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OneFS uses up to three levels of read cache, plus an NVRAM-backed write cache, or coalescer. These cache levels and their high-level interaction are illustrated in the following diagram.
The first two types of read cache - level 1 (L1) and level 2 (L2) - are memory (RAM)-based, and analogous to the cache used in processors (CPUs). These two cache layers are in all Dell PowerScale and Isilon storage nodes.
An optional third tier of read cache, called SmartFlash or level 3 (L3) cache, is also configurable on nodes that contain solid state drives (SSDs). L3 cache is an eviction cache that the L2 cache blocks populate as they are aged out from memory.
Each cache level is optimized for a specific purpose and operations, as summarized in the following table. These cache levels are described in more detail in the next few sections.
Name | Type | Persistence | Description |
L1 Cache | RAM | Volatile | Also called front-end cache. It holds clean, cluster-coherent copies of file system data, and metadata blocks, requested from clients through the front-end network. |
L2 Cache | RAM | Volatile | Back-end cache containing clean copies of file system data and metadata on a local node. |
SmartCache/Write Coalescer | DRAM/ NVRAM | Nonvolatile | Persistent journal cache that buffers any pending writes to front-end files that have not been committed to disk. |
SmartFlash/L3 Cache | SSD | Nonvolatile | Cache that contains file data and metadata blocks evicted from L2 cache, increasing L2 cache capacity. |