Balancing act -- Match your data to the correct disk

Implementing a tiered storage environment involves aligning the appropriate type of disk and storage medium to an application's service requirement and data value. This tip takes a look at matching your data to the correct disk or storage system to meet different service requirements. Service requirements for matching your data to the correct disk drive include performance characteristics, availability, capacity and cost, among others. A common mistake made is to evaluate and choose disk drives based on dollar per capacity, for example, dollar per gigabyte (GB), terabyte (TB) or petabyte (PB) or simply based on the capacity size of the disk drive.

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For those looking for performance, the typical evaluation metric is either the revolution per minute (rpm) rotational speed or simply the disk drive interface (ATA, Fibre Channel, SAS, SATA or SCSI). However, the key to deciding what type of disk drive to use is aligning the disk to the data, and more importantly, to how the data will be used and accessed. Some other factors that come into play include your existing environment and operational considerations, such as available electrical power, cooling and ventilation, floor loading, available space and existing storage, servers and network components, and management tools.

Deciding what type of disk drives to use for different applications can be daunting with the various options. Deciding what type of disk drive to buy is somewhat like deciding what type of TV set to buy with the myriad of options and manufacturers.

For example, with TV you have standard definition (ST), digital TV (DTV), high definition (HD), DLP, Plasma, LCD, rear projection, screen size, resolution, 1080i, 720p, 1080p, component video, svideo or HDMI interfaces, among other options. While the most common type of disk drive for storage applications is the magnetic hard disk drive (HDD), there are also semiconductor RAM- and FLASH-based solid state disk (SSD) devices, as well as optical (MO, DVD, CD) and magnetic tape for secondary, nearline and offline storage needs.

Determining the disk drive to use involves understanding your applications storage requirements for online, nearline and offline categories. For example, are your online storage requirements performance intensive (I/O or throughput), storage capacity intensive or both I/O and storage intensive. It is important to understand your availability requirements for storage and data protection needs.

Staying focused on HDD technology, your options include high performance (rpm, transfer rates, latency), and high capacity (147 GB, 250 GB, 500 GB, 750 GB and soon 1 TB). Other options include low cost, high reliability (MTBF), duty cycle, form factor (1 inch, 1.8 inch, 2.5 inch, 3.5 inch), target markets (consumer, notebook, desktop or enterprise), amount of cache, different interfaces and manufactures (Fujitsu, HGST, Samsung, Seagate, Toshiba, Western Digital)

Characteristic	Disk Drive Type
Category or type of HDD	ATA/IDE (aka PATA)	SATA	Fibre Channel nearline	Fibre Channel	Parallel SCSI	SAS
Typical applications	Primary storage in SMB or entry-level systems, D2D backup, archiving, nearline, remote data replication, VTL, video and image storage, and retention			OLTP, large-scale file sharing, large email, data collection, surveillance, video, enterprise general purpose storage, high-end server system disk, database and I/O intensive files
General characteristics	Low cost, high capacity, good availability, primary storage for smaller environments			High performance (IOP or throughput), high reliability, mission critical, enterprise class, high-end servers
Where found	Notebook and PC servers, older storage systems	Notebook and PC servers, some storage systems	Some newer storage systems	Entry, midrange and enterprise storage	Servers and some entry storage systems	Blade, rack and other servers, entry and midrange storage
Class and form factor	2.5" or 3.5" desktop or enterprise class	2.5" or 3.5" desktop or enterprise class	3.5" Fibre Channel nearline	3.5" Enterprise	2.5" and 3.5" Enterprise	2.5" and 3.5" Enterprise
High performance		Some 10K or 7.2K NCQ	7.2K	10K and 15K	10K and 15K	10K and 15K
High reliability	Lower MTBF, lower duty cycle	MTBF up to around 1M hours with enterprise, low-duty cycle	High MTBF around 1M hours, high-duty cycle	High MTBF above 1M hours, high-duty cycle	High MTBF above 1M hours, high-duty cycle	High MTBF above 1M hours, high-duty cycle
Dual port	External device	External device	Built in	Built in	Built in	Built in
Trend	Shifting to SATA or SAS	Larger capacities, shifting to 2.5" smaller form factor	Being adopted by some in place of SATA	Shifting to 2.5" smaller form factor	Shifting to SAS or SATA or Fibre Channel	Shifting to 2.5" smaller form factor

Table-1: HDD characteristic (Note: Specifications subject to rapid change)

Note that not all 2.5-inch disk drives are desktop. There is a new class of high-performance enterprise-class small form factor (SFF) 2.5-inch disk drives. Likewise, not all 3.5-inch disk drives are enterprise class, as many high-capacity SATA disk drives are 3.5 inch. For I/O intensive applications where multiple HDD are used to provide increased aggregated I/O per second, SSD can in many instances be a better fit when measured on a dollar per I/O or dollar per transaction basis instead of on a storage capacity basis, as is the case with HDDs.

While some SATA disks support 10,000 rpm speeds, they may lack performance-enhancing capabilities found on perceived slower 7,200 rpm disks. Similar to Fibre Channel and SCSI disks, there are premium SATA disk drives and economical (low-cost) versions. There are enterprise-class and desktop SATA disk drives with enterprise drives having better features for reliability and performance. Many of the SATA disk drives deployed have been desktop category adding fuel to the fire that SATA disk drives perform poorly and are unreliable. Ask your storage solution provider if it is using enterprise-class or desktop-category disk drives. Also, ask if it has added any special firmware features in their controllers to support and leverage disk drive features and differences for ATA, Fibre Channel, SAS, SCSI and SATA.

For online applications with high performance and availability requirements, 10,000-15,000 rpm Fibre Channel and SCSI (U160 and U320) disk drives a good choice. Enterprise and open systems distributed storage subsystems will continue to support Fibre Channel disk drives for several years. SAS disk drives, including 2.5-inch SFF, will begin replacing SCSI disk drives in 2005 for high-density applications requiring good performance and availability. Similar to how some storage subsystems support a mix of Fibre Channel and SATA (or ATA) disk drives, some storage subsystems will eventually include support for SAS along with Fibre Channel and SATA. Even with this shift to SAS disks, there will remain various storage interface options for attaching to servers via DAS or networked connection via SAN (iSCSI and Fibre Channel) or NAS.

Many of the same considerations that go into aligning the applicable type of disk drive to your data will also apply to aligning different storage systems to your application and business needs; however we will leave the storage system discussion for a separate tip.

About the author: Greg Schulz is founder and senior analyst with the IT infrastructure analyst and consulting firm StorageIO. Greg is also the author and illustrator of "Resilient Storage Networks" (Elsevier) and has contributed material to "Storage" magazine and other TechTarget venues.

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