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How much hard disk space should I actually use?

It was once a common practice to use only up to 80% of the total hard disk drive capacity for optimal performance -- beyond 80% of capacity usage and it was time to buy more disks. Is still a good formula to follow? If so, does it apply to different RAID configurations?
The answer to this question really depends on a variety of things:

Main things to consider:

  1. What is the growth rate of the data in this storage pool? What applications are using the storage pool, and how do they each use them? This may be a place to look at adding some SRM product to the mix to understand the exact usage patterns of the storage pool and then make some informed decisions about the layout of the data. Should the data be on separate disk pools based on growth or importance? This will also help you justify the costs associated with managing the storage environment.
  2. What type of disk subsystem are you using? Does it allow for storage growth on demand, or is it a direct-attached storage system, or JBOD, that requires the system to be shutdown for growth of the storage pool, this also relates to the volume manager being used if the solution is a blocks based solution, or NAS with a file system that is capable of growth on the back-end without interruption to the user's work.
  3. What is the layout of the data on the disks? Is it stripped, mirrored, or is it a single disk being accessed? The reason that this is important is that growing a RAID stripe correctly may require one to relay out the stripping to gain maximum space and performance capabilities, or add another set of disks to match the correct RAID group size.
  4. What is the file system fragmentation level like? Check the level of fragmentation and usage patterns of the file system. Sometimes you might find that the file system is severely fragmented and could use a good ole defrag.
  5. Migrate data/information that is of less value to another storage medium. Information Lifecycle Managemenet (ILM) is a buzzword that usually equates to a simple process of classifying data based on its value to the company. Is it old or stale data that isn't accessed on a regular basis, or is the data part of a project that is finished? These are cases where it may be appropriate to migrate the data using a product that is capable of managing the recall of the data and real-time as well as presenting a common interface to the user. Network Appliance works with a company called NuView to provide a product called Virtual File Manager, or VFM, which provides this capabilities as well as a host of other great capabilities, like file synchronization and a global namespace using Microsoft's own DFS.
There is no right answer to your question about a general formula for a percentage, rather the right answer is to understand the growth of the data, what it is composed of, the requirements for the storage (HA, replicated, etc.), correlated with the capabilities of your storage environment and then plan according.

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