White Paper: Understanding the Relationship Between SSD Endurance and Over-Provisioning. Solid State Drive

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1 White Paper: Understanding the Relationship Between SSD Endurance and Over-Provisioning Solid State Drive

2 2 Understanding the Relationship Between Endurance and Over-Provisioning Each of the cells inside an SSD that stores one to four bytes has a limited lifespan. Each cell can be read an unlimited number of times, but only written to so many times. In addition, each time a write is made to the drive, changing a single byte results in an entire block being erased and then re-written an effect known as write amplification. Over-provisioning which employs some of the storage capacity in a solid state drive (SSD) to improve performance designates some of the cells of the SSD for use in wear leveling operations and for garbage collection, which involves taking cells with old data and readying them to be used for new data. It s also key to pay attention to wear leveling, a process that involves moving data around on the drive to make sure that all the cells on the drive are erased and written equally. Without it, some cells might wear out quickly when storing files that change often, while others that change infrequently would last much longer.

3 3 Optimizing Read-Write Operations In a drive with lots of write operations, these operations are necessary to free up space for new writes, and too little overprovisioning can restrict performance. Too much overprovisioning wastes space that could be used for additional storage. Older SSDs tend to have a fixed amount of over provisioning, but the new Samsung 860 DCT allows users modify the amount of over-provisioning through the Samsung SSD Toolkit or Linux HDParm disk management command lines. While the 860 DCT is designed for primarily read-intensive applications, it can be optimized for heavier writes if needed. The combination of write amplification and wear leveling can make it difficult to predict exactly how long an SSD will last. >_

4 4 NAND and Block Management Raw NAND Capacity and NAND P/E cycles are both fixed values based on hardware specifications and are not affected by over-provisioning. However, Usable Capacity and Write Amplification Factor (hereby WAF) change depending on over-provisioning level. Usable capacity changes because increasing over-provisioning reduces the usable capacity, which contributes to increase in DWPD (i.e. fewer gigabytes per drive write). NAND can be programmed at the page level (each page is usually 8KB or 16KB in modern NAND), but erase can only be done at the block level (typically several hundred pages). Solid State Drive In order to explain WAF in detail, it is necessary to understand the operation between the operating system (OS) and SSD. The OS uses LBAs (Logical Block Addresses) to keep track of where data is. The Flash Translation Layer (FLT) in the drive then translates the LBAs into physical addresses in the NAND. Drive endurance is determined by the following equations: Total Bytes Written (TBW) = Raw NAND Capacity * NAND PE Cycles Write Amplification Factor * Wear Leveling Factor Total Bytes Written Drive Writes per Day (DWPD) = TBW * 1000 Warranty in # years * 365 * Usable Capacity (in GB)

5 5 Standard NAND Block Management Usable space Over-provisioning Valid data Invalid data Empty space OP1 OP OP OP3 OP OP3 OP4 The graphic above illustrates a NAND block with 12 pages (physical NAND addresses), of which eight are visible to the OS (i.e. eight LBAs and effective over-provisioning of 33.3%). In an enterprise environment, drives are often operated in full state, meaning that the eight LBAs (grey boxes) are already associated with data. While NAND cannot be overwritten, the OS can still overwrite data to an LBA and in such case the data is written to the over-provisioned space. In this example, the OS writes data to LBA7, which the FTL then maps to OP1 in the physical NAND address space. In reality there are no separate OP pages within a block (all pages and NAND addresses are the same and can be used for all data), but the usable and OP space have been separated in this example to simplify the explanation.

6 6 Over-Provisioning with SSDs When more writes come in from the host, more data is written to the OP space and the corresponding LBAs are marked as invalid. Once the block is completely full of data (both usable and OP space), the only way to write more data is to read the whole block to DRAM cache, erase the block and write the old valid data along with the new data. This is called a read-modify-write process and it is part of the garbage collection process. With 16.7% over-provisioning, there are 10 pages of valid data and 2 pages on invalid data in a block. As a new write comes in, there will still be 9 pages of valid data that need to be rewritten during the read-modify-write cycle. In other words, in order to write just one page of data the drive actually needs to write 10 pages. The Write Amplification Factor (WAF) would be 10 in this case. With 33.3% over-provisioning, there are now 8 pages of valid data and 4 pages of invalid data. Instead of having to rewrite 9 pages of old valid data, there are only 7 pages with valid data, meaning that the drive only needs to write 8 pages to program one page, dropping the WAF to % 33.3% over-provisioning over-provisioning

7 7 Conclusion This is a very simplified version with the purpose of helping to understand why increasing over-provisioning increases endurance. In reality, the process and garbage collection algorithms are far more complex because there are millions of NAND blocks in an SSD with each having hundreds of pages. The IO sizes and patterns also vary, making it impossible to calculate a fixed value for write amplification. The key takeaway is that the garbage collection process is more efficient with higher over-provisioning because less valid data needs to be rewritten during the read-modify-write cycle, which reduces write amplification that in turn increases endurance. The optimum amount of over-provisioning on a drive is dependent on the type of use the drive will see a write-heavy application will benefit from more over-provisioning than would be needed for a read-intensive application. The SMART attributes ID 245, ID 246 and ID 247 allow the administrator to monitor actual usage of the drive, including the wear with an actual workload rather than attempting to predict wear using software. Once the user determines the actual wear with their typical workload, they can establish the optimum level of over-provisioning that will maintain drive performance without wasting space. Get insights on the latest topics in Enterprise Data Storage with Samsung: samsung.com/enterprisessd 2018 Samsung Electronics America, Inc. All rights reserved. Samsung is a registered trademark of Samsung Electronics Co., Ltd. All products, logos and brand names are trademarks or registered trademarks of their respective companies. This white paper is for informational purposes only. Samsung makes no warranties, express or implied, in this white paper. Learn more: samsung.com/enterprisessd SAM4BIZ Follow us: WHP-SSD-ENDURANCE-AUG18-SW