Pros and cons of file- and block-based flash caching

Can you compare file-based caching and block-based flash caching? How do they work? How are they installed? And, what are the pros and cons of each?

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File-based caching is designed to cache specific files, either the whole file or blocks within that file. Block-based caching, on the other hand, caches blocks of data on a storage volume, regardless of what files are associated with those blocks.

File-based flash caching should be more accurate and use cache resources more effectively because it can focus on specific files. But it needs to be installed within the confines of the operating system or even within the applications themselves. This means that the implementation and operation of a file-based cache may require more administrator interaction because multiple instances need to be installed. Furthermore, each instance needs to be programmed to know which files it should focus on for cache utilization. It is ideal when the performance demands of an application are well-known and when the investment in flash capacity needs to be kept at a minimum. Database applications are typically a good candidate for file-based caching.

Block-based flash caching is installed at either the operating system level or more typically at the virtualization hypervisor level. This allows for a single installation to be leveraged across every virtual machine running on a server. This means less administrator interaction and ongoing fine tuning. Block-level caching is ideal when the cost of flash capacity is less of an issue and in virtual server environments.

More on this topic:

How do flash caching software products differ from one another?
Marc Staimer, independent storage expert, discusses flash caching software in this Expert Response. 

Duplessie: Deploying SSDs as a storage cache
In this Storage Decisions video, Steve Duplessie discusses the options for using solid-state data, particularly as a storage cache. 

Auto tiering vs. caching in hybrid storage systems
Auto tiering and caching provide similar benefits in hybrid storage systems, but are different technologies with different capabilities and use cases.

This was first published in February 2014

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