Using virtual disks for disk system checkpointing

A method of checkpointing data stored on one or more storage devices in a computer employing a plurality of processors and a plurality of distributed disk storage devices, comprising the steps of:

• classifying all storage space associated with the processors as one or more logical disks;
• assigning a plurality of virtual disks to a processor, the virtual disks together comprising a subset of the logical disks;
• assigning functions to the virtual disks of each processor such that each function is assigned to one or more virtual disks of each processor;
• storing a checkpoint of the distributed disk system in one or more virtual disks, portions of the checkpoint being distributed among one or more of the virtual disks, wherein the checkpoint represents the state of the distributed disk system at a particular time;
• periodically updating the checkpoint.

A method of constructing a checkpoint in a multiprocessor system, comprising:

• maintaining a first block list corresponding to a group of blocks stored in memory for an active version of a file;
• creating a second block list, identical to the first block list, and maintaining the second block list as a checkpoint;
• modifying data in the first group of blocks, comprising the steps of:
• copying data residing in a first disk location to a new disk location;
• overwriting some or all of the data in the new disk location with new data;
• replacing the entry in the first block list which corresponds to the first disk location with the address of the new disk location;
• accounting for the number of blocks addressed by the first and second block lists, comprising the steps of:
• providing used and reserved block counters, and a maximum limit for the processor;
• for each file, providing a new block counter and a reserved block counter;
• setting the used block counter of each file to be the union of blocks used by the checkpoint and user block lists;
• setting the reserved block counter of each file to be equal to the number of blocks in the checkpoint block list that are not also in the user block list of the file;
• when a block of the user block list is overwritten, incrementing the used block counter of that file and of the system by one and decrementing the reserved block counter of that file and of the system;
• setting the total number of used blocks to be equal to the sum of the used block counts for each file;
• setting the total number of reserved blocks to equal the sum of reserved block counts of each file;
• maintaining the counters when files are deleted, overwritten, resized or checkpointed.