Selecting a SSD for Fusion Drive My aging Early 2009 Macbook White is getting a bit long in the tooth, so  after learning about Fusion Drive, I'm considering combining the existing 500GB HDD with a budget-to-medium-class SSD drive, to prolong the life of the machine.

As far as I understand, Fusion Drive will fill up the drive to within 4-5 GB of its capacity. 

In other news I read that some mainstream SSD controllers (eg. Sandforce) have very poor write performance when the disk is used to its full capacity.

So my questions are:

Is the "fullness" of a Fusion Drive SSD considered "full capacity" in the context of controller performance degradation? 

Which aftermarket SSDs are supposed to perform consistently fast in the Fusion Drive scenario? So far I have looked at Samsung 840, OCZ Vertex 2, Agility 3, Intel 330, based on a combination of price, brand reputation and reviews - but several of them are Sandforce drives.  

Thirdly, does it make sense to create an empty 8-10 GB partition on the SSD before creating the Fusion Drive, to keep the drive from filling up? And is it even possible to create a Fusion Drive on a partitioned SSD?

Cheers!
It is pretty close to "full capacity" (the first 117 of 128 GB are filled), but SSDs do overprovisioning for this reason.
As long as you have enabled TRIM on your SSD, you should be fine. I keep my (non-Fusioned Crucial RealSSD 300) SSD near full capacity and don't experience degradation, as long as I have TRIM turned on (if it's turned off it gets terrible fast).
Yes, you can create a Fusion Drive by specifying the partitions, e.g.:

diskutil cs create fusion /dev/disk0s2 /dev/disk1s2


If you are looking for non-Sandforce controlled devices, you have many options now: Crucials M4/M500, Samsung SSDs, OCZ Agility/Vertex 4, and more.

For further reading, check out AnandTech's article with more details and pretty graphs.