"In typical Apple fashion, the announcement had lots of dazzle and little technical detail, but a few specifications emerged. Fusion Drive uses separate SSD and HDD to provide 128Gbytes of flash and 1 to 3Tbytes of spinning disk... When an SSD and hard drive are combined into a single volume in Apple's core storage, the volume manager will dedicate 4GBbytes of the SSD as a write cache and place newly written data there. Frequently-accessed data will be promoted from the hard disk to the SSD as well. The 128GBbytes of flash Apple uses should be enough to hold all the programs and data most desktop users access more than once in a blue moon, giving those programs SSD-like performance.""
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