TRIM to the rescue
This is where TRIM comes in. Supporting OSes query the drive for its rotational speed, and if the returned value is 0, the OS identifies the drives as a SSD. Thus the OS enables the TRIM command for the drive when files are deleted from it. The TRIM command tells the SSD controller to delete the pages on the NAND flash block when the user deletes the data contained in the pages in the OS. The entire block containing the data is copied into the memory cache, then the block is erased. After this procedure, the data sans the data deleted by the user is written back to the block from the memory cache. This results in longer delete times, but allows the write performance to be like the drive in its new state. Look at the example below.
What if you can’t TRIM ?
Not all the drives and OSes support TRIM. Most of the manufacturers are coming out with firmware updates for their SSDs to add the TRIM support. You will also need a TRIM supporting OS like Windows 7 to reap the benefit of this feature. If you cannot use this feature, there is another way by which you can restore your drive to its peak performance. HDDErase is a freeware utility that securely erases data on hard drives using the security erase unit command built into the firmware of ATA and SATA drives. This utlilty will clear every page on the drive, restoring the performance to like-new state. This utility can be downloaded from here.