NetApp has issued a document explaining how best to configure its products that aims to addresss "common disinformation that generally surrounds these topics, such as how NetApp competitors and disk vendors state capacity information differently than NetApp and more."
The new "Storage Subsystem Configuration Guide" is billed as an update to 2005's “Aggregate and RAID Group Sizing Guide,” and includes a substantial section on "Marketed Capacity" that says "Competitors may inappropriately use marketed capacity to imply that they have an advantage over NetApp storage solutions."
The document goes on to offer a lengthy explanation of how NetApp calaculates disks' capacity and how this differs from the way drive manufacturers market their products. The key difference, the paper argues, is that drive manufacturers use a headline figure to market drives, but that once sector sizes and other factors are taken into account different vendors' drives that are marketed as having the same raw capacity will actually offer different usable capacities to arrays.
The document explains the problem in this way:
"Manufacturer’s physical capacity is important to understand for the following reasons:
- Individual disk drive manufacturers publish their own version of physical disk capacities that are different from what is seen in Data ONTAP®.
This type of physical capacity is based purely on the number of sectors present on the disk drive before it is used by any system. It is very common to see variations in the number of sectors on like-capacity disk drives from different manufacturers. Generally SATA disk drives use 512-byte sectors, whereas FC and SAS disk drives use 520-byte sectors."
The document also offers the following explanation for the usable disk capacity NetApp users can access:
"There are several additional factors that affect usable capacity when additional configuration, such as creating aggregates and flexible volumes, is applied to a storage configuration. Some of these factors include:
10% WAFL system reserve
5% aggregate Snapshot™ reserve
20% default volume Snapshot reserve"
NetApp does not name the competitors it believes may be spreading fear, uncertainty and doubt.
SearchStorage ANZ briefed storage analysts on the contents of the document, and their comments suggested it is an honest and frank explanation.
It would seem that NetApp is being quite upfront about their suggested default reserves," wrote one. "Customers need to understand that modern arrays do have quite an overhead," another commented in an email.
The document is available in NetApp's online communities.