Embodiments of the invention relate to block layout and block allocation in a file system to support write transactions. Regions in a cluster file system are defined to support a block allocation, include both write affinity and wide striping region. An allocation map is maintained to define and support the block allocation regions. For each write transaction, a copy of data is written to both regions, and in the event there is a shortage of blocks in one of the regions, at least one block in the other region is dynamically converted to support the write transaction.