Galaxy formation at its peak phase, during the first few billion years of cosmic history, at redshift z~2, was far more violent than today. The higher density of the Universe at earlier times makes all the timescales relevant for assembly, galactic dynamics and star formation shorter, so the processes associated with galaxy formation and evolution were intensified, making this period ideal for studying galaxy evolution. Dense streams of cold gas impact galaxies at intense rates along the 'cosmic-web' of intergalactic structure, providing fuel for high star-formation rates and causing a state of 'violent disk instability' (VDI) where several giant clumps, each containing a few percent of the disk mass and a tenth of its star formation, are continuously formed. In this talk, I will discuss the formation and the evolution of these giant clumps in high z disk galaxies. I will present a simple analytical toy model for the evolution of clumps in disks, accounting for migration, star-formation and outflows due to stellar feedback. This model is compared to high resolution simulations, both of isolated galaxies and zoom-in cosmological simulations, and to observations. I will then discuss the origin of VDI and clump formation in the context of classical Toomre instability and the compressible nature of turbulence.