Vdev eviction openzfs series#

The ZFS adjustable replacement cache (ARC) is one such caching mechanism that caches both recent block requests as well as frequent block requests. So, it would seem that the obvious approach would be to somehow combine the two- have an LRU and an LFU simultaneously. Thus, there are a great amount of disk requests, which kind of defeats the purpose of a cache to begin with. However, they suffer from the problem that newer data could be evicted from the cache if it's not read frequently enough. There do also exist least frequently used (LFU) caches. Of course, it's eventually replaced as newer data blocks are requested. Thus, from the cache perspective, it ends up with a lot of worthless, useless data that is no longer needed.

When large sequential reads are read from disk, and placed into the cache, it has a tendency to evict more frequently requested pages from the cache. Image showing the traditional LRU caching scheme. As more blocks are read, the push the older blocks toward the least recently used portion of the cache, until they fall off the conveyor belt, or in other words are evicted. Blocks are put into most recently used portion of the cache. Think of the whole process as a conveyor belt. Even if those older pages are accessed more frequently. Thus, when the cache is full, the older pages will be pushed out of the cache.

However, the cache is a FIFO (first in, first out) algorithm. The cache will fill as more and more data is read, and put into the cache. The way the LRU algorithm works, is when an application reads data blocks, they are put into the cache. Traditional CachesĬaching mechanisms on Linux and other operating systems use what is called a Least Recently Used caching algorithm. Our previous post discussed the ZFS Intent Log, or ZIL and the Separate Intent Logging Device, or SLOG.

Vdev eviction openzfs series#

Our continuation in the ZFS Administration series continues with another zpool VDEV call the Adjustable Replacement Cache, or ARC for short.