Storage tiering mode balances performance and capacity by dividing storage devices into a cache tier and a capacity tier based on the storage media configured in the cluster and business requirements. This mode mainly applies to two types of scenarios. The first is when the cluster contains a combination of different types of physical disks, such as NVMe SSD + SATA SSD or NVMe SSD + HDD. The second is when all physical disks in the cluster are SSDs of a single media type, but space optimization features such as erasure coding need to be enabled. In the first scenario, ABS allocates most of the space on higher-performance physical disks to the cache tier, while lower-performance, capacity-oriented physical disks are pooled into the capacity tier. The cache tier is further divided into a write cache for the cluster-wide unified cache tier and a read cache for read acceleration of the local disk-based capacity tier. In the second scenario, the cache tier and the capacity tier share the same set of physical disks. Each physical disk is divided into a cache partition and a data partition at a capacity ratio of 1:9. In this case, the cache tier is used only as a write cache.
Under different media tiering modes, when writes occur to a storage volume, the system typically prioritizes writing data to the cache tier. If the logical data area of the volume is updated frequently, the data remains in the cache tier for a longer period; otherwise, infrequently updated data is periodically sunk to the capacity tier. For data that is not frequently updated but is often read, the system stores it in the read cache of the capacity tier to ensure high-speed access. Under the same media tiering mode, I/O of replica volumes is written directly to the capacity tier, while I/O of EC volumes is first written to the cache tier and then sunk to the capacity tier.
Data of the write cache is stored as multiple replicas, while the organization of data in the capacity tier is determined by the storage policy set by the user for the volume. In the tiered storage mode, for critical volumes that require performance assurance, ABS also supports configuring the volume pinning mode. In this mode, all data of a volume is retained in the cache tier, regardless of whether it is hot or cold. This meets the application scenarios where certain data always requires high-performance, low-latency access.
