Write Anywhere File Layout (Walf) - IBM N Series Hardware Manual

11.1 Write Anywhere File Layout (WALF)

Write Anywhere File Layout (WAFL) is the N series file system. At the core of Data ONTAP is
WAFL, N series proprietary software that manages the placement and protection of storage
data. Integrated with WAFL is N series RAID technology, which includes both single and
double parity disk protection. N series RAID is proprietary and fully integrated with the data
management and placement layers, allowing efficient data placement and high-performance
data paths.
WAFL has these core features:
WAFL is highly data aware, and enables the storage system to determine the most
efficient data placement on disk, as shown in Figure 11-1.
Data is intelligently written in batches to available free space in the aggregate without
changing existing blocks.
The aggregate can reclaim free blocks from one flexible volume (FlexVol volume) for
allocation to another.
Data objects can be accessed through NFS, CIFS, FC, FCoE, or iSCSI protocols.
Figure 11-1 WAFL
WAFL also includes the necessary file and directory mechanisms to support file-based
storage, and the read and write mechanisms to support block storage or LUNs.
Notice that the protocol access layer is above the data placement layer of WAFL. This layer
allows all of the data to be effectively managed on disk independently of how it is accessed by
the host. This level of storage virtualization offers significant advantages over other
architectures that have tight association between the network protocol and data.
To improve performance, WAFL attempts to avoid the disk head writing data and then moving
to a special portion of the disk to update the inodes. The inodes contain the metadata. This
movement across the physical disk medium increases the write time. Head seeks happen
quickly, but on server-class systems you have thousands of disk accesses going on per
second. This additional time adds up quickly, and greatly affects the performance of the
system, particularly on write operations. WAFL does not have that handicap, and writes the
metadata in line with the rest of the data. Write anywhere refers to the file system's capability
to write any class of data at any location on the disk.
The basic goal of WAFL is to write to the first best available location. "First" is the closest
available block. "Best" is the same address block on all disks, that is, a complete stripe. The
first best available is always going to be a complete stripe across an entire RAID group that
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IBM System Storage N series Hardware Guide