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Be prepared
The best way to prepare for scaling a storage infrastructure is to buy the best parts in the first place. Here are some questions you should ask vendors when considering SAN and SAN array products:
  • What's the supported fan-out ratio per port?
  • What's the maximum number of logical unit numbers that can be presented per port?
  • How many front-end host directors

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  • or adapters can be added?
  • How many back-end disk directors or adapters can be added?
  • How many disks are supported per adapter? What's the recommended average?
  • What's the average IOPS supported by the host port? (Some vendors may not publish this information.)
  • How many disks are supported in a RAID group? Does it vary by RAID level? What's the recommended number of disks in a RAID group for each RAID level?
  • Does replication or snapshots/clones cause a performance penalty on the production devices? If yes, to what degree?
  • Does the array support a mix of different drive types?
  • What sort of multiplatform support is available? Are there any issues with connecting the array to multiple platforms?

Scaling and applications
Capacity and scalability requirements come in all forms, but they're ultimately reduced to a single consideration: Do your applications function in a manner that suits the business? For example, the I/O profile changes resulting from adding more disk space vary considerably from application to application. A database growing in size (and I/O) may need more disk space to generate more logs or to create bigger indices. Once the additional space has been made available, the array's performance may be affected because the application now demands more I/O. This may be very different from an Internet-based application that requires more space only to store more content.

Effective scaling is based on the correct sizing of all the parameters involved. If you don't understand the requirements of your applications, you may impact the larger environment.

Understanding application requirements for your storage infrastructure is a part of the application lifecycle management process. As applications change, so do their requirements. The application management process starts during the initial design of the storage system. Understanding these parameters--and how they'll likely grow--is the key to avoiding being locked into a disk solution that doesn't scale with the apps it hosts.

This was first published in June 2006

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