During garbage collection, writes to objects being copied (relocated) are monitored (tracked) using a write barrier that uses a thread-local write barrier buffer. In the preferred embodiment, soft synchronization is used for reading the thread-local write barrier buffers. In response to detecting a write to an object, the object may be re-copied, the copying may be made to fail, the write may be propagated to another copy of the object, or, e.g., another node in a distributed system may be notified of the write.