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Don't overlook hosts
In addition to the network, how hosts use available storage will have a direct affect on any storage scaling efforts. The way that hosts access storage will largely dictate how applications running on them perform. Some of the host-related design issues that can have a bearing on storage scaling include:

  • The number of I/O paths (HBAs) to the storage ports and the fan-in ratio per path
  • The type of multipathing software (and load-balancing algorithm) used
  • Volume management (volume layout) and file system
  • SCSI stack tuning

The way in which an application's objects use the storage resources assigned to them can also make a huge difference. For example, putting log and data files on separate file systems (and spindles) for an Oracle database is common practice. All of the above parameters also change based on how powerful the host is in terms of its CPU, memory and backplane. Vendors' engineering documents should be checked to see how much I/O a host is capable of driving through each HBA.

Planning is essential
Detailed planning and design before you invest in a storage infrastructure

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will make it more likely that future scaling projects will be successful. It's critical to understand and be able to quantify the scalability limits of your storage infrastructure--storage arrays as well as the network.

Adding capacity to an environment may appear to be a linear and superficial task, but the consequences can be considerable. A storage array is complex, and its components--disk controllers, cache, processors and front-end controllers--all need to function within vendor-recommended ranges for the array to perform at its best.

 

This was first published in June 2006

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