Undo saved in ram or hard drive ?

Is the undo levels saved in ram or hard drive, if the hard drive is it possible to prevent them form saving to the hard drive to improve performance ?

The undo levels are held in the system’s RAM (not sure what happens if the available RAM is depleted and the system starts swapping to disk, though), otherwise Blender would have to access the hard drive on every single user interaction to save the undo state, which would bring the whole system to a screeching halt.

What exactly are the performance issues you see?

When i use undo in sculpting blender freezes for around 3-6 seconds which is a long time when it happens often, it defenitly reminds me of the autosave freeze which i minimized with a long autosave timer, that i changed from the 2 default to 30 min, if ti wa ram i dont see how it would interrupt blender like this, it can also happen when working with modifiers but mostly in sculpting mode.

Specs are:

CPU: Corei7 2600k 3.4 Ghz
RAM: 24 GB ddr3
SSD: Corsair neutron Gtx 240 GB
GPU: Geforce Gtx 550ti 1 GB vram

I have a feeling this is to a certain degree inherent to the system: Depending on your level of subdivision/multires (Dyntopo, even?) there can be a lot of vertex positions to restore on undo while sculpting. And I think that the undo system is only single-threaded (not sure, though).

Don’t know what you are undoing, but perhaps it is more efficient not to use undo at all, but rather try to “correct” erroneous strokes with the sculpting tools at hand (smooth, especially). Like a real world sculptor would do with his/her clay.

Blender’s performance with very complex and/or high-poly scenes has never been exactly stellar unfortunately, be it viewport performance or even scene handling…