Veritas File System (VxFS) supports an option to do lazy copy-on-write when a region of a file referred to by a shared extent is overwritten. A typical copy-on-write implementation involves reading the old data, allocating a new block, copying or writing the old data to the new block synchronously, and writing the new data to the new block. This results in a worst case possibility of one or more allocating transactions, followed by a read, followed by a synchronous write and another write that conforms to the I/O behavior requested for the overwrite. This sequence makes typical copy-on-write a costly operation. The VxFS lazy copy-on-write implementation does not copy the old data to the newly allocated block and hence does not have to read the old data either, as long as the new data covers the entire block. This behavior combined with delayed processing of shared extent accounting makes the lazy copy-on-write complete in times comparable to that of an allocating write. However, in the event of a server crash, when the server has not flushed the new data to the newly allocated blocks, the data seen on the overwritten region would be similar to what you would find in the case of an allocating write where the server has crashed before the data is flushed. This is not the default behavior and with the default behavior the data that you find in the overwritten region will be either the new data or the old data.