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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.
After the completion of this operation, a hybrid volume can be decommissioned or the size of a cache device can be reduced.
Dm-cache is a device-mapper level solution for caching blocks of data from mechanical hard drives to solid state SSDs.
The stochastic multiqueue (smq) policy performs in a similar way as the multiqueue policy, but requires fewer resources to operate; in particular, it uses substantially smaller amounts of main memory to track cached blocks.
Later, Joe Thornber, Heinz Mauelshagen and Mike Snitzer provided their own implementation of the concept, which resulted in the inclusion of dm-cache into the Linux kernel.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.Acting as a mapping target, dm-cache makes it possible for SSD-based caching to be part of the created virtual block device, while the configurable operating modes and cache policies determine how dm-cache works internally.
The operating mode selects the way in which the data is kept in sync between an HDD and an SSD, while the cache policy, selectable from separate modules that implement each of the policies, provides the algorithm for determining which blocks are promoted (moved from an HDD to an SSD), demoted (moved from an SSD to an HDD), cleaned, etc.
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.