Introduction to Hard Disk Drives
HDDs are sealed units used for nonvolatile data storage. Nonvolatile, or semipermanent, storage means that the storage device retains the data even when no power is supplied.
HDDs contain rigid, circular platters, usually constructed of aluminum or glass.
Modern interfaces offer data transfer rates of up to 133MBps for Parallel AT Attachment (ATA), up to 600MBps for Serial ATA (SATA) and Serial Attached SCSI (SAS), and up to nearly 2,000MBps for SATA Express. These interfaces were typically much faster than the individual drives they supported, meaning that the true transfer rate you will see was almost entirely limited by the drive and not the interface you choose. Parallel ATA (PATA) drives have been replaced by SATA drives in new systems.
Form factor standards ensure that available drives will fit in the bay, the screw holes will line up, and the standard cables and connections will plug in. Without these industry standards, there would be no compatibility between different chassis, motherboards, cables, and drives. Currently, 3 1/2-inch drives are the most popular for desktops, whereas 2 1/2-inch and smaller drives are popular in laptops and other portable devices.
Magnetic Hard Drives
Magnetic drives were once the main type of hard drive used. The drive itself is a mechanical device that spins a number of disks or platters and uses a magnetic head to read and write data to the surface of the disks.
One of the advantages of solid-state drives (discussed in the next section) is the absence of mechanical parts that can malfunction.
The basic hard disk geometry consists of three components: the number of sectors that each track contains, the number of read/write heads in the disk assembly, and the number of cylinders in the assembly. This set of values is known as CHS (for cylinders/heads/sectors). A cylinder is the set of tracks of the same number on all the writable surfaces of the assembly. It is called a cylinder because the collection of all same-number tracks on all writable surfaces of the hard disk assembly looks like a geometric cylinder when connected together vertically. Therefore, cylinder 1, for instance, on an assembly that contains three platters consists of six tracks (one on each side of each platter), each labeled track 1 on its respective surface.
The basic physical construction of an HDD consists of spinning disks with heads that move over the disks and store data in tracks and sectors. The heads read and write data in concentric rings called tracks. These are divided into segments called sectors, which typically store 512 or 4,096 bytes each.
5400 RPM
The rotational speed of the disk or platter has a direct influence on how quickly the drive can locate any specific disk sector on the drive. This locational delay is called latency and is measured in milliseconds (ms). The faster the rotation, the smaller the delay will be. A drive operating at 5400 RPM will experience about 5.5 ms of this delay. The RPM stands for revolutions per minute.
7200 RPM
Drives that operate at 7200 RPM will experience about 4.2 ms of latency. As of 2015, a typical 7200 RPM desktop hard drive has a sustained data transfer rate up to 750 Mbps. This rate depends on the track location, so it will be higher for data on the outer tracks and lower toward the inner tracks.
10,000 RPM
At 10,000 RPM, the latency will decrease to about 3 ms. Data transfer rates (about 1.5 Gb) also generally go up with a higher rotational speed but are influenced by the density of the disk (the number of tracks and sectors present in a given area).
15,000 RPM
Drives that operate at 15,000 RPM are higher-end drives and suffer only 2 ms of latency. They operate at just under 2 Gb. These drives also generate more heat, requiring more cooling to the case. They also offer faster data transfer rates for the same areal density (areal density refers to the amount of bits that can be stored in a given amount of space.).
Solid-State Drives
Although hard disk drives continue to be the most important mass storage devices used in personal computers, many high-performance desktops and smaller (under 15-inch screen size) laptops use a flash memory-based mass storage device known as a solid-state drive (SSD) instead of an HDD. SSDs are much faster than HDDs because they use flash memory rather than mechanical parts.
Solid-state drives (SSDs) retain data in nonvolatile memory chips and contain no moving parts. Compared to electromechanical hard disk drives (HDDs), SSDs are typically less susceptible to physical shock, are silent, have lower access time and latency, but are more expensive per gigabyte.
Introduction to Solid State Drives (SSD)
Hybrid Drives
A hybrid drive is one in which both technologies, solid-state and traditional mechanical drives, are combined. This is done to take advantage of the speed of solid-state drives while maintaining the cost effectiveness of mechanical drives.
There are two main approaches to this: dual-drive hybrid and solid-state hybrid. Dual-drive systems contain both types of drives in the same machine, and performance is optimized by the user placing more frequently used information on the solid-state drive and less frequently accessed data on the mechanical drive or in some cases by the operating system creating hybrid volumes using space in both drives.
A solid-state hybrid drive (SSHD), on the other hand, is a single storage device that includes solid-state flash memory in a traditional hard drive. Data that is most related to the performance of the machine is stored in the flash memory, resulting in improved performance.
Hot-Swappable Drives
If a drive can be attached to the server without shutting down, then it is a hot-swappable drive. Drive types that are hot-swappable include USB, FireWire, SATA, and those that connect through Ethernet. You should always check the documentation to ensure that your drive supports this feature.
Introduction to Hard Drive Partition Types
What is Hard Disk Drive Cache
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