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Solid State Drives (SSDs) are used as a drop-in replacement of HDD's. The SSD technology advantages over traditional mechanical drives are commonly known and understood. The rate of the technology replacement is capped however by the higher $/Gigabyte cost of SSD solution. While the price of the consumer grade SSDs decreased drastically within the last few years, it is still more expensive to buy an SSD than a comparable capacity HDD solution. The consumer grade SSD price reduction was possible thanks to new technologies such as QLC and TLC NAND. These technologies however are not suitable for applications with 24/7 write requirement and most of all where Industrial grade operating temperatures are required. For these applications, much more expensive SLC or at least Industrial grade MLC NAND should be used.e.
The SSD market is currently growing at 15% CAGR and the market size is expected to go well above $50B by 2023 from about $30B in 2020. The key volume growth is in the Consumer and Enterprise market segments. The growth rate however is high for all storage market segments.
Let's first review basic Flash memory organization and NAND operations. It helps to understand the relationship between application and SSD life expectancy prediction.
The past of SSD shipments reflected how many applications required absolute system and data reliability and/or ruggedness. The cost was an insignificant part of the deployment equation.
On the other hand, HDD is very cost effective and for a long time did not have a serious competitor, mostly because of the cost. The vast majority of storage requirements were therefore written based on what HDD can and what HDD cannot do.
The Flash price decline changes this way of thinking. While cost remains an important factor, other technology becomes more significant in the deployment equation.
HDD replacement rate due to disk failure reaches 8% already during the second year of disk operations (source: Google). Typical life expectancy of the HDD is 40-70K hours. Preventive maintenance procedures are widely used as a vehicle to manage low life expectancy (and reliability) of HDD's. Due to the large deployment base, the cost of maintenance is very high.
The HDD life cycle length is driven by consumer market and rarely exceeds 2 years and is much shorter than for SSD's. For industrial, or telecommunication applications, the storage qualification or re-qualification cost is around $250,000. Each re-qualification due to technology update will add a substantial cost to each drive.
Typical life expectancy of SSD's is 300-1000K hours. Preventive maintenance is typically not required.
Wear leveling uses blocks within the boundaries of one wear leveling zone. Some of those blocks may contain so called "static" data. The "static" indicates rarely modified data. Examples may include OS or user files.
The dynamic wear leveling excludes the blocks with the "static" data from the wear leveling. Consider a hypothetical 4000 wear leveling zone where 3500 blocks contain "static" data and the remaining 500 blocks are part of the wear leveling pool. The dynamic wear leveling would spread the writes among the 500 blocks only. The drive could fail prematurely because wear leveling was unable to spread the use among the blocks containing the "static" data.
When "static" data is modified however, wear leveling moves the entire block content to a new location and the block will be placed in the wear leveling block pool.
The dynamic wear leveling could be compared to a tire maintenance process that uses tire rotation and spare tires. The tires installed on a car are an equivalent of blocks in the wear leveling pool. The spare tires are an equivalent of blocks with the "static" data. Dynamic wear leveling action is like effecting a tire rotation. This evens out the wear of tires installed on the car.
Writing to a block with the "static" data is like replacing the tire installed with the spare tire. This helps to even out the wear among spare and installed tires.
Bottom line for the dynamic wear leveling is that if drive content changes from time, all blocks will experience similar usage during SSD life time.
Some applications however, such as those that use file system, may push to the limit the dynamic wear leveling capability. For example, the drive area storing FAT and metadata may experience many more erases/writes than other areas of the wear leveling zone and/or disk.
The static wear leveling would help to address this challenge. It ensures that all blocks within the wear leveling zone, regardless if they contain "static" data or not, are subject to same level of usage. The static wear leveling would move the "static" data from one location to other, transparently to the host depending exclusively on block usage criteria.
While static wear leveling benefits MLC NAND based storage, virtually all industrial grade flash products use today dynamic wear leveling. When combined with the SLC NAND, it provides a very good SSD life expectancy for most high end applications.
Sequential write across the entire drive makes wear leveling irrelevant. Every memory section experience the same level of usage. The sequential writing acts like a perfect wear leveling maximizing life expectancy calculation. It should not be a surprise that SSD manufacturers typically calculate life expectancy, expressed in years of operations, based on this model.
Consider a 64GB drive that is written to at 25MB/s rate. It will take about 40 min to overwrite the drive. Other word, each block will be written every 40 min. Assuming 100,000 write endurance limit and 24/7/365 operations, the drive would reach end of life in about 8 years.
Conversely, SSD manufactures would not be able to claim higher number of erase/write cycles than 100,000 guaranteed by the SLC NAND vendors, as the application uses equally all the blocks and the wear leveling does not have anything to level. Conversely, SSD manufactures would not be able to claim higher number of erase/write cycles than 100,000 guaranteed by the SLC NAND vendors, as the application uses equally all the blocks and the wear leveling does not have anything to level.
Solid state and no mechanical system latencies will ensure SSD leadership in terms of raw read/write speeds and especially where random access is required.
With falling flash prices, the performance has become the third most important factor, after ruggedness and reliability, behind deployment of flash SSD's.
Storage designers are using, in parallel to $/GB, cost benchmarks that include performance aspects, such $/IOps, or $/MBps. For some applications requiring very high performance, these metrics indicate a clear advantage of SSD's by making possible designing into the system less drives and consequently less servers resulting in lower initial cost, less power consumption and less heat, lower cooling requirements, smaller footprint, less inventory and lower recurrent cost of system maintenance.
Data Centers are spending a fortune on air conditioning. One enterprise class hard drive may require as much as 11 W during write/read operation. Data Centers may have thousands of disks installed.
SSD's use 1-2W during write/read operations that translate into potential energy savings.