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.

Entry HDDs are used in limited configurations and for non-critical applications. That usage would be for servers typically operating in normal business hours and with read/write patterns not exceeding 55TB/year. These drives would not be for data center operations, but better suited for small business office operations.

These drives do not have power loss protection, which means if there is an unexpected power disruption to the system, data that has been sent to the drive, but not yet actually written to the media, may be lost. The underlying size of the smallest group of data on these drives is 4K bytes. However most operating systems work with 512 bytes, so the drive has a way to emulate or make it look like the data is available in 512 byte groups. That emulation (512e), aka AF (Advanced Format) of 512 bytes (512n) on a 4K byte (4Kn) HDD can have performance impacts based on the operating system and the applications being executed.

Below is a table illustrating the differences between Entry HDDs and Enterprise SATA/SAS HDDs*
 Entry SATA HDDs Enterprise SATA / Nearline SAS Business Critical HDDsEnterprise SAS Mission Critical HDDs
RPM 5900/7200 RPM7200 RPM 10K/15K RPM
SATA Interface6Gb 3Gb-6Gb 6Gb
HDD Sector Format 512 byte AF(advanced format)
(no power loss protection) 
  • 512 byte
  • 512 byte emulation
          (power loss protection)
  • 4K byte 
  • 512 byte
  • 512 byte emulation
    (power loss protection)
  • 4K byte
  • MTBF 750K hours 1.2M hours 2.0M hours
    @Workload Rating 55 TB/year 550 TB/year 550 TB/year
    @Usage hours 2400 hours/year 8760 hours/year 8760 hours/year
    UBER(unrecovered error rate) 1X10141X10151X1016
    Performance Variation up to +/- 35% +/- 5% +/- 5%
    Vibration RV(Radians/sec/sec) 5.5 13.5 (3.5”) 38.4 (2.5”)
    RV Sensor No Yes Yes
    Online Firmware Download No Yes Yes 

    Below are a number of charts comparing the various Enterprise HDD drive types. 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. The charts below show the advantages of 2.5” MC vs. 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 consider including 2.5” 10K/15K over 3.5” 15K HDDs

    Input/Output operations per second (IOPs) is a recognized measurement of random I/O performance for HDDs. The chart below illustrates the differences between the various HDDs from 2.5” 15K at the top and 3.5” 7.2K at the bottom. To see all the characteristics of the drive types, please refer to the URL mentioned in the Overview tab.


    Power requirements show the advantage of 2.5” 7.2K drives and the disadvantage of 3.5” 15K:


    Combining these two charts into an IOPs/Watt measurement results in the following:

    Iops Watt

    If there is concern about how much performance can be gained from drives in a given amount of rack space, consider the following graph where it is assumed twice as many 2.5” drives can fit in the same space as 3.5” drives. Clearly IOPs for the 2.5” mission critical drives dominate.

    Iops 2u volume

    NLSAS and SATA business critical drives have clear advantages over mission critical for considerations of cost and maximum capacities. The chart below on cost illustrates the lower cost of 3.5” 7.2K drives and the higher cost of 2.5” 15K drives. The capacity chart shows the much larger capacities of 3.5” 7.2K drives. Except for 3.5” 15K, all of the categories will have capacity increases moving into the future.


    Relative capacity points for each drive category are shown in the chart below. Note that it is expected that capacities will increase for each category but is not anticipated for the 3.5” 15K drives.

    If you are interested in an SSD for the enterprise environment, there are different types of drives, depending on the access and performance that you require.

    Dell Value 2.5” SATA SSDs are suitable for customers that have mixed workloads such as Data warehousing, virtual desktop and high-performance computing applications.These drives come with a 3 year warranty and are offered in capacities of 100GB and 200GB and have been designed for compatibility and ease of integration into Dell servers.

    The Dell Mainstream 2.5” SAS SSDs are a good solution for customers who manage online transactional processing (OLTP), databases, Business Decision Support Systems and data caching applications, which may have extremely high write intensive workloads. These drives come with a 5 year warranty and are offered in capacities of 200GB and 400GB. They have been designed for compatibility and ease of integration into new or existing tier-0 enterprise storage systems, including servers, direct-attached storage and network-attached storage.