Why must keep some space in the SSD? can do this by leaving some unallocated partition space for the SSD?

so cheap. i bot 860 evo last 1-2 months for RM350. but only use the SATA2 port. Using old socket 775 core2 duo motherboard.

In practice, an SSD’s performance begins to decline after it reaches about 50% full. This is why some manufacturers reduce the amount of capacity available to the user and set it aside as additional over-provisioning. For example, a manufacturer might reserve 28 out of 128GB and market the resulting configuration as a 100GB SSD with 28% over-provisioning. In actuality, this 28% is in addition to the built-in 7.37%, so it’s good to be aware of how vendors toss these terms around. Users should also consider that an SSD in service is rarely completely full. SSDs take advantage of this unused capacity, dynamically using it as additional over-provisioning.

Some SSD manufacturers provide software tools to allow for over-provisioning of drives by the user. Actually, even without special software, any user can set aside a portion of the SSD when first setting it up in the system by creating a partition that does not use the drive’s full capacity. This unclaimed space will automatically be used by the controller as dynamic over-provisioning.

There is one obvious drawback to over-provisioning: the more unused capacity one reserves to increase writing speeds, the less capacity there is for storage. With hard drives, the somewhat similar practice of short stroking, which confines reads and writes to the fastest outer tracks of the drive platters, is less penalizing because the cost per gigabyte is lower with magnetic media. At $1 to $3 per gigabyte for enterprise-class SSDs, the decision to give up 25% or more of a drive’s capacity becomes more difficult. There had better be some real benefit when giving up those expensive bytes!