I have 2 Intel X-25M SSD units in my system. One of them is 80GB, with Windows7 installed on it and it's 60% full. The second SSD is 160GB X-25M, contains only Microsoft Flight Simulator X reaching 98% full. My question is - will the FSX SSD performance drop if I fill it up to nearly 100% (it's a non-OS SSD, the W7 is on another SSD). To what extent can I stuff this SSD? I read that writing performance are dropping, is this a case for me to be concerned?
Read performance doesn't care. Write performance on SSDs is a tricky thing. To an extent, the more free space, the faster it can write. This is because it can't just find available cells to write in, but has to locate a used-but-deleted cell and erase it first. SSDs actually "over-provision," maintaining some capacity that you don't see, so there is a little more free space than you think (but you can't use it).
In response to "My question is - will the FSX SSD performance drop if I fill it up to nearly 100% [?]" Well, write will. But then again, you won't be doing much writing to a full disk. As for "To what extent can I stuff this SSD?" You can fill it 100%. At that point, writing will not work, because there is no space to write to.
If your access to that drive is almost exclusively read, fill the last 2%. (are we really discussing the last 2% of drive space?) At least you won't have file fragmentation issues. If you have active writing to that drive while applications are running, it's worth your effort to do a write benchmark for that drive and an HDD, to see if it's time to use the HDD instead.
Flight Simulator takes up 160 GB? Is that just the SW, or thousands of game saves?