1-2-1 I SSDs applications they may be suited to over time?
MK: As SSD technology and its manufacturing processes evolve, the price/GB of Flash storage will continue to fall and eventually propagate across virtually all data centre applications, including cloud applications, as well as laptop applications and other mobile based applications.
GW: There is reason to believe that economical flash-based SSDs with high write-cycle durability, fast write performance, and high MTBF will arrive on the market during the next 12 months. Huge strides have already been made by the leaders in this space during just the last year or two.
JC: Solid-state will always be focused on those 10% of the applications that make IT Budgets disappear into in a desperate “1 step forward, 2 steps back” attempt at securing scalable performance such as: Virtualization - VDI; Virtualization - Server; SQL; Oracle; OLTP;
Q Delving into SSD technology in more detail, what are the differences between multi-level cell (MLC) and single-level cell (SLC) SSDs?
MK: A root challenge with NAND Flash is that it wears out and can only sustain so many erase/write cycles before it fails. SLC memory stores one bit in each cell, leading to faster transfer speeds, lower power consumption and higher cell endurance. The only disadvantage of SLC is the manufacturing cost per MB, meaning that the SLC flash technology is used in enterprise level solutions where high-performance and endurance are important. MLC memory stores two bits in each cell. By storing more bits per cell, a MLC memory card will achieve slower speeds, higher power consumption and lower cell endurance than a SLC. The advantage of MLC is the lower manufacturing cost but is realistically only viable for consumer memory devices where sustained heavy writes are unlikely. eMLC memory is a recently emerged technology which combines MLC level production and capacity with enterprise level endurance.
GW: From the user’s perspective, MLC flash tends to be used in consumer applications such as laptops and iPhones. It’s cheap and dense, but lacks the write performance and durability needed for enterprise applications. SLC flash is used in most credible enterprise or data center applications. Recently, a new technology known as eMLC has arrived which aims to use MLC technology plus various bits of technical magic to achieve performance and reliability levels that are suitable for midrange RAID applications, while delivering performance that’s a tier below SLC (but still far better than spinning disks).
JC: It comes down to basic physics. MLC Flash is based on a multi-level cell design that can accommodate less overall WRITES to each cell, thus it’s life expectancy is less than that of SLC. The massive adoption of MLC Flash by the consumer
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electronic community has driven it’s cost down by a compelling order-of-magnitude. In the past most vendors either pushed a DRAM or a hybrid version including SLC. This was fine for clients such as Facebook, the Federal government or large financial services firms, but left the majority of enterprises struggling to find the path to successful performance TCO.
Q Similarly, what are the pros and cons of non-volatile NAND flash memory as opposed to DRAM volatile memory when it comes to SSDs?
MK: DRAM volatile memory was used in early SSDs and was not considered by many as an alternative to HDDs because the SSDs required a permanent power source to retain the data and the DRAM was expensive. By comparison, NAND Flash memory delivers persistent data storage and does not require permanent power source to retain the data, it is cheaper, and is therefore a higher performance alternative to HDDs.
GW: DRAM based systems have been around since the 1980’s at least, and are usually extremely fast and have infinite write durability, but tend to be expensive for a given amount of capacity. Since they are inherently volatile, they must be surrounded by electrical systems that de-stage the data to disk or flash in the event of an unexpected shutdown. In practice, they are usually stand-alone rack devices, but recently they have been packaged in a pluggable carrier. We use one of the latter type in our E5000, to hold frequently changing write cache data. It has a small battery on-board to provide non-volatility.
DRAM modules such as these tend to be much faster than flash, but are also relatively expensive and low capacity.
JC: DRAM is fast, but expensive and fragile. Similar to the concept of HDD’s, the marketspace is thristy for an alternative and the recent focus on SSD’s is directly tied to this. FLASH can handle heat, cold, vibrations and power-loss elegantly. Q Other SSD choices to be made include cache or buffer, and battery or super capacitor - can you say bit about each?
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