Data ONTAP can report alignment problems with properly aligned LUNs. A certain amount of misaligned I/O occurs on properly aligned LUNs and can be harmless.
For proper performance, it is important that LUNs be correctly aligned with the underlying WAFL block boundaries. For physical hosts, selecting the correct ostype value when creating the LUN is usually sufficient. For virtualized server environments, an additional alignment step is sometimes needed.
However, the alignment detection features of Data ONTAP sometimes report misalignment for properly aligned LUNs because Data ONTAP detects misaligned I/O.
The presence of misaligned I/O does not necessary mean the LUN or datastore itself is misaligned. Certain types of applications, primarily databases, generate misaligned write I/O that does not harm performance.
The performance problem with misaligned I/O is that additional reads are required to complete a write. For example, a 4 KB block that straddles a WAFL block boundary requires an update to parts of two WAFL blocks. Before the misaligned write is committed to disk, two partial reads are required to retrieve the data which is not being changed and to allow Data ONTAP to complete the write. These additional reads increase the I/O load on the storage system and can increase latency as seen by the host system. An aligned 4 KB block requires only an overwrite.
Databases and other applications which incrementally update a log file with variable sized blocks sometimes trigger misalignment warnings. Although these applications sometimes perform a 4 KB misaligned write, the next write performed completes the partial block. As the sequential write continues, each new block written eliminates the need for any partial reads caused by prior write operations.
In general, misalignment warnings on LUNs or datastores can be disregarded as long you are confident that the LUN or datastore is properly provisioned and any partitioning table is correct.
If further investigation is required, technical support can run diagnostic commands that show detailed I/O alignment statistics to confirm the presence or absence of true LUN misalignment.