Flash Storage Economic Ecosystem
Flash storage can deliver impressive performance, especially for random I/O, by eliminating rotational and seek latencies that are common in all HDD storage systems. This is particularly relevant in today’s virtualized application world where I/O access patterns are much more random due to the “blender effect” of virtualization.
Furthermore, flash storage is available now at a much more affordable price point than cache memory. As aresult flash storage can be deployed in larger capacity than cache, more like HDD storage. With the increasing adoption of flash storage in the data center, some industry pundits are projecting that the economies of scale will eventually drive down the cost of flash storage to the same level as HDD storage and flash storage will, over time, replace HDD storage altogether.
Not all flash storage drives are created equal
Although flash storage can be readily substituted for HDD storage in a data center, it is fundamentally a different medium– silicon or electronic NAND gates vs. magnetic media – with very different performance, cost and data retention characteristics that may impact the economics and operations of your application workloads in your datacenter. Hence, it is important to understand some basics of flash storage technology before considering its adoption.
• Write Endurance: Unlike magnetic media on HDD storage, data stored on flash needs to be erased before something new can be written or “programmed” – this is known as the Program-Erase Cycle (PE/C). The maximum number of PE/Cs of NAND gates on flash storage is in the order of a few thousand, after which, the performance and reliability of the flash storage drive drops dramatically - up to 10 times performance degradation may occur. This characteristic of flash technology limits the number of write operations that can be performed on a flash drive. This limitation of flash drive is called the Write Endurance and is expressed in full drive writes per day (or drive fills per day).
• Write Cliff: The requirement to erase a block of NAND gates in a flash drive before they can be written means that a flash drive needs to keep pre-erased blocks in order to improve performance – even for a small block write – by eliminating the latency that would be incurred if the erase process was performed every time on an as-needed basis. If a flash drive runs out of pre-erased blocks, especially when the drive fills up or during a sustained write intensive operation, it is possible to experience a large (50+ms) delay while the flash drive tries to create a free block of NAND gates. This write performance degradation in a flash drive is called a Write Cliff.
There are many ways to deal with write endurance and write cliff in a flash drive, impacting its cost and life span. For example, the write cliff is typically avoided / delayed by over-provisioning capacity for internal housekeeping (increasing cost/GB) and a process known as “wear leveling” which spreads out re-writes to the same block to new locations on the flash storage thus ensuring that the drive wears out evenly (increasing drive lifespan).
Today, there are three common types of enterprise flash drives in the market based on write endurance. Your flash drive selection for each of your application workloads may impact your data center economics and operations.
• Read Optimized (RO) or Multi-Layer Cell (MLC) Drive: This type of drive is optimized for read operations. Typically, MLC flash drives have a Write Endurance of 1-to-3 drive fills per day. They are also the least expensive of the enterprise-class flash drives.
• Write Intensive (WI) or Single Layer Cell (SLC) Drive: This type of drive has a higher Write Endurance and are typically more over-provisioned than the RO drives. Write Endurance of SLC flash drives is typically 20-to-30 drive fills per day. The increased write endurance comes at a cost; making this the most expensive and most reliable enterprise flash drive.
• Mixed Use (MU) or Enterprise MLC (eMLC) Drive: These type of drives attempt to strike a balance between SLC and MLC drives both from a Write Endurance and cost perspective. Typical Write Endurance for MU drives is somewhere between 5-to-10 drive fills per day.