Symptoms
Veritas InfoScale Storage Foundation for Windows before version 8.0 may exhibit a higher number of misaligned I/Os on PowerMax devices.
Cause
The default offset for InfoScale Storage foundation for Windows (SFW) 8.0 and before is "64" for a "Symmetrix" type disk. A Symmetrix type disk includes PowerMax arrays.The volume offset in the product (SFW) is set in blocks. For a 512-byte FBA disk, the 64 block settings results in an offset of 32 KiB. In the history of the array (Symmetrix > DMX > VMAX > PowerMax), internal cache block size has changed twice, now being at 128 KiB.The difference between the offset of the volume on the disk being at 32 KiB vs. the cache slot size of the array being 128 KiB on a PowerMax sufficiently explains a higher number of misaligned I/Os seen on the hosts using SFW.A misaligned I/O is a terminology that Dell uses to describe a situation where a data entity that is written to disk, blocks more array cache slots, than the entity size would formally require. However, in general, it follows from that definition, that a misalignment is an expected behavior. It can happen at times, and despite its unlucky name, it is not a functional error as such.What may cause an effect, is the ratio between misaligned I/Os and the overall number of I/Os. If that ratio is mostly pointing towards misaligned I/Os, we have occasionally seen performance impact in the past, due to increased locking activity in the array cache elements (slots).To proactively prevent this locking overhead, the data section on the disk, that is the volume, should start at a 128 KiB boundary from the beginning of the physical disk.
Resolution
SFW can align the volume that is based on the storage class detected. To mitigate any potential risk of this having an impact, aligning the SFW volumes at 256 blocks instead of 64 blocks aligns them at exactly at a 128KiB boundary.The default setting can be changed in Veritas Enterprise Administrator and is considered when a new volume is created.
