Comparison of NAND Flash Technologies Used in Solid- State Storage

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1 An explanation and comparison of SLC and MLC NAND technologies August 2010 Comparison of NAND Flash Technologies Used in Solid- State Storage By Shaluka Perera IBM Systems and Technology Group Bill Bornstein Procurement Engineer, IBM Integrated Supply Chain

2 Page 2 Executive Overview Consumer products such as flash drives, cell phones, MP3 players and cameras have increased the need and the demand for nonvolatile memory that is, the ability to write data to a device and have that data remain in memory even with the power off. Both hard disk drives and NAND flash memory are nonvolatile storage devices. In most consumer applications, flash drives are selected because of their smaller size and lower cost. Because of breakthroughs in NAND flash technologies, companies now sell high-end storage solutions with NAND technologies. Solidstate drives 1 (SSDs) are also being used in PCs and servers (including IBM System x, BladeCenter, and System x idataplex servers) because of several benefits over standard HDDs, including higher IOPS (I/O operations per second) and bandwidth 2, higher reliability 3, low energy draw 4, silent operation, fast boot-up times, and reduction in rack space consumed. This white paper will discuss the differences between two NAND flash technologies, Single Level Cell (SLC) and Multi Level Cell (MLC), their trends, and an overview of their respective performance. Then we will discuss the advantages and disadvantages of each of these technologies as they are utilized in storage applications and SSDs. NAND Technologies SLC and MLC are the two most common NAND technologies in use today. An SLC device can store a single bit of data in one cell. That corresponds to a 0 or a 1, which allows an SLC cell to store only two different program states. See Figure 1 for the Vt distributions of an SLC cell. Figure 1. Vt distributions of an SLC cell An MLC device can store two bits of data in a single cell. That corresponds to 4 different program states: 00, 01, 10, and 11. See Figure 2 for the Vt distributions of an MLC cell. For a given technology node an SLC or MLC device will have the same Vt voltage range. The key difference is that a SLC cell is divided into two states and an MLC cell is divided into four. 1 We use the term solid-state drives for NAND devices that use standard HDD interfaces, as well as PCIe adapter-based storage solutions. 2 For example, according to internal IBM testing, the typical maximum IOPS of a 600GB 15K 3.5-inch hot-swap SAS HDD is ~400 (4K random reads), while the typical IOPS of a 640GB High IOPS SSD PCIe adapter is ~200,000, or ~500x. The typical bandwidth of the same HDD = ~195MBps (64K sequential reads), while the typical bandwidth of a 640GB High IOPS MLC adapter is ~1.5GBps, or ~7.7x. These adapters offer near-dram performance and RAID-level data protection, using N+1 chiplevel redundancy and 11-bit ECC. 3 Approximately 3,000,000 hours MTBF for SSDs vs. approximately 1,000,000 hours for enterprise 3.5-inch 15K SAS HDDs. 4 On a performance-per-watt basis, these adapters outperform HDDs by up to 445x: 97,014 IOPS / 9W = 10,779 IOPS per watt (160GB/320GB High IOPS adapters). 196,000 IOPS / 12W = 16,333 IOPS per watt (640GB High IOPS adapter). 400 IOPS / 16.5W = 24.2 IOPS per watt (600GB 15K 3.5-inch hot-swap SAS HDD).

3 Page 3 Figure 2. Vt distributions of an MLC cell This difference in Vt voltage ranges for each state causes the MLC device to be slightly slower and less reliable than the SLC device. However, the benefit of MLC over SLC is you get double the density of storage on the same amount of silicon. Flash technologies have shrinking ground rules just like all semiconductors technologies. The problem with flash components is that each time data is programmed and erased in a cell there is small amount of damage to the storage structure, creating trap sites. After many program/erase (PE) cycles on the cells, electrons get trapped changing/shifting the Vt distributions. The Vt distributions get larger, shift and start to overlap. This Vt overlap causes read errors which, in most cases, are correctable with Error Correction Code (ECC). However, after some finite number of PE cycles, error rates are too high to be correctable and the data is no longer good. The number of PE cycles, or endurance, a device can withstand is determined by two factors: The first concerns the technology ground rules, and the second is the cell structure/layout (SLC or MLC). Table 1 shows examples of endurance performance for both SLC and MLC for a few different technology nodes. Some MLC suppliers have been able to make modifications to increase the endurance of their MLC devices. They have found that by slowing down the programming time (t prog) endurance can be increased from only 3,000-5,000 read/writes to as many as 30,000 but at the cost of performance. Slowing down the programming reduces the generation of trap sites and damage, which improves cell reliability. This type of device is called e-mlc NAND and has just been offered in the latest 3X nm technologies. Technology SLC MLC e-mlc 5X nm 100K write/erase cycles 10K NA 4X nm 100K 5K NA 3X nm 50K 3-5K 20-30K Table 1. Endurance performance for various technology nodes Solid-State Drives The first generations of solid-state drives (SSDs) were generally made with SLC technologies. As the need for additional capacity grew, SSD suppliers introduced MLC-based SSDs. This allowed for double the density for almost the same cost as the SLC-based drives. This was good news for the cost-sensitive applications. However, suppliers have not spent enough time explaining the downside of using MLC NAND flash in SSDs. This is where this white paper will help customers who have cost-sensitive applications assess the feasibility of MLC-based SSD solutions.

