copies of volumes on a group of servers for higher availability and reliability. The list of the servers that hold a copy of the replicated volume is the VSG or Volume Storage Group. Each replicated volumes VolumeId is also replicated. The replicated VolumeIds keep the VSG and the local volumes of each member together.Connected OperationThe inner workings of a file system operation in CODA begin in much the same way they would in many other file systems. The client will request a file and a few system calls will be made in relation to the file, that is, the program will ask the kernel for service. The kernel will attempt to find the inode of the file and return a file handle associated with the file. The inode contains the information needed to access the file data and the file handle is used for the opening program. The call to open the file is handled by the Virtual File System (VFS) in the kernel, once the VFS realizes the requested file is in the coda file system the call is passed to Venus. Venus is CODAs cache manager. IT will first check the client disk cache to see if the file is already in local cache. If the file resides in local cache it is read immediately from the local disk. In the event the file is not in the client disk cache, Venus must contact the server for the file. The file must come from one of the VSG, but not all VSG are available to each client. This can be due to security considerations, downed server or a myriad of other reasons. Because of this each client has an Available Volume Storage Group (AVSG) that it can contact. Clients also have a preferred server among the AVSG. The preferred server is chosen by factors such as physical proximity, load and/or server CPU power. Venus also verifies with the other AVSG that the preferred server has the latest copy of the requested data. If this is not true the data is fetched from the server with the latest copy and it is made the preferred ser...
