OCZ, Crucial, and several other manufacturers specifically list in their information and FAQ pages that you should not have your operating system solid state drive greater than 80% full capacity. This is because the drive needs extra space to place files in this open space while it goes through and temporarily erases flash memory blocks to rewrite data. This reorganization of data is a part of TRIM, supported on nearly all SSDs today, and is a major necessity to keep the drive performing at its full speed after years of usage and to help improve longevity. Additionally, MLC based SSDs normally will have a section of NAND memory set aside for failing memory cells as they age or become defective which is normal wear for a long-running SSD. Sometimes the manufacturer will automatically provision this space out of the drive so it cannot be formatted, but this is not always the case. If no over provisioning is done, and the entire drive capacity is formatted, and then becomes completely full, then you run the very likely chance of losing critical data and destroying your SSD as the controller will have no space to reallocated data for the read/erase/write tasks, and more importantly will have no place to reallocate memory to as other sections wear out from usage meaning loss of data.
There are numerous ways in which different SSD manufacturers have come to address these issues throughout the years of course. Some SSDs include onboard cache for the controller to make writes to as its having to perform TRIM cleanup commands and writing back blocks of data, while others again have intentionally provisioned a part of the NAND to never be formatted for usage.
