Each AC caches file data to improve file system performance. Since it is necessary to manage discard operation of cache to ensure consistency in each file system, the GFS Shared File System uses file-exclusive tokens (referred to hereafter as tokens) for management.
A token guarantees that the AC can access files. The token contains information that indicates how it accesses a particular file. An AC must have a token of a file when the AC accesses the file. Two types of tokens are provided: one that ensures meta-data access and another that ensures file data access.
Meta-data access and token management
File data access and token management
Tokens are used to access file data in ordinary files. The two types of available tokens (write authority and read authority) are managed in units of logical blocks. Such operations as simultaneously reading from the same block and simultaneously reading from/writing to different blocks in the same file can be performed simultaneously by multiple shared nodes without degrading performance. The file data token is required to access ordinary files.
