significant overhead compared with writing data into an empty page, potentially crippling write performance. SSDs store data in flash memory cells that are grouped into pages typically of 4 to 16 kiB, grouped together into blocks of typically 128 to 512 pages. Example: 512 kiB blocks that group 128 pages of 4 kiB each. NAND flash memory cells can be directly written to only when they are empty. If they happen to contain data, the contents must be erased before a write operation. An SSD write operation can be done to a single page but, due to hardware limitations, erase commands always affect entire blocks; consequently, writing data to empty pages on an SSD is very fast, but slows down considerably once previously written pages need to be overwritten. Since an erase of the cells in the page is needed before it can be written to again, but only entire blocks can be erased, an overwrite will initiate a read-erase-modify-write cycle: the contents of the entire block are stored in cache, then the entire block is erased from the SSD, then the overwritten page(s) is written into the cached block, and only then can the entire updated block be written to the flash medium. This phenomenon is known as write amplification. Operation The TRIM command enables an operating system to notify the SSD of pages which no longer contain valid data. For a file deletion operation, the operating system will mark the file's sectors as free for new data, then send a TRIM command to the SSD. After trimming, the SSD will not preserve any contents of the block when writing new data to a page of flash memory, resulting in less write amplification (fewer writes), higher write throughput (no need for a read-erase-modify sequence), thus increasing drive life. Different SSDs implement the command somewhat differently, so performance can vary. TRIM tells the SSD to mark an LBA region as invalid and subsequent reads on the region will not return any meaningful data. For a very brief time, the data could still reside on the flash internally. However, after the TRIM command is issued and garbage collection has taken place, it is highly unlikely that even a forensic scientist would be able to recover the data. Implementation Operating system support The TRIM command is beneficial only if the drive implements it and the operating system requests it. The table below