Flash SSD Cost of Ownership
Flash SSD's are used as a drop-in replacement of HDD's. The addressable market is the HDD market. This market is around 400M units shipped in 2006 expected to double by 2010.
Even if flash SSD specification looks very attractive, nobody expects that flash SSD will entirely replace HDD in the foreseeable future.
Cost of raw solid state memory, in terms of $/GB, has always been several times higher than cost of HDD and unit shipments reflected this ratio. In 2005 one flash SSD was shipped for every 900 HDD's. This ratio is expected to be around 1:20 in 2010 due to the rapid flash memory price erosion.
The figure above helps to visualize this deployment dynamics and how large is the defined market opportunity for the flash SSD technology.
Flash market appears is elastic and the cost is one of key attributes governing the deployment. Since flash SSD displaces HDD, it is important to rate key values for each technology.
Managing write endurance limit in Flash SSD
Let's first review basic Flash memory organization and NAND operations. It helps to understand the relationship between application and Flash SSD life expectancy prediction.
Flash SSD vs. HDD - some cost of ownership factors:
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 the flash SSD. 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 flash SSD 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 Flash 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 flash 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 Flash 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, flash 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, flash 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 flash 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 flash SSD 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.
Flash SSDs use 1-2W during write/read operations that translate into potential energy savings.