What is the exact meaning of 'acala sukha'? Is it mentioned in the Pali-canon? If so, where ? In what context ?
The PTS dictionary says,
Cala (adj.) [see calati] moving, quivering; unsteady, fickle, transient S iv.68 (dhammā calā c' eva vyayā ca aniccā, etc.); J ii.299; iii.381; v.345; Miln 93, 418; Sdhp 430, 494.
-- acala steadfast, immovable S i.232; J i.71 (ṭṭhāna); Vv 514 (˚ṭṭhāna=Ep. of Nibbāna); acalaŋ sukhaŋ (=Nibbāna) Th 2, 350; cp. niccala motionless DhA iii.38
So cala means "unsteady", acala means "not unsteady", and acala sukha means "unwavering happiness" i.e. nibbana.
Stuffs turn Vaṭṭa on (vaṭṭa) = appearing stuffs in paṭiccasamuppāda = non stopping cycling of stuffs = anicca (unstable) & dukkha = cala (unstable) = whole 5 aggregates (stuffs)
Stuffs turn vaṭṭa off (vivaṭṭa) = disappeared stuffs in paṭiccasamuppāda = stopped cycling of stuffs = niccatā (stable) & sukkha = acala (stable) = nibbāna
Whole 5 aggregates and nibbāna are anatta.