of SSDs in a given PXIe SSD RAID module, so a 16 TB SSD RAID module would support a TBW of 8 x 1,200 or 9,600 TB. DWPD is related to TBW but considers the warranty period of the SSD. Specifically, TBW = DWPD x warranty period.
- Clean Up (garbage collection) using SSD TRIM: Because NAND flash SSDs have limited write cycles, it’s important to treat them differently when deleting data and files. Operating systems like Microsoft Windows 10 that support NVMe “know” to use SSD “TRIM” as opposed to HDD “defragmentation” to minimize unnecessary SSD writes. Note that support for TRIM under MS Windows is limited to NTFS file systems. Also note that conventional RAID controllers (as opposed to PCIe switches) can interfere with TRIM command execution.
- Encryption: Besides being small and fast, M.2 and other NVMe SSDs are usually smart, too. Most recent M.2 and U.2 (2.5-inch NVMe format) SSDs support built-in AES-256 encryption using the Opal storage specification for self-encrypting drives (SED) developed by the Trusted Computing Group (see https://bit.ly/307IltF). If one enables Opal encryption, the data stored on the SSD or SSD module is encrypted with AES-256, which can significantly decrease the chance of unintended disclosure of sensitive data.
- Sanitizing SSDs: With frequent use, or for use in applications where the data is sensitive, one will eventually need to sanitize the SSD prior to disposal. NVMe SSDs support a “user data erase” command. Also, if one has enabled Opal encryption, one can also use “cryptographic erase,” which will make the data unusable since it destroys the keys. For government users, NIST recommends that one always verify after erasing. For more info on the NIST recommendations, visit https://bit.ly/2Nrol3I.
Because of the additional complexities associated with NAND flash-based devices, NVMe SSD device manufacturers publish free software tools that simplify SSD management. To check on the remaining life of the SSD, enable Opal encryption, or sanitize the SSD, one can either use operating system commands or utilities or use these tools from the manufacturer. Below are links to the tools from three of the leaders in M.2 SSDs: Intel, Samsung, and Micron:
- Intel: https://downloadcenter.intel.com/download/28111/Intel-Solid-State-Drive-Toolbox?v=t
- Micron: https://www.micron.com/products/solid-state-storage/storage-executive-software
- Samsung: https://www.samsung.com/semiconductor/minisite/ssd/download/tools/
Summary
NAND flash-based M.2 SSDs—when packaged as COTS, high-performance, single-slot, PXIe SSD RAID modules—enable wideband RF RPA developers, system integrators, and users to eliminate external RAID subsystems (and data recorders) to dramatically reduce costs and SWaP while improving performance and reducing complexity. And because COTS PXIe SSD RAID modules are based on commercial M.2 SSDs, one can fully expect their capacities and performance to improve over time, while prices should also continue to fall. Lastly, with standards like NVMe, new PXIe SSD RAID modules based on NVMe M.2 SSDs should be backward compatible with current modules, eliminating the need for expensive lifetime-buys. In short, M.2 NVMe-based PXIe SSD RAID modules exemplify the power of modular, COTS products.