When it comes to modern computing and data storage, one of the most fundamental components is the disk. Disks are used in computers, servers, and storage arrays to store digital data magnetically or optically. A single disk on its own has limited storage capacity. To increase overall capacity, multiple disks are often combined together into what’s known as a disk array or disk subsystem. This enables storing much more data across the array of disks.
In this article, we will explore the collective terms used to describe multiple disks configured to work together as part of a larger storage system. We will look at the history of disk arrays, different types, usage contexts, benefits, and challenges. By the end, you will have a clear understanding of the various terms used for arrays consisting of more than one disk.
Definition of a Disk
A disk is a flat, round, magnetic data storage device used in computers and data storage systems (1). Disks come in two main forms – hard disks and floppy disks. Hard disks have a rigid platter made of metal or glass inside a protective casing, while floppy disks have a flexible plastic film platter inside a square plastic casing (2).
Disks store data magnetically on the flat surface of the platter. The disk spins rapidly while a read/write head floats just above the surface, allowing data to be written to and read from the disk as needed. Disks allow random access, meaning any piece of data can be accessed directly without having to pass sequentially through all the data (3).
Disks have been a core component of computer data storage since the 1960s. While newer solid state drives are becoming more common, disks continue to offer high capacity storage at low cost, making them essential for data centers, servers, desktop computers and laptops (2).
Overall, a disk is a ubiquitous data storage device characterized by its flat, round magnetic platter that can be read from or written to by a read/write head.
(1) https://www.computerhope.com/jargon/d/disk.htm
(2) https://www.webopedia.com/definitions/disk/
(3) https://www.collinsdictionary.com/us/dictionary/english/computer-disk
Types of Disks
Disks are used as data storage devices in computers and other electronic devices. The most common types of disks include:
- Hard Disk Drives (HDD): Use rotating magnetic disks to store data. HDDs have large capacity and are non-volatile, meaning they retain data when powered off. Their performance is limited by the moving parts. Common form factors include 3.5″ and 2.5″ drives.
- Solid State Drives (SSD): Use integrated circuits and have no moving parts. SSDs offer much faster performance than HDDs, at lower latency and power use. However, their capacity is typically lower and cost per gigabyte remains higher than HDDs. Common form factors include 2.5″ and M.2 drives.
- Optical Discs: CDs, DVDs and Blu-ray discs store data optically. Their capacity ranges from 700MB (CD) up to 50GB (Blu-ray). Optical discs are removable, portable and durable, but have slower read/write speeds. Their usage has declined with the rise of high-capacity HDDs and flash drives.
- Floppy Disks: Early magnetic storage media that could be easily removed and transported. Their low 1.44MB capacity led to obsolescence as storage needs grew. The last Apple computer with a floppy drive was sold in 1998.
What is a Disk Array?
A disk array, sometimes called a storage array, is a data storage system that combines multiple disk drives together. The drives are arranged and managed in a way to improve performance, capacity, resilience, and flexibility compared to using single drives.
There are several methods to configure multiple disks in an array:
- RAID (Redundant Array of Independent Disks) offers data protection by replicating data across multiple drives. Popular RAID levels include RAID 0, RAID 1, RAID 5, and RAID 10.
- JBOD (Just a Bunch of Disks) simply combines drives together into a larger storage pool without any data protection. Drives appear as a single large disk.
- Concatenation stripes data sequentially across multiple disks similar to JBOD.
A disk array controller manages the configuration of the disks and how data is distributed across them. Advanced disk arrays include features like caching, tiering, thin provisioning, and snapshots to further optimize performance.
By grouping disks together, disk arrays can increase capacity, speed, and resilience well beyond single disk limitations. Data can be replicated across drives for fault tolerance. The total bandwidth is multiplied with parallel access. Disk arrays are vital for storing and managing vast amounts of data in enterprise environments.
Common Terms for Multiple Disks
There are several common terms used to refer to collections of multiple disks in computer systems:
Disk array – A disk array is a data storage system that contains multiple disk drives (Merriam-Webster). Disk arrays allow large amounts of data to be stored across multiple disks and accessed in parallel for faster performance.
RAID – RAID stands for Redundant Array of Independent Disks. It is a data storage technology that combines multiple disk drives into a logical unit (TechTarget). RAID provides increased storage performance and fault tolerance compared to single disk systems. Some common RAID levels include RAID 0, RAID 1, and RAID 5.
JBOD – JBOD stands for Just a Bunch of Disks. It refers to a collection of disks in a computer system that are not arranged according to any RAID configuration (TechTarget). The disks are available individually without any combining into logical units.
History of Disk Arrays
Though the earliest computers used drum memory and other forms of storage, the first commercial disk drive was introduced by IBM in 1956 with the RAMAC 305 system. As noted by Wikipedia, “Each generation of disk drives replaced larger, more sensitive and more cumbersome devices.”
