As high performing storage solutions based on block protocols (e.g. iSCSI, FC), SANs excel at optimizing block access. SANs work at a storage layer underneath the operating systems file system; usually NTFS when discussing Microsoft Windows®. That dictates that a SAN is unaware of “file” fragmentation and unable to solve this issue.
Fig 1.0: Diagram of Disk I/O as it travels from Operating System to SAN LUN.
With file fragmentation causing the host operating system to generate additional unnecessary disk I/Os (more overhead on CPU and RAM) performance suffers. In most cases the randomness of I/O requests, due to fragmentation and concurrent data requests, the blocks that make up the file will be physically scattered in uneven stripes across a SAN LUN/aggregate. This causes even greater degradation in performance.
Fig 1.1: Sample Windows Performance Monitor Report from fragmented SAN-attached NTFS volume.
Fortunately there are simple solutions to NTFS file system fragmentation; fragmentation prevention and defragmentation. Both approaches solve file fragmentation at the source, the local disk file system.
Learn more, read: SAN best practices .