Save the whole buffer for undo when reloading it. This applies to the ":e!" command and reloading for when the buffer changed outside of Vim. FileChangedShell The save only happens when this option is negative or when the number of lines is smaller than the value of this option. Set this option to zero to disable undo for a reload.

When saving undo for a reload, any undo file is not read.

Note that this causes the whole buffer to be stored in memory. Set this option to a lower value if you run out of memory.

(default 10000)