Please add spaces between words: ThisisbecauseSSDdrivescannotactuallyoverwritedata–theyhavetocopythedata(withyourchanges)toanewlocationandthenerasetheoldblockentirely.AnoptimisationinthisareaisthatanSSDmightnotevenerasetheoldblockbutrathersimplymakeanotetoerasetheblockatalatertimewhenthingsaren’tsobusy.TheendresultisthatSSDdrivesfitverywellwithaCoWfilesystemanddon’tperformaswellwithnon-CoWfilesystems.
This is because SSD drives cannot actually overwrite data – they have to copy the data (with your changes) to a new location and then erase the old block entirely. An optimisation in this area is that an SSD might not even erase the old block but rather simply make a note to erase the block at a later time when things aren’t so busy. The end result is that SSD drives fit very well with a CoW filesystem and don’t perform as well with non-CoW filesystems.