Help Me Choose: Hard Drives and Solid-state Drives

Hard Disk Drives (HDD) and Solid State Drives (SSD) come in several varieties designed to address differing enterprise customer needs. These include:
Solid State Drives: Fast storage medium for random IO

10K & 15K SAS drives: Performance and availability for performance optimized applications

Nearline SAS and Enterprise SATA drives: High capacity and exceptional $/GB for capacity optimized applications

Entry Drives: Least expensive drives used in applications where low usage and limited total number of drives are applicable. Due to restrictions on use, these drives are available in a small number of systems and configurations
As an enterprise systems user, you are most concerned about the security of your data and the reliability of the medium holding that data.

Dell puts its Enterprise Hard Drive and Solid State Drive offerings through a rigorous validation process before they are ever considered as additions to our line of server and storage systems. Dell has stringent drive specifications for performance, duty cycles, Mean Time Between Failure (MTBF) and rotational vibration, which are all necessary for an Enterprise-level environment .These types of exacting specifications must be met before we will make them available for customer usage.

Hard Disk Drives for Dell Enterprise server and storage products are available in several different types, including Mission Critical / Performance Optimized, Business Critical / Capacity Optimized, and Entry. Emerging categories include drives designed for data needed less frequently and data stored for archived purposes. Dell is evaluating these later categories for inclusion in the HDD offerings.

Generally speaking, the Mission Critical (MC) or Performance Optimized drives (10K and 15K SAS) are used in those application requiring the highest reliability and performance. Business Critical (BC) or Capacity Optimized drives (7.2K Nearline SAS and SATA) may provide much higher capacities, but lower reliability and performance when compared to Mission Critical.

The direction of the Mission Critical drive industry is towards 2.5” or Small Form Factor drives only. The 3.5” drives or Large Form Factor have reached their maximum capacity (600GB), will not have additional development activity and will cease production in the near future. 2.5” MC are advantaged over 3.5” MC in most cases, and these advantages will increase in the future as further development on 2.5” MC occurs.

Our recommendation to customers reviewing standard configurations for Dell™ servers is to consider including 2.5” 10K/15K over 3.5” 15K HDDs

There is a fundamental transition occurring in the HDD industry. The standard size of a basic unit of data (a sector on the drive) is changing from 512 bytes to 4K bytes. Fortunately there are ways to maintain the 512 byte sector size for quite a while into the future, but eventually all drives will have the 4K byte standard size. Below is some information on that change and what a user should consider:

  • Beginning in late 2009, accelerating in 2010, and hitting mainstream in 2011 for client-based HDDs, hard drive companies began migrating away from the legacy sector size of 512 bytes to a larger, more efficient sector size of 4,096 bytes, generally referred to as 4K sectors, and now referred to as Advanced Format by IDEMA (The International Disk Drive Equipment and Materials Association). Enterprise HDDs are also moving to this format, but will be slower in adoption. The first Advanced Format enterprise HDD became available in 2012, with a limited set in 2013 and a more general distribution in 2014 and beyond
  • Customers have been using applications and operating systems / file systems built on 512 bytes (512n) for decades. This move to 4K byte sector size will impact those software stacks and will result in additional validation work as well as possible structural changes to software as the transition is made. New, higher capacity drives will need to be created in this 4K format. Recognizing that customers may be reluctant to make a change like this quickly, an emulation model of these drives is being created – the drive is built from 4K technology, but allows for 512 byte addressing and transfer at the interface. These drives are known as 512e.
  • Below is a table describing these concepts:
    Format typeBytes per sector valueBytes per physical sector value
    512n512512
    512e5124,096
    4Kn4,0964,096
  • Because the 512e drive has extra work to do in reading and writing 512 byte chunks in a 4K byte sector, there can be performance impacts. Those may be mitigated by internal buffering and caching employed by the drives, but those techniques can be overwhelmed by high utilization rates. In order to avoid that condition, it is paramount for customers to use operating systems and driver additions and possibly applications which force the reading/writing to be started on 4K address boundaries

The Dell HDD offering contains the 512n drives, 512e drives, and drives in the 4Kn format. The 512n format is available for customers interested in maintaining the same drive type they have used over time. The 512e drives provide a 512 byte sector size for those capacities not available in 512n. The 4Kn formatted drives are for those customers interested in adopting the latest, highest capacity HDDs and for those getting prepared for the future direction of the HDD industry.

Dell offers different solid-state drive (SSD) solutions to meet different customer needs. Enterprise SSDs as a class are unique to client or consumer-based SSD in terms of reliability, performance and architecture. While consumer-based SSDs, such as those utilized in notebooks are designed with a focus on consumer-based workloads, rigidity and battery life, enterprise-class SSDs are designed around enterprise application I/O (input/output) requirements with the primary attribute focus being on random I/O performance and reliability.

Understanding the basics of enterprise-class SSDs allow customers to make informed decisions when comparing solutions:
Over-provisioning: The Achilles' heel of SSD are their write performance. To rewrite an area of an SSD that has already been written, the data must be erased and then written. In order to overcome a portion of the write performance penalty, Dell enterprise SSD found across Dell PowerEdge and Dell Storage products, all employ a practice known as over-provisioning of Flash. This practice keeps native Flash capacity beyond the user-defined capacity, and utilizes the additional space as a scratch pad of sorts to quickly put down application write data on areas of Flash that are already in an erased state. The SSDs perform cleanup functions of this over-provisioned Flash space during time periods typically not impacting application performance.

Write Endurance: Write endurance is the number of program/erase (P/E cycles) that can be applied to a block of flash memory before the storage media becomes unreliable. Due to different data center workloads and read/write needs, Dell offers different enterprise SSDs with different endurance ratings so customers can design the right solution for their needs.

Below are the different categories (swim lanes) of enterprise SSDs Dell offers:
Write Intensive (WI) -50/50 read/write workloads with highest endurance. HPC, Database logging, and caching are example workloads.

Mix Use (MU)
-70/30 read/write workloads with medium endurance. E-mail/messaging, OLTP, and E-commerce are example workloads.

Read Intensive (RI)
-90/10 read/write workloads with low endurance. Database warehousing, media streaming, and VOD solutions are example workloads.

Boot Optimized (Boot)
-Lowest cost/small capacity SSDs designed to be used as a boot device in servers. Very low endurance.

Host Interface:
Dell enterprise SSD support three kind of host interface options:

SATA SSD: SATA SSDs are based on the industry standard SATA interface. SATA SSDs provide reasonable performance for enterprise servers.

SAS SSD:
SAS SSDs are based on the industry standard SAS interface. SAS SSDs combine superior reliability, data integrity, and data fail recovery making them suitable for enterprise applications.

PCIe SSD:
The Dell PowerEdge Express Flash PCIe SSD is a high performance solid state storage device that enables IOPS performance of up to 500X more than conventional rotating hard drives.