The ever-increasing demand of storage capability leads to scaling requirement in RAID-structured storage systems. Previous approaches to RAID scaling mainly focus on minimizing data migration, without considering the user-level application accesses. However, the mixed scaling I/Os and user accesses in practical systems will interfere with each other, which results in significant performance degradation of both the data migration time and the user response time. In this paper, we divide the whole storage space into multiple zones and measure the popularity (mainly using the metric of access frequency) of each zone. Based on the measured popularity, we propose an online scheme, namely Popularity-based Online Scaling (POS), to scale RAID-structured storage systems. The main idea of POS is to scale storage areas with high popularity first so as to better exploit workload locality. POS can efficiently alleviate the performance degradation of user response time and data migration time during the scaling process. It can be readily deployed atop various conventional RAID scaling approaches to improve their performance. To evaluate the performance of POS, we implement FastScale and FastScale with POS (POS-FS) in the same system. Through extensive benchmark studies on real-system workloads, we show that POS can efficiently reduce the response time to user requests and scaling I/Os and improve the sequentiality of data accesses.