A disk is a data storage device used in computers and other electronic devices. The two main types of disks are magnetic disks and optical disks. Magnetic disks store data on a thin coating of magnetic material, while optical disks store data in small pits on the surface that are read by a laser. Magnetic disks include traditional hard disk drives (HDDs) with spinning platters as well as newer solid state drives (SSDs) that have no moving parts. Optical disks include CDs, DVDs, and Blu-ray discs.
Disks provide long-term storage for the operating system, applications, documents, media files, and other data on a computer. They have much greater capacity than volatile storage like RAM. Disks allow data to persist even when the power is off, making them critical for long-term storage and retrieval.
Magnetic Storage
Magnetic storage like hard disk drives (HDDs) store data by magnetizing tiny sections of the platter’s surface (Folder101). The platters inside the HDD are made of thin metal, usually aluminum, glass, or ceramic, that are coated with a magnetic material. A read-write head hovers just above each platter surface and uses an electromagnet to set the magnetic polarity of tiny sections of the platter as it spins. By aligning magnetic polarity in certain patterns, binary 1s and 0s can be encoded on the disk (Quizlet). The read-write head senses the magnetic fields on the platter to read the data later. HDDs use multiple platters stacked on top of each other to increase storage capacity. The platters spin at very high speeds while the actuator arm moves the heads back and forth to access data stored in concentric tracks on the platter (Brainscape).
Optical Storage
Optical storage devices like CDs, DVDs, and Blu-ray discs store data optically using a laser to read and write data. They work by reflecting laser light off of the surface of the disc in order to read the microscopic pits and lands arranged in a spiral track (https://www.techtarget.com/searchstorage/definition/optical-disc).
During the writing process, a laser beam burns microscopic pits into the organic dye layer of the disc. These pits and the spaces between them (lands) represent binary data. When reading the data, a laser beam with lower power reflects off the surface of the disc and a photodiode sensor detects the transitions between pits and lands. The reflected beam is then decoded to read the stored data.
Compact discs (CDs) store up to 700MB of data while DVDs can store 4.7GB (single layer) to 17GB (dual layer). Blu-ray discs, which use a blue-violet laser, can store up to 25GB per layer. The storage capacity of optical discs increased over time as the wavelength of the laser decreased, allowing smaller pits to be written on the surface (https://m.youtube.com/watch?v=WFwo_aZYQGQ).
Overall, optical storage allows large amounts of data to be stored securely and read back reliably using laser technology.
Solid State Drives
Solid state drives (SSDs) use flash memory to store data, instead of mechanical platters like traditional hard disk drives (HDDs). Flash memory in SSDs typically come in one of two types: NAND flash or NOR flash.
NAND flash is most commonly used in consumer SSDs. It stores data in an array of memory cells made from floating-gate transistors. NAND flash is cheaper than NOR flash but also typically slower. However, modern NAND flash offers excellent speeds for most consumer workloads.
Writes to NAND flash memory must be done at the block level, so SSD controllers use techniques like wear leveling to distribute writes across all cells. Reads can be done at the byte level. NAND flash cells also deteriorate with use, so SSD controllers use garbage collection to reclaim unused blocks.
Overall, the lack of moving parts, fast access times, and resilience against physical shocks have made SSDs the preferred storage choice for consumer devices like laptops. Prices have also fallen steadily, making SSDs affordable for more applications.
Disk Formatting
Disk formatting is the process of preparing a storage device like a hard disk drive (HDD), solid-state drive (SSD), or USB flash drive for initial use. Formatting configures the logical structures on the disk that organize data storage. Common file systems used for formatting include NTFS, exFAT, FAT32, HFS+, and ext4.
The file system defines how data is stored, organized, and retrieved. For example, NTFS uses advanced data structures to improve performance, reliability, and disk space use compared to older file systems like FAT32. exFAT allows support for larger file sizes compared to FAT32 while maintaining compatibility across operating systems.
Formatting erases existing data on a disk by overwriting the logical structures, allowing it to be reused for new data storage. Common tools for formatting storage devices include the operating system’s built-in disk utility, command line tools like diskpart, and third-party formatting utilities.
It’s important to back up data prior to formatting, as the process erases all user data on the disk. Formatting is often required to change file systems, repair logical errors, or prepare a new disk for use.
