Terminology
Terms and Definitions
| Term | Definition |
|---|---|
| cache | The transparent storage of data so that future requests for that data can be served faster. |
| cache instance | A single occurrance of the caching engine mapping a core device and a cache device. |
| cache hit | When requested data is contained in (and returned from) the cache. |
| cache miss | When requested data is not in the cache, and therefore must be retrieved from its primary storage location. |
| core device | The device to be cached. |
| cache device | The device caching for a core device. |
| dirty data | This refers to data that is modified within the cache but not modified in primary storage. |
| IO | Abbreviation for input/output as it relates to the flow of data. |
| lazy write | The process of mirroring to primary storage. |
| pass-through | A caching mode in which the cache will be bypassed for all operations. |
| primary storage | As it relates to caching, the storage system or location (DAS, SAN, NAS, etc.) where the data is stored. |
| SAN | Storage Area Network. Framework used to attach remote computer storage devices to servers. Storage devices appear as if they were attached locally to the operating system. |
| SSD | Solid State Drive. A device used for data storage that uses memory chips instead of a revolving disk. |
| tiered storage | A data storage technique that moves data between two or more kinds of storage, which are differentiated by four primary attributes: price, performance, capacity, and function. |
| write-around (wa) | A caching mode in which some write operations are not cached. Writes to blocks that do not exist in cache are written directly to the core device, bypassing the cache. If a write operation is issued to a block that is already in cache (because of a previous read operation), then writes are sent to both the core device the cache device. Write-around cache improves performance of workloads where write operations are done rarely and no further read accesses to that data are performed, so there is no benefit in caching it. |
| write-back (wb) | A caching mode in which data is written first to the cache and then mirrored to primary storage when IO bandwidth is available. The process of mirroring to primary storage is known as a lazy write. |
| write-through (wt) | A caching mode in which every write to the cache causes a synchronous write to primary storage. |
| write-only (wo) | A caching mode in which data is written first to the cache and then mirrored to primary storage when IO bandwidth is available. Reads can be served by the cache device only if previously written otherwise reads are bypassed and read from primary storage. |