In cloud storage systems, caching is a commonly employed method to enhance system performance. Utilizing Solid State Drives (SSDs) as caches for Hard Disk Drives (HDDs) can improve overall system performance at a relatively low cost. However, traditional caching systems suffer from inefficiencies in write operations due to their overwrite approach. Furthermore, when slow disks are present in cloud storage systems, caching algorithms are often unable to detect them, resulting in degraded performance of the storage system. To address these challenges, we proposed SynergiCache, a novel cluster caching system for cloud storage systems that significantly enhances performance and efficiency. Initially, we convert the traditional overwrite approach to an efficient append-only method, leveraging Remote Direct Memory Access (RDMA) technology to enhance data transfer efficiency. Furthermore, we employ a composite Key-Value (KV) storage mode and implement a cooperative garbage collection mechanism to optimize data access and storage performance. Subsequently, we introduce a slow-disk-sensitive adaptive (SDSA) caching algorithm that optimizes the flow of data between SSDs and HDDs, thereby reducing the adverse impact of slow disks on cloud storage systems. We implemented SynergiCache, adapted it for integration with Ceph, and conducted comprehensive experiments. The experimental results demonstrate that SynergiCache significantly enhances cloud storage performance, reducing average latency by 89.55% and increasing IOPS by 9.52 \(\times \) compared to traditional caching systems.