Logical Structure
The data on a hard disk is organized logically into sectors and clusters. A sector is the smallest unit of storage and represents 512 bytes of data. Multiple sectors are grouped together into clusters, which are the units that the operating system allocates to files. The number of sectors per cluster varies depending on the operating system and disk size, but is commonly between 8 and 64 sectors per cluster [1].
Data is written to the disk in a concentric ring structure. The disk platters are divided into tracks, which represent the concentric circles where data is stored. Tracks are further divided radially into sectors. The operating system keeps track of where files are stored on the disk by mapping clusters to cylinder, head, and sector locations. Each cylinder represents a track on all the platters in the drive. The head represents the platter surface while the sector specifies the offset of the cluster within the track [2].
This logical structure allows the operating system to locate files efficiently on the disk by translating file requests to physical cylinder-head-sector addresses. The virtual mapping also allows bad sectors to be masked and remapped without changing the logical disk structure.
Cylinders and Tracks
Physically, hard disk drives consist of circular platters coated with magnetic material that spin at high speeds. Read/write heads are positioned above and below each platter to access the data. The platters are organized into tracks and cylinders for locating data.
A track is a circular path located on the surface of a disk platter. As the platter spins, the read/write head remains stationary over the track. Each platter surface has many concentric tracks. Tracks located at the same radial position on every platter surface are collectively called a cylinder. So a cylinder consists of all tracks located at the same distance from the spindle on every platter surface. https://tinf2.vub.ac.be/~dvermeir/mirrors/sunos-wks.acs.ohio-state.edu/sysadm_course/html/sysadm-30.html
The purpose of this physical layout is to allow the read/write heads to access data locations quickly. With tracks and cylinders, the head only needs to move in or out radially while the platters spin to access new data. This is much faster than physically moving the heads across the full width of each platter to reach new tracks.
Access Methods
There are two main methods for accessing data stored on a disk: sequential access and direct access. Sequential access involves reading or writing data in a serial order, starting from the beginning of the disk and accessing each bit in sequence. Direct access allows jumping directly to the desired location to read or write data without having to go through the entire disk sequentially.
With sequential access, the drive’s read/write head starts at the beginning of the disk and accesses each sector until it gets to the target location. This method is efficient for accessing large amounts of contiguous data, like reading an entire file from start to finish. However, it can be quite slow for locating and accessing smaller, non-contiguous pieces of information.
In contrast, direct access allows the drive head to move directly to the cylinder and track containing the desired data, without having to pass over the intermediary tracks. This random access ability provides much faster retrieval of non-sequential data. However, it requires more complex and costly hardware to properly position the R/W head on the disk platter. Overall, direct access provides more flexibility and efficiency for accessing data in any order.
Performance Factors
There are several key factors that affect the performance of a hard disk drive:
Latency refers to the time it takes for the read/write head to move into position over the correct track on the disk. It is impacted by the rotational speed of the disk. Lower latency means faster access times. Source
Transfer rate is the rate at which data can be read from or written to the disk. It is measured in megabytes per second (MB/s) and depends on factors like the disk’s rotational speed and recording density. Higher transfer rates allow faster data access. Source
Seek time refers to the time required for the actuator arm to move the read/write heads to a specific location. Lower seek times allow data to be accessed more quickly. Seek time is impacted by track width and the speed of the actuator arm.
Conclusion
In summary, data storage on disks has evolved tremendously over the decades, from early magnetic storage methods like floppy disks and hard drives, to optical media like CDs and Blu-ray discs, to the latest solid state drives utilizing flash memory chips. As technology continues advancing, storage capacities keep increasing while physical sizes keep shrinking. Emerging storage technologies like DNA digital data storage and atomic-scale storage show great promise for the future. With exponentially growing data generation rates across personal, business, and scientific realms, ongoing innovation in high-capacity, compact data storage will remain crucial.
Looking ahead, we can expect faster, denser, and more reliable forms of data storage to emerge. Cloud-based storage solutions are already gaining popularity for their accessibility, while local storage may incorporate DNA or atomic-level tech for ultra-high densities. Quantum computing could also revolutionize storage by enabling extraordinarily fast search and retrieval of data. Overall, the future looks bright for developing storage technologies that keep pace with our collective data growth.