Mass Storage is any data storage medium that retains its contents permanently. Typically slower, less compact and more power hungry than RAM, but far cheaper and more capacious, and unlike RAM they generally do not require constant power to retain their data. Some mass storage devices are Read Only Memory (these are also referred to as Write Once, Read Many, or WORM, formats), but many are fully rewriteable.
Mass storage is the main factor of two things:
- Load times. Programs need to load into RAM before they can run, so the faster they can do it, the shorter load times are.
- Response times. If the program streams data a lot (Wide Open Sandbox do this), then the program feels snappier if response times are low.
Storage HierarchyThere is an effective hierarchy in storage types, with newer, faster types of storage not just being significantly when first developed, but remaining so for decades after. This not only means choosing a compromise between performance and price for a given use, but also means many systems will contain multiple different kinds of storage, just as they do different kinds of memory.
Magnetic Tape It may surprise many people to discover that not only is magnetic tape still in use, it's actually more popular than ever. It's extremely slow to save or access data, and can only do so sequentially. However, it's incredibly cheap, has good data density, and is stable in at least the medium term (although it does degrade over the course of decades). This makes it by far the most popular choice of backup storage medium, and with people and businesses creating ever more data, there is more tape in use than ever. Although most will be written once and then never touched again, it's an extremely important part of disaster recovery.
Magnetic Disk The classic hard disk drive or HDD. Using similar technology to tapes but with much faster access times and, most importantly, random access allowing data to be read or written to any part of the disk. Although expensive when first introduced (invented in the 1950s, but still rare in the 1980s due to cost), they eventually became the primary storage for almost all computers. However, although the demise of tape has been predicted many times, tape and disk are based on the same technology and so tape has remained cheaper and in use where access speed is not an issue.
HDDs are slowly being replaced by flash-based SSDs, however, flash memory is much more expensive and so HDDs remain popular when all-out performance is not so important.
Solid-state Drives This covers a range of different storage formats, but all relying on the same basic Flash Memory technology. The most familiar is what has traditionally been called an SSD, which uses a similar form-factor to an HDD plugged in using the same SATA connection. SSDs have much faster data rates, if the interface allows, but are significantly more expensive than magnetic drives. It is therefore common for computers to have a relatively small SSD to store the operating system and some programs, while having a much larger HDD to store things like music and video which do not benefit from being accessed at high speed.
Some manufacturers have produced hybrid drives which couple a standard HDD with a small SSD, as little as 8GB. This can be used to cache frequently used data, thus allowing SSD-like speed for opening common programs without the associated cost.
More recently, flash memory has been used in NVMe drives, which use the same PCI-Express interface as things like graphics cards, allowing even higher transfer speeds than regular SSDs. These usually come in a much more compact format, such as the common M.2, which consists of little more than a PCB card with some chips attached. This makes them very attractive in space-limited applications such as laptops. It also means that the cost is not really different from a larger SSD, so the latter may be dying out before it even managed to replace HDDs.
3D XPoint A brand name rather than a storage class, but only because it's effectively created an entirely new class of its own. 3D XPoint is a technology that attempts to cross the gap between storage and RAM. It is much faster than even flash-based SSDs (although not as much faster yet as initially predicted), and although still slower than normal DRAM, it is available in a memory-module format for use in that role. Even a few years after its introduction, it remains far too expensive for the average home computer, but its use in servers has been growing, effectively creating a fourth level in the hierarchy.
Removable StorageRemovable storage has not generally formed the same hierarchy, with one kind largely dominating at any given time, or overlapping with another that will replace it.
Magnetic Tape Using the same basic technology as mass-storage tape, but usually in a much smaller and more manageable cassette, these were used for some early computer systems. Importantly, cassettes could be written to as well as read from, so once a game had been loaded, the tape could be swapped for a blank one and saving the game state was possible. Competing systems using cartridges could not usually offer similar functionality. Of course, this came with the downside that tapes could be easily copied using consumer audio equipment, leading to the first rise of video game piracy.
Cartridges Cartridges were used for many games consoles, largely because of the aforementioned piracy - making copies of ROMs onto a proprietary cartridge format was much more difficult than simply copying a normal tape. Since cartridges also used chips, they could add extra functions and even additional processors. However, this also made them much more expensive to produce, and only Nintendo stuck with them for long after the rise of optical disks.
Magnetic Disk Similar to magnetic hard drives, but containing only the magnetic disk itself in a protective sleeve, with the reading head, motors, and other parts in a separate drive. Often called "floppy disks", this was to contrast with hard disks not because the disks were necessarily floppy (although the 5 1/4" format certainly were that as well). Although tape was cheaper, it was slower, did not allow random access and, most importantly, tape uses a contact head while a disk reader is non-contact. This made floppies much more durable with regular use, and ultimately relegated tape to long-term storage duty. Having some of the same issues as tape meant that it never fully took over from cartridges in consoles, but rapidly took over the entire personal computer market.
Optical Disks Optical discs cover a few different formats, but all based on essentially the same technology. The CD was first released in the early 1980s, but cost meant they didn't really become popular until the 1990s. Part of the problem was that they were simply too big - a CD can hold around 600MB of data, while hard drives of the time could maybe manage 10MB at most. Tape was just as good for bulk storage but much cheaper, and there just wasn't much use for portable media that held two orders of magnitude more data than your computer could work with. As computers became more powerful and hard drives larger, the CD, and later DVD, found their place, replacing the floppy drives that couldn't compete on size or speed. They also finally killed off the cartridge, with price, capacity and speed so much better that the drawbacks were no longer enough.
Flash Memory Using the same technology as SSDs, but in a format that can be carried around and plugged to a variety of computers. By far the most common is the USB stick. Far more portable than optical disks, or indeed any other portable medium preceding them, and in most ways also more durable and less error-prone in normal use. They quickly replaced disks of all kinds for almost all re-writeable uses, however, they never really caught on for software distribution.
Ultimately, both optical disks and flash memory have been out-competed by the internet. With cloud storage and streaming services increasingly available, end users simply don't need access to large amounts of portable storage in the same way as before. Both still see use, DVDs for game distribution and USB sticks for re-writeable uses, but neither is likely to really conquer the market. Of course, those cloud and streaming services all ultimately use the storage hierarchy listed above, they can only replace portable storage.