One disadvantage of the migration algorithm is that only the threads on one host can access data contained in the same block at any given time. Replication can reduce the average cost of read operations, since it allows read operations to be simultaneously executed locally ( with no communication overhead ) at multiple hosts.

However, some of the write operations may become more expensive, since the replicas may have to be invalidated or updated to maintain consistency. Nevertheless, if the ratio of reads over writes is large, the extra expense for the write operations may be more than offset by the lower average cost of the read operations.

Replication can be naturally added to the migration algorithm by allowing either one site a read/write copy of a particular block or multiple sites read-only copies of that block. This type of replication is referred to as multiple readers/single writer replication.

For a read operation on a data item in a block that is currently not local, it is necessary to communicate with remote sites to first acquire a read-only copy of that block and to change to read only the access rights to any writable copy if necessary before the read operation can complete. For a write operation to data in a block that is either not local or for which the local host has no write permission, all copies of the same block held at other sites must be invalidated before the write can proceed. The read-replication algorithm is consistent because a read access always returns the value of the most recent write to the same location.

In the read replication algorithm, DSM must keep track of the location of all the copies of data blocks. One way to do this is to have the owner node of a data block keep track of all the nodes that have a copy of the data block. Alternatively, a distributed linked list may be used to keep track of all the nodes that have a copy of the data block.

                     
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