Allocation and Data Placement Using Virtual Contiguity

We describe an allocation and data placement technique called virtual contiguity that keeps the blocks of a file “near” each other so that a file system can read many blocks of a file at the same time with a single disk head movement. This technique avoids the disk seek penalties incurred when reading discontiguous block allocations, without requiring allocations to be strictly contiguous. At the same time, virtual contiguity provides for the finegrained allocation and writing of data required to support memory-mapped I/O and efficiently perform copy-onwrite for file systems that snap-shot data. Preliminary experimental results show that virtual contiguity groups data effectively and reduces the number of disk seeks required to read a file.