L2ARC

L2ARC, or Level 2 Adaptive Replacement Cache, is a caching component of the ZFS filesystem designed to extend the in-memory ARC by storing a subset of cached blocks on a fast second-level storage device, typically an SSD. The purpose is to improve read performance for data that is accessed frequently but does not fit entirely in RAM.

How it works: When a block is read, ZFS serves it from the ARC in memory. If the block is evicted from ARC but still likely to be reused, ZFS may cache a copy of the block in the L2ARC on the SSD. Should the same block be requested again, it can be served from L2ARC rather than requiring a disk read, reducing latency and I/O. L2ARC is a read-side cache and does not cache writes directly.

Management and considerations: L2ARC is largely automated by ZFS, with tunables to adjust its size and behavior on supported platforms. It requires a suitable fast SSD or similar device to be effective. Benefits are most pronounced on read-heavy workloads with data locality and when RAM is insufficient for ARC. If the SSD is slow or the workload has little locality, L2ARC may offer limited improvement and can add extra I/O and wear on the caching device.

Relation to ARC: L2ARC complements the in-memory ARC by providing a second-level cache layer. It is one component of ZFS’s broader caching strategy, alongside the RAM-based ARC and, in some deployments, metadata caching.

See also: ZFS, ARC, OpenZFS.