Solid State Drives (SSDs) brought performance to the forefront of computing technologies, and their adoption is increasing not only in the data center but also in consumer-grade products. Unlike its traditional spinning hard disk drive (HDD) counterpart, SSDs comprise a collection of computer chips (non-volatile NAND memory) with no movable parts. Therefore SSDs are not kept busy seeking to new drive locations and, in turn, introducing latency. As great as this sounds, SSDs are still more expensive than HDDs. HDD prices have settled to around $0.03/GB; SSD prices vary but sit at around $0.13-$0.15/GB. At scale, that price gap makes a world of difference.
To keep costs down and still invest in the needed capacities, one logical solution is to buy a large number of HDDs and a small number of SSDs and enable the SSDs to act as a performant cache for the slower HDDs.
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