There are several basic operations upon which the entire caching logic is constructed. Getting familiar with them is essential to understanding how OCF caching engines work, and how individual I/O requests are handled depending on the current cache state. The following section provides a brief introduction to these operations.
Mapping
Mapping is an operation of linking up a core line with a cache line in the cache metadata. During mapping the cache storage area of size of a single cache line is being assigned to the core line, and during cache operation this area can be used for storing core line data, until the cache line is being evicted.
Mapping updates core id and core line number in cache line metadata as well as hash map information.
Insertion
Insertion is an operation of writing core line data to a cache line. During insertion the data of I/O request is being written to an area of cache storage storing data of the cache line corresponding to the core line being accessed. The valid and dirty bits of the cache line in the metadata are being updated accordingly during this operation.
Insertion doesn’t change cache line mapping metadata, but only valid and dirty bits. Because of that it can be done only after successfull mapping operation. However, unlike mapping, which operates on whole cache lines, insertion is performed with sector granularity. Every change of valid bit from 0 to 1 is considered an insertion.
Insertion may occur for both read and write requests. Additionally, in the Write-Back mode the insertion may introduce a dirty data. In such situation, besides setting a valid bit, it also sets a dirty bit for accessed sectors.
Update
Update is an operation of overwriting cache line data in the cache storage. It’s performed when the core line accessed during a write request is mapped into the cache and the accessed sectors are marked as valid. When some of the written sectors within the cache line are invalid, then those sectors are inserted, which means that update and insertion can be done together by a single I/O request.
Update can be performed only during a write request (a read can never change the data) and similarly to insert it can introduce dirty data, so it may update dirty bit for accessed sectors in the cache line. However it will never change valid bit, as update touches only sectors that are already valid.
Invalidation
Invalidation is an operation of marking one or more cache line sectors as invalid. It can be done for example during handling of discard request, during purge operation, or as result of some internal cache operations. More information about cases when invalidation is performed can be found here.
Eviction
Eviction is an operation of removing cache line mappings. It’s performed when there is not sufficient space in the cache storage for mapping data of incoming I/O requests (when cache occupancy is close to 100%). The cache lines to be evicted are selected by an eviction policy algorithm. Currently OCF supports one eviction policy, the Least Recently Used (LRU) which selects cache lines that have not been accessed for the longest period of time.
Flushing
Flushing is an operation of synchronizing data in the cache storage and the backend storage. It involves reading sectors marked as dirty from the cache storage, writing them to the backend storage, and setting dirty bits in cache line metadata to zero.
Flushing is a cache management operation, and it has to be triggered by the user, and it’s typically done in order to safely detach the core from the cache without risking data loss.
Cleaning
Cleaning is an operation very simillar to flushing, but it’s performed by the cache automatically as a background task. Process of cleaning is controlled by a cleaning policy. OCF currently supports three cleaning policies:
- Approximately Least Recently Used (ALRU),
- Aggressive Cleaning Policy (ACP),
- No Operation (NOP).