Method for Migrating Memory and Checkpoints in a Fault Tolerant System

1 . A method of migrating memory from a primary computer to a secondary computer comprising the steps of:

a) waiting for a checkpoint on the primary computer;

b) pausing the primary computer;

c) selecting a group of pages of memory to be transferred to the secondary computer;

d) transferring the selected group of pages of memory and checkpointed data to the secondary computer;

e) restarting the primary computer;

f) waiting for a checkpoint on the primary computer;

g) pausing the primary computer;

h) selecting another group of pages of memory to be transferred to the secondary computer;

i) transferring, to the secondary computer, the other selected group of pages of memory and any data checkpointed since the previous checkpoint;

j) restarting the primary computer; and

k) repeating steps (f) through (j) until all the memory of the primary computer is transferred.

2 . The method of claim 1 wherein the checkpointed data that is transferred to the secondary computer since the previous checkpoint is only checkpointed data from a previously transferred group of pages of memory.

3 . The method of claim 1 wherein the number of pages in a group of pages of memory transferred at each checkpoint varies.

4 . The method of claim 1 wherein in addition to the selected group of pages of memory and checkpointed data being transferred, a predetermined number of additional pages is also transferred.

5 . The method of claim 4 wherein the predetermined number of pages of memory are marked as dirty pages whether the pages have been accessed or not.

6 . The method of claim 4 wherein the selected group of pages are selected from a pool of pages.

7 . The method of claim 6 wherein a sweep index determines which pages are selected from a pool.

8 . The method of claim 7 wherein the sweep index ranges from 0 to the highest page number of memory.

9 . A computer system comprising:

a primary computer having a primary computer memory and a primary computer checkpoint controller;

a secondary computer having a secondary computer memory; and

a communications link between the primary computer and the secondary computer;

wherein when the checkpoint controller of the primary computer declares a checkpoint, the primary computer is paused, a group of pages of memory of the primary computer is selected to be transferred to the secondary computer, the selected group of pages of memory and checkpointed data is transferred to the secondary computer over the communications link, and the primary computer is restarted; and

wherein when another checkpoint is declared by the primary computer checkpoint controller, another group of pages of memory to be transferred to the secondary computer is selected, and the other selected group of pages of memory and any data checkpointed since the previous checkpoint is transferred to the secondary computer.

10 . The computer system of claim 9 wherein the pages selected for transfer are determined by a sweep index counter.