Research On Cache Technique Of OBS

Storage systems have faced more and more challenges with the development of computer technique and networks, including higher I/O speech, larger storage capacity, security, expansibility and usability. The OBS (Object Based storage System) fits in with the next generation internet. The model of data access is object interface for OBS, there is an exclusive ID for every data object, constructing a flat three-level namespace. The OBS make up of computer node, object-based storage device(OSD), object-based file system, metadata server and network. The object-based file system (OBFS) is the major factor of affecting the performance with OBS. Making full use of the difference among the memory speed, network bandwidth, and disk bandwidth, building caches in the file system is one of the major methods to enhance the file system performance.In order to improve the performance of OBFS, a hierarchical cache system has been designed, including metadata cache level zero, cache level one in computer node, cache level two in OSD.Cache level zero is present at computer node. With strategy of improving metadata access protocol, the metadata cache reduces the delay of data accessing and one communication with metadata server once the cache hits. The cache has high hit-ratio, because of the metadata hold little space.The cache level one in computer node is implemented using shared memory. The application processes attaches the shared memory to its own memory space, then they can access the cache directly. The shared memory cache reduces the times of copying data between tow processes, and realizes data truly shared among the processes in a node.In order to improve the whole cache hit-ratio for cache level two, we design a replace algorithm called MQU. The algorithm classifies the blocks which often be accessed together as an access group, and replaces the blocks of an access group in bulk. MQU algorithm relieves a single cache level two missed which reduces the cache efficiency, and homogenizes the cache hit-ratios of all the storage nodes caches.The test results indicate that the hierarchical cache system improves the I/O capacity. The metadata cache level zero can raise the metadata operation speed for above 100%; the computing nodes cache level one can enhance the system performance of parallel read and parallel write for more than 35%; the hit-ratios of MQU algorithm is better than traditional algorithms such as LFU and LRU etc.