4 Page 4 Solid state drives are mainly made up of NAND flash memory and a controller. Almost all SSDs have spare or extra NAND flash memory above and beyond its stated capacity. This is call overprovisioning. That means a 320GB drive may actually have 420GB (or more) of raw NAND, so that as memory cells wear out, others are there to take their place. The controller controls the writing and the erasing of data to the flash. Most or all controllers are designed to maximize the life of the NAND flash PE cycles. This is done primarily with wear-leveling algorithms. Wear leveling is the equal distribution of data writes/erases across the available NAND (including spares). This assures that both active and spare NAND devices do not wear out prematurely. The bottom line is that the SSDs will receive write commands, but the controller will manage the program/erase cycles of the NAND. SSD suppliers have other methods of maximizing memory cells besides wear leveling, such as compression of data and caching, which can also be used to minimize PE cycles on the NAND devices. Minimizing the number of PE cycles by consolidating individual SSD writes extends the life of the SSDs and NAND flash devices, and is therefore extremely beneficial for the lower PE cycles on MLC devices. However, SLC-based SSDs will always have a better endurance performance than a MLC-based drive for a given technology node. Application of Solid-State Drives Now it is time to assess NAND Flash technology choices based on the user s application needs. The SSD suppliers will state and publish basic timing performance specification for their products. However, it is up to the user to understand how much data will be written to the drive daily, workload (write/read ratio) and what type of data (sequential or random). These factors play a key role in how well the controller can manage the PE cycles of the drive. There are several ways to calculate SSD lifetime. One of the simplest ways is total data written to the NAND flash. One SSD supplier has established baseline maximums for total data/bytes written. The total number of bytes written to the flash will be equal to the total bytes written to the SSD drive plus housekeeping overhead. This overhead, as we mentioned before, is the controller writes for data refreshes and block management, which increase the total number of bytes written to the NAND Flash. Present solutions allow for a total of 4 petabytes (10 15 ) written (PBW) for the MLC technologies being manufactured today (320GB drive). As one would expect SLC drives have a much higher PBW. The PBW is 75PB for a similar 320GB drive made with SLC. The PBW scales linearly as drive capacity increases or decreases. The main message here is that SSDs have a finite lifetime based on total bytes written to the NAND flash. Most MLC-based storage solutions have a one- to three-year warranty. However, it is critical to understand the use conditions because a MLC based SSD could have a lifetime of three years or less if excessive writes are performed. On the other hand, SLC based solutions have greater than a three-year life even under heavy PE cycles. SSD Selection Tools These tools have been developed by most suppliers and may be different from supplier to supplier. However, we need to discuss SSD technology selection tools because of the use of MLC technologies. Before an SSD is purchased, key bits of information are needed to aid in proper selection of the drive. The three most critical parameters are total expected terabytes written daily (TBW or ), write/read ratio, and target lifetime of the drive (in years). The combination of these parameters should guide the user to the proper capacity and NAND flash technology. If the time and effort is put into the selection process, there should be no unexpected wear-out of the SSD.

5 Page 5 In addition to the selection tool, most SSDs have customer user interfaces (UI) to allow the monitoring of the drive status. This will allow the user to see where they stand with respect to the drive lifetime and/or total bytes written. This tool can also act as an early warning if the user s application conditions have changed and the lifetime is being used up at a greater-than-expected rate. Conclusion/Summary As we discussed, there are pros and cons to both MLC and SLC NAND flash technologies. MLCbased solutions provide higher capacities at a lower cost. The downside to MLC is that it has a shorter useful life. SLC is typically faster than MLC and has a much greater useful life. The downside to SLC is cost. In conclusion, each customer must assess the application parameters closely using the appropriate tools to see if MLC/e-MLC can be used without negative impact to their system lifetime expectations.

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