How to over provision free space for SSD?

Also, the DU GUI does still leave ~262144 sectors after the final partition, most of my drives don't have it because I often partition them manually using the Terminal.

DU Disk Utility in El Capitan GM leaves 262151 sectors after the final partition and 262144 in between partitions you create with it.

Allan

Interesting. I have two drives I'm nearly certain I partitioned using DU GUI that have 262149 and 262144 at the end of the final partitions respectively. There are no gaps at all between partitions.

What I haven't tested, and perhaps what you are seeing, is what happens when you create one partition using DU GUI, then another, then another etc. as opposed to erasing the drive and creating 3 partitions in one action.

I created the partitions all at once. I am using a firewire/usb dock on firewire for the external I'm experimenting with in case that makes a difference.

I doubt that the type of connection would be a factor - I'm using Thunderbolt everywhere fwiw - but it suggests that your hardware is older and there's a possibility that makes a difference.

In your case, given that you are clearly prioritising data integrity highly enough to consciously over-provision, I suggest you play it safe and partition with the buffer everywhere as this is doing some over-provisioning already, and so it can't be bad for your use case.

I'm also going to remove my earlier comment about having buffers between partitions as being unnecessary. Better safe than sorry.

I just booted from my El Capitan GM installer USB drive and completely set up my 500GB Samsung 840 Evo SSD from scratch. I used DIsk Utility off the USB Installer to erase and partition the drive (400GB Macintosh HD and 50GB Untitled). I then did a full new install of El Capitan GM onto Macintosh HD. It created a Recovery Partition as well. Upon completion I booted the new system, ran Terminal, and saw there was no 262144 sector buffer between any of the partitions, just a 262151 sector one after the four partitions.

Yes, that matches my OS X installs on external and internal drives too (bar very slight differences in the final buffer size).

Also wanted to point out that the 262144 sector buffer between Macintosh HD and Untitiled does exist until the Recovery HD partition gets created as part of the install, and then it is gone.

Interesting. Thanks.