A RAM-Disk or Ramdisk can be either:
- Another name for volatile types of solid state drives, a specialized piece of data storage hardware.
- A software abstraction that treats a segment of random access memory (RAM) as secondary storage, a role typically filled by hard drives.
In either case, access time is greatly improved. Due to the lack of moving parts, RAM can be several orders of magnitude faster than hard drives. However, the volatility of RAM means that data will be lost if power is lost, e.g. when the computer is turned off. This is sometimes desirable, for example when working with a decrypted copy of an encrypted document. In many cases, the data stored on the RAM disk is only a copy of or is derived from data permanently stored elsewhere, and thus will be re-created when power is restored and the system reboots. Due to the higher price of RAM over hard drive storage, in some situations, RAM is used only for very frequently accessed data, or that which is creating a performance bottleneck. This might include swap space for virtual memory, temporary files used for scratch space which experience high-frequency reads and writes, or frequently accessed data from a database. RAM disks can also be used to hold uncompressed programs for short periods.
Hardware RAM disks
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It has been suggested that this article or section be merged into Solid state drive. () |
Hardware RAM disks are also known as ramdrives, and are a type of solid state drive. Newer models carry lithium-ion batteries as backup in case of a power outage or transfer to another computer. RAM disks were popular as boot media in the 1980s, when hard drives were expensive, floppy drives were slow, and a few systems, such as the Amiga series and the Apple IIgs, supported booting from a RAM disk. At the cost of some main memory, the system could be soft-rebooted and be back in the operating system in mere seconds instead of minutes. Some systems had battery-backed RAM disks so their contents could persist when the system is shut down. A proper disk cache in the operating system may lessen the performance motivation for a RAM disk; a disk cache fulfills a similar role (fast access to data that is notionally stored on a disk) without the various penalties (data loss in the event of power loss, static partitioning, etc.). RAM disks are, however, indispensable in situations in which a physical disk is not available, or where access to, or changing a physical disk is not desirable (such as in the case of Live CDs). They can also be used in a kiosk-style device where any changes made to a system are not committed and the original configuration is to be loaded each time the computer is turned on. The advent of Serial ATA has meant that RAM disks can be interfaced as a normal hard drive, although with extremely high transfer speeds. An example is the Gigabyte i-RAM, Hybrid Ram Disk, which uses standard DDR modules and connects to its host via SATA; this card also can make use of the system's standby power (also used for Wake-on-LAN and similar features) to maintain its RAM contents even with the system powered off, and even includes a battery that can retain the data when the system is completely disconnected from the power supply.
Software ramdisks
Software ramdisks use the normal RAM in main memory as if it were a partition on a hard drive, using special mechanisms in the operating system kernel, rather than actually accessing the data bus normally used for secondary storage. Usually no battery backup is needed due to the temporary nature of the information stored in the ramdisk, but an uninterruptible power supply can keep the entire system running during a power outage, if necessary. Some ramdisks use the cramfs filesystem, to allow compressed data to be accessed on the fly, without uncompressing it first. This is convenient because ramdisks are often small due to the higher price per megabyte than conventional hard drive storage. The first commercially available software RAM disk for microcomputers was the Silicon Disk System from Microcosm Ltd. This appeared in 1980, initially for the CP/M operating system and later for MS-DOS. Microsoft added a RAM-disk to MS-DOS (version 2.0) in 1983.
