Bookkeeping

One major change required of the file system is how it handles a fundamental problem with range writes which we refer to as delayed address notification. Specifically, only as each write completes does the file system know the target address of the write. The file system cares about this result because it is in charge of bookkeeping, and must record the address in a pertinent structure (e.g., an inode).

Figure: File Create Time (Empty File System). The figure plots the average write time during a file create benchmark. The benchmark creates 1000 4-KB files in the same directory. Range writes are either used or not, and the files are either created by a single process or multiple processes. The y-axis plots the average write time of each write across the 1000 data block writes that occur.

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In general, this may force two alterations in file system allocation. First, the file system must carefully track outstanding requests to a given region of disk, in order to avoid sending writes to a full region. However, this modification should not be substantial.

Second, delayed notification forces an ordering on file systems, in that the block pointed to must be written before the block containing the pointer. Although reminiscent of soft updates [12], this ordering should be easier to implement, because the file system will likely not employ range writes for all structures, as we discuss below.