In a computer system and mostly for cache systems, there are two most important localities. First locality is spatial locality and the second locality is spatial. They are important because the system improves the performance by storing frequently accessed or soon to be accessed in the cache, and evict those that are not going to be used.
Spatial locality describes the phenomenon where a resource that is going to be accessed next has high likelihood of being near the previous access. For example, a resource on position i+1 is likely to be accessed after referencing a resource on position i.
Temporal locality, on the other hand, describes the case where a resource in a location accessed once has high chance of being referenced again in the near future. For example, a variable in a loop will be accessed as many as the loop count.
Understanding the locality is not only important in cache systems but also in log-structured file systems, especially when a log-structured file system that employs hot/cold separation mechanism, performs garbage collection. After cleaning a segment in the file system, the valid blocks within the segment has to be allocated to one of hot or cold segments. Typically, cold data represents image, audio, visual, and write-once-read-only files, and frequently updated blocks occupies the hot segment.
Say a file is as large as few hundred Gigabytes which is used as storage for a virtual machine guest. One can argue that log-structured file system is not suit for such application, but let’s not discuss the best local file system for a virtual machines. Let’s assume that the guest performs a lot of random write on various and wide range of Logical Block Address (LBA) on the guest storage. However, to host system, it is just a random access to different locations which does not affect or make difference for the garbage collection and treat them specially. To garbage collection, all the write requests to the large is as if there are no localities in the workload.
The question is that do we have to consider reference counts of large files as well as the localities?
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