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For SSDs in Datacenters, there are many arguments

Jul. 1. 2013

There are many convincing arguments in favor of SSD and its eventual high adoption rate into data centers. All these arguments build a strong case in terms of TCO that favors SSD adoption in caching and tier0 storage environment:

  1. Up to 20 times and increasing higher bandwidth and 1000 faster latency compared to HDD
  2. 50% Lower power consumption than HDD
  3. Predictable reliability and lacking mechanical parts vs. unpredictable and high failure rate of HDDs
  4. Comparable price per GB in low density enterprise class storage devices

But there are still Myths and barriers that slow down the adoption rate of SSD in data center industry:

  1. Using SSD as a drop-in replacement of HDD, without knowledge of various types of SSD
  2. Lack of education in dealing with SSD as Flash based device
  3. Usage of consumer SSD in server environment , due to easy access and high adoption rate in PC environment, led to many bad experiences and fails, spreading the idea of low reliability of SSD
  4. Many vendors covering various level of value change ownership, many Brands, different level of quality and pricing, etc. makes the choice for the user not easier, rather increase the level of hesitation
  5. Low level of knowledge and hesitation of traditional server OEM Sales force on usage of SSD as a non-commodity and application dependent device

This article aims to tackle the adoption barriers and provide an easy broad knowledge on topics in relation with SSD.

Filling the performance Gap between Hard disk and Main Memory

HDD is not dead, it is part of the solution; tiered Storage is the best solution today dealing with big amounts of data

When it comes to data access, the only economically reasonable approach is storage tiering, i.e. using various storage medium in cascade. The faster the memory, the closer it should be to the central processing unit (CPU). The typical metric used to measure this is latency – the time needed to fetch data from a storage medium when the data is requested. DRAM latencies are in a range of 10s of nano-seconds, for SSD in the range of micro seconds (factor of 1000), and for HDD we are talking about milli-seconds (again a factor of 1000).

In this sense each medium is caching the data placed in the next level. The most wanted data is in DRAM, the next priority of data in SSD and the least used data in HDD.

SSD is not a drop-in HDD replacement but a different animal, requiring a different approach

SSD is based on NAND flash memory. NAND flash cells have a limited life-span defined on the number of times data could be written into the cell. When this number is reached the cell is dead and the data in each cell is potentially compromised. In this sense, SSDs have a completely different behavior compared to HDDs.

The different features and behavior of SSD require a different approach compared with Hard Disks. The applications can get more out of this new device if they fit their software patterns to the special features of NAND flash, instead of using SSD as just another storage device and as a HDD drop-in replacement.

Here below are two aspects, which need to be considered to make the right choice of SSDs for your environment.

  • The writing behavior of your environment is the key for the choice of the SSD. You need to know how many data you expect to write on the SSD over the life cycle of the product and how many data your application writes every day on your storage device, and whether these are random or sequential writes.
  • SSD density: The higher the density, the more total bytes could be written on an SSD and therefore the longer the life cycle. It is worth calculating if buying a higher density of a specific SSD category resulting in increased endurance and life cycle is cheaper than moving to the next higher category with longer endurance. This optimizes the performance per dollar of your investment.

Right Choice of Solid State Drive

SSD Value chain – quality and the brand

Quality is a criterion that needs to be built upon and is a matter of trust. It is also about the behavior of the product over time, something that is not obvious from the beginning or by comparing the products side by side.
SSD is based on Flash components and the main criterion on choice of SSD is its capability of total bytes written(TBW). In other words, how often a flash cell can be rewritten before it fails and this comes down to the quality of the flash cells.

In the production process, not all the components have the same quality. A flash manufacturer typically tests and chooses the best components for SSD production and uses the other components for less critical devices or even sells it into the market for other purposes.
Samsung with its approx. 40% market share in global flash output has developed a sophisticated process to pick the very best parts for its SSD production.

Consumer SSD in Enterprise Environment

A consumer SSD has firmware optimizing the interaction of the SSD with a client, assuming it is operating in a PC/Note book environment. However an enterprise server expects 24/7 availability from its SSD and the enterprise Firmware is designed with this in mind.

You will notice the difference when you decide using Client SSD behind HBA or Raid Card.

A consumer SSD disconnects from the outside world for just a few seconds to clean itself and restructure the data internally: its own so called “garbage collection” process. For a Raid Card the lack of responsiveness above a few milliseconds is considered as a failure of the device. Hence, an Enterprise SSD will typically switch down its performance while making the garbage collection, but it will never fail to response to a Raid card.

Warranty period

SSD life cycle strongly relies on amount of data written on it. The best target for warranty of a SSD is a set Target on the “total amount of data” that can be written on SSD. This is typically provided by “Total Bytes Written(TBW)”. Asking for a specific time period as a sign of warranty is what most customers expects based on their previous experience on Hard Disk drives, but it is not what in the SSD world could be seriously given.

Still since most end customer without the right education of SSD expects a time period for warranty, the best given Warranty can only be provided by a combination of Time and TBW. E.g. “2PB or 5 years”. For this example, the warranty provided can be translated as, if the user writes total amount of Data in the size of 2PB on the disc before 5 years is achieved the warranty is void. In other words, the device has a warranty period of 5 years, if during that period the amount of data written on SSD would not exceed 2PB.

Samsung's SSD offering for Data Centers
Product name PM843 SM843 SM843T SM1625
Applications* Boot Drive
Read intensive
Read Cache Read & Write Cache
+ Storage
Storage
Price Ball park $/GB 100% 120% 130% 300%
Interface SATA 6G SATA 6G SATA 6G SAS 6G Dual Port
NAND TLC MLC MLC eMLC
Densities 120 - 480GB 120 - 480GB 120 - 960GB 100 - 800GB
Over provisioning 7% 7% 7% 50% - 28%
TBW 200 - 800TB 1 - 4PB 2 - 16PB 4 - 24PB
Work load per day
(seq. Write)
2.5 4 8 12
Data protection
(in case of power shut down)
No No Yes Yes
SMART ID
(suport and Linux based magician tool)
Yes Yes Yes No

Figure 1) Samsung Enterprise class SSD portfolio, reflecting various requirements in Date Centers in dealing with Data and writing volume. From left to right, the TBW value increases. With 2 focus products we believe our customers could cover up to 80% of target applications.

Samsung SM843T is designed to provide a trade-off between low GB/$ to become attractive for Design-ins, and high endurance to get attraction of enterprise requirements. SM843T comes with a Firmware based on whole knowledge of Samsung and its long term engagement with Storage OEMs to be the ideal product for caching applications. And as market leader in Flash manufacturing, Samsung has access to vast resources to choose the highest level of reliability component to be integrated into SM843T. The Long term support of ~2 years makes the product ideal for project design-ins and data center applications.

Right choice of SSD for Target application: narrowing down the focus product
SSD req. By application & Bandwidth PM843 SM843T SM1625
portion of read/write 90% / 10% 90-50% / 50-90% 10% / 90%
HPC Boot
OS
streaming
reading Data
Metadata ; Scratch
or dump data;
Read/write cache
storage
Big data In-Memory
Hosting
Interface SATA 6G SATA 6G SAS 6G Dual port

Figure 2) right choice of SSD for Target application: narrowing down the focus product

Source: Markt & Technik

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