System and method for log-structured garbage collection are provided. In particular, some embodiments use write pointer to read pointer offsets to enable reclamation of space within a log-structured storage medium (e.g., sequential forward only write mechanisms such as SSD, Tape, Shingled Drives, Flash Drives, etc.). These techniques allow the garbage collection system to reclaim space without copying data from one storage medium to another. Instead of copying the data, various embodiments reset the write and read pointers. In addition, different retention policies can be easily enforced while allowing for efficient garbage collection. For example, in a backup storage, each log-structured media can be designated with a retention time and only allow workloads with the specified retention time to be stored. As a result, the garbage collection incurs only read pointer movements.