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SCM, somewhere between DRAM and NAND, will eventually replace flash memory as the preferred high-speed storage medium

Published :11/12/2019 6:13:09 AM

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Over the past few years, many storage technologies and markets have been in the pipeline, and more and more enterprise storage systems have recently started using SCM storage-level memory technology.So how much impact will this technology have on the storage industry?

SCM is between DRAM and NAND

SCM media itself is faster than NAND SSD and slower than DRAM memory. Because it also supports byte addressing, it does not need to erase the whole block first when writing, greatly reducing write amplification, and it has many inherent advantages over NAND in that it has a much lower latency and a longer life span.

Simply put, SCM is the intermediary between DRAM and SSD, a high-speed read-write non-volatile memory technology used to improve the overall I/O performance of a system.

Ideally, it would be a new storage technology that rivals DRAM in speed but costs closer to traditional hard drives.Of course, at present, only the reading speed can match DRAM, but the writing speed still lags behind, and with SSD unit cost approaching the traditional hard disk, SCM does not have enough cost performance as the underlying storage device.

In the existing AFA storage system, in order to pursue the extreme performance of NVMe SSD, the delay caused by the software stack itself cannot be ignored.

Compared with SSDS, SCM media has access delays of several orders of magnitude (from hundreds of microseconds to hundreds of nanoseconds), and the software stack delay problem will become more prominent.For example, the traditional software stack from application to kernel has a clear decomposition hierarchy of functions, which is suitable for slow storage media, but becomes the bottleneck for ultra-fast media like SCM.

For the same reason, the proportion of network delay in SCM system also becomes the main contradiction that affects the system delay.How to build a fast and stable network has become a key factor of whether SCM media performance can be fully utilized in the system.


SCM has advantages over NAND

Memory level SCM's ability to retain its content as much as NAND flash, as well as its dram-like speed, means that it will eventually replace flash as the preferred high-speed storage medium.

SCM is much better in this area because of the inherent design of flash memory.One of the biggest causes of performance problems and flash memory delays is the use of garbage collection to meet new writes.Unable to overwrite old information while writing data to a flash drive.It must write a new block of data somewhere else and delete the old file when disk I/O pauses.

· very low latency

· higher durability

· byte addressing similar to DRAM

Nonvolatile storage based on NVMe/PCIe byte addressing opens a new chapter in storage architecture innovation.SCM is often used as an extended Cache or as a persistent store for the highest performance Tier.

So most of the time, SCM is positioned to fill gaps in NAND, not replace it.HPE has announced that it will use Intel's Optane as an extension to its DRAM cache. As can be seen from HPE 3Par 3D cache test data, the delay is reduced by 50% while IO is increased by 80%.


SCM technical goals and potential

The goal and potential of SCM technology is to bridge the gap between DRAM and SSD read and write speeds.In theory, modern information systems are short of the overall performance due to the gap of internal device performance and the time consumed for data round-trip. Therefore, a temporary storage device and a cache are set up between processor and memory. SCM is introduced as memory buffer or SSD cache to solve such problems.


Basically SCM is used in two ways: as a cache and as a persistent store.

(1) HPE SCM as a cache, from the implementation, this way of reading and writing cache implementation is relatively simple.

It is said that the delay of 3PAR and Nimble can be kept under 300 microns, and the majority of IO delay can be kept under 200 microns.

HPE used Optane for caching in 3PAR, resulting in a latency reduction of twice as much, and said it was 50% faster than DELL EMC's PoweMAX with NVMe SSD.

Unlike HPE, PowerMAX of DELL EMC USES SCM as the storage layer.

PoweMAX connects to the server with a low-latency nvme-of, because with SCM, data access is faster.

In the implementation of PowerMAX, each port is fully utilized, each port has its own queue to handle more IO, and PowerMAX can provide separate queues for read, write, and small block requests, large block requests, and various loads.

In addition, different ports can be configured with different controllers, which makes RAID more efficient. For example, when a hard disk failure requires reconstruction, two controllers can participate simultaneously.In the past, it took 7-8 hours for a single controller to reconstruct a 7200 RPM hard disk, but it took only 2.5 hours for the dual-controller operation, which tripled the time.


SCM exploded as a trend in the storage industry

In the long run, NVMe and SCM are both key steps toward a new data center architecture, and we recommend that enterprise technicians do some NVMe and nvme-of technology updates by 2020.After 2020, enterprise technicians should plan the route of technology update around the composable data center architecture.

Storage technology and the market are both in a state of gestation,SCM is sometimes referred to as "persistent memory". In order to take full advantage of SCM, the industry must agree on a new interface.As expected, the 2018 persistent memory SCM is about to explode, with Optane and 3D XPoint coming into play and top storage oems remaining stable

Persistent memory SCM exploded, and while applications can treat SCM the same way they treat existing system memory, using persistent memory has the added benefit of effectively integrating ssd-driven storage capabilities with memory bus DRAM semantics.

To take full advantage of SCM, the industry must agree on new interfaces and re-architect applications to take advantage of them.The storage network industry association (SNIA) is the standard body for the storage industry.SNIA has been developing driver standards around persistent memory for many years, and these standards are starting to mature

Intel will officially launch the DIMM slot's 3D XPoint product this year, called Persistent memory (PM).According to Intel, 3D XPoint will help data center operators and developers overcome historic barriers to capacity and performance by changing the design rules for applications and systems, reversing more than 50 years of thinking about small, expensive and volatile memory.

Both the Microsoft and Linux communities have taken Intel's PEM library and integrated it into SCM products.Microsoft began supporting SCM on Windows Server in late 2017 and has recently tested the next version of Windows Server with more sophisticated support.In addition, Microsoft now supports SQL Server 2016 SP1 technology to enhance performance.


Current difficulties in SCM development

SCM costs about four times more per byte than flash memory.SCM is currently produced by only two suppliers: Intel and Samsung

Because of the inherent design of flash memory, memory latency at the storage level is much lower.One of the biggest reasons for flash memory performance problems and delays is garbage collection used to satisfy new writes.

When data is written to a flash drive, it cannot overwrite old information.It must write a new block of data somewhere else and later delete the old file when disk I/O is sluggish


This year will be the year when the industry begins to adopt SCM as a core part of its architecture in memory computing, large databases, and analytics applications.It may take longer for mainstream applications to reap the benefits of SCM, but eventually it will become a reality.

SCM, which overcomes the limitations of NAND flash storage and is able to store its content like NAND flash with DRAM speed, will eventually replace flash as the preferred high-speed storage medium, but it won't happen overnight.