Patent ID: 7464116

Claim:
An apparatus comprising: a read-only storage system to contain a read-only data block; a read-write storage system to contain a modified data block; a filesystem to indicate a location of a data block of a file; and logic to copy a data block from the read-only storage system to the read-write storage system and update the filesystem if the data block is to be modified, wherein each data block indicated by the filesystem is stored in exactly one of the read-only storage system and the read-write storage system; the read-only storage system is shared between a first processor and a second processor, the first processor having read-only access to the storage system and the second processor having read-write access to the storage system, wherein the filesystem comprises: a metadata container to contain a block number of a data block; and a block map to indicate which one of two storage systems contains the data block; and the block map comprises a list of numbers of blocks that are present on the read-write storage system if a size of the list is less than a threshold size, and the block map comprises a bitmap of blocks of the read-write storage system if a number of blocks on the read-write storage system is greater than or equal to the threshold size.