The comment in the c file says **lookahead_size must be a multiple of 4** https://github.com/ARMmbed/littlefs/blob/v2-alpha/lfs.c#L3222 And comment in headers says **lookahead_size must be multiple of 64-bits (8 bytes)** https://github.com/ARMmbed/littlefs/blob/v2-alpha/lfs.h#L208