Dm-cache

dm-cache is a component (more specifically, a target) of the Linux kernel's device mapper, which is a framework for mapping block devices onto higher-level virtual block devices. It allows one or more fast storage devices, such as flash-based solid-state drives (SSDs), to act as a cache for one or more slower storage devices such as hard disk drives (HDDs); this effectively creates hybrid volumes and provides secondary storage performance improvements. The design of dm-cache requires three physical storage devices for the creation of a single hybrid volume; dm-cache uses those storage devices to separately store actual data, cache data, and required metadata. Configurable operating modes and cache policies, with the latter in the form of separate modules, determine the way data caching is actually performed. dm-cache is licensed under the terms of GNU General Public License (GPL), with Joe Thornber, Heinz Mauelshagen and Mike Snitzer as its primary developers. dm-cache uses solid-state drives (SSDs) as an additional level of indirection while accessing hard disk drives (HDDs), improving the overall performance by using fast flash-based SSDs as caches for the slower mechanical HDDs based on rotational magnetic media. As a result, the costly speed of SSDs becomes combined with the storage capacity offered by slower but less expensive HDDs. Moreover, in the case of storage area networks (SANs) used in cloud environments as shared storage systems for virtual machines, dm-cache can also improve overall performance and reduce the load of SANs by providing data caching using client-side local storage. dm-cache is implemented as a component of the Linux kernel's device mapper, which is a volume management framework that allows various mappings to be created between physical and virtual block devices. The way a mapping between devices is created determines how the virtual blocks are translated into underlying physical blocks, with the specific translation types referred to as targets.