A modular block allocator receives a cleaner message requesting dirty buffers associated with an inode be cleaned. The modular block allocator provides at least one bucket cache comprising a plurality of buckets, wherein each bucket represents a plurality of free data blocks. The dirty buffers are cleaned by allocating the data blocks of one of the buckets to the dirty buffers. The allocated data blocks are mapped to a stripe set and when the stripe set is full, the stripe set is sent to a storage system. In one embodiment of the invention, a modular block allocator includes a front end module and a back end module communicating with each other via an application programming interface (API). The front end module contains write allocation policies that define how blocks are laid out on disk. The back end module creates data structures for execution of the policies.