The concept of combining multiple disk drives together into an array emerged in the 1970s and 80s as a way to improve performance and capacity. According to Wikipedia’s article on disk arrays, some of the early work on disk arrays was done at UC Berkeley in the late 70s. In the 1980s, RAID (Redundant Array of Independent Disks) technology was pioneered to improve reliability through redundancy.
As storage needs grew in the 80s and 90s, especially for enterprise and server applications, disk arrays became more widely adopted. Vendors began selling dedicated hardware disk arrays, and the technology improved to add caching, better performance, and network connectivity. Over time, disk arrays evolved into complex storage systems with advanced software functionality for virtualization, automation and data services.
Usage Contexts
Disk arrays are commonly used in enterprise and data center environments where large amounts of storage and high performance are required. Some of the main usage contexts and applications for disk arrays include:
Data warehouses – Disk arrays provide the fast access and high availability needed for large-scale analytics and business intelligence applications. Parallel access to multiple disks increases query performance. according to https://www.techtarget.com/searchstorage/definition/array
Database servers – Databases like Oracle, SQL Server, MySQL, etc rely on disk arrays to store and access vast amounts of structured data while supporting many concurrent transactions. Disk arrays help optimize database performance. according to https://en.wikipedia.org/wiki/Disk_array
Virtualization and cloud computing – Virtual machines and cloud-based applications need shared storage with high capacity and I/O performance. Disk arrays allow allocating storage resources efficiently across many VMs and cloud workloads. according to https://www.techtarget.com/searchstorage/definition/array
High performance computing – Applications like data mining, seismic imaging, video rendering, etc require fast parallel storage access. Disk arrays are designed to deliver very high throughput and IOPS for these demanding HPC workloads.
Business continuity – Disk arrays support features like RAID, replication, and snapshots for data redundancy and disaster recovery. This provides continuous availability in case of disk failures or outages. according to https://www.quora.com/What-is-the-purpose-of-using-different-types-of-hard-disk-arrays-RAID-What-are-the-benefits-and-drawbacks-of-each-type
Benefits of Disk Arrays
Using multiple disks in an array configuration provides several advantages over using a single disk, including improved performance, redundancy, and flexibility.
Performance is enhanced through techniques like data striping, which spreads data across multiple disks. This allows read/write operations to occur in parallel, increasing throughput. Some RAID levels like RAID 0 can double disk performance.
Disk arrays provide redundancy through techniques like disk mirroring, where data is duplicated on separate disks. If one disk fails, data integrity is maintained. RAID 1, RAID 5, RAID 6 and other levels provide redundancy. This protects against data loss due to hardware failures.
Disk arrays also provide flexibility. Disks can be added to expand storage capacity. Different RAID levels can be implemented to optimize for performance or redundancy as needed. Disks can be hot swapped in case of failure. Overall, disk arrays provide more options than single disks.
Challenges of Disk Arrays
Managing multiple disks in an array presents several challenges that need to be considered. According to research from Varki (Issues and challenges in the performance analysis of real disk arrays, 2004), some key challenges include:
Performance modeling and analysis – With multiple disks and large caches, accurately modeling and analyzing the performance of disk arrays can be difficult.
Scalability – As the number of disks increases, scalability becomes a concern. The array architecture must be designed to handle expansion efficiently.
Reliability – More disks means more opportunities for failure. Maintaining data reliability and redundancy across many disks is challenging.
Data migration – Moving data across disks is complex, especially while maintaining availability. Data migration strategies must be robust.
Utilization – Evenly distributing data and workloads across all disks is key for optimization but hard to achieve in practice.
Troubleshooting – Identifying and resolving issues is more difficult with many interdependent components.
In summary, while disk arrays provide benefits like increased capacity and performance, managing multiple disks introduces difficulties related to modeling, scalability, reliability, migration, utilization, and troubleshooting that must be taken into account.
Conclusion
In summary, there are various common terms used to refer to multiple disks in computing. The main collective terms are:
- Disk array – A disk array is a data storage system that contains multiple disk drives. The disk drives can be arranged and accessed in various ways to provide different configurations and benefits.
- RAID – RAID (Redundant Array of Independent Disks) refers to specific standardized architectures of disk arrays that provide increased performance, reliability and redundancy.
- JBOD – JBOD (Just a Bunch of Disks) refers to the architecture of directly attaching multiple disks to a computer and accessing them independently.
- Disk pack – Historically, a disk pack referred to removable packs of stacked disks used in older computer systems.
Using multiple disks together in arrays provides key advantages like improved performance, capacity, redundancy and flexibility compared to single disks. Disk arrays now play an integral role in enterprise storage systems, servers and data centers. Understanding the different terms and configurations of multi-disk systems remains important knowledge for IT professionals managing storage infrastructure.
