Assessing the cost/benefit ratio for storage quotas

Reader Paul Hilton provides several useful considerations and scenarios for calculating the costs/benefits of storage quotas.

Editor's Comments: After reading Bruce Backa's recent Storage Management tip, "Why have storage policies," this...

SearchStorage subscriber had more to share on how best to assess the risks and advantages of implementing quotas in your end-user organizations. We found his analyses and examples useful, and thought other members of our audience could also benefit. What's been your experience with quotas? Let us know. E-mail us.

Quotas are just one of many storage management techniques available for use in meeting service level objectives and ensuring sufficient storage for all users. Most of these techniques can be policy based. The techniques include:

Some quota considerations

There are a number of items to consider prior to implementing quotas. Here are a few:

Storage management costs associated with quotas. There is a cost to managing storage. This cost should be considered in any storage solution. Different storage management techniques place costs in different areas of an organization. Quotas place storage management costs primarily in user areas because quotas force users to constantly look at their data and delete duplicate or redundant data. Other storage management techniques create higher costs in storage management or systems administration groups. Organizations need to determine where storage management can be optimally accomplished.

Quota size. This is a difficult issue. It needs to be considered before quotas are implemented and it will probably be revisited many times after quotas are implemented. Does one size fit all? Probably not, since different types of users usually need different quantities of space to do their job. Different parts of the organization probably have different needs and are able to fund more or less storage for their users.

Quota size impacts the justification for quotas. The size needs to be small enough that it forces users to clean up. If the quota size is too large and most users don't need to worry about it, why bother having quotas in the first place? If the quota size is too small, there will be too many requests for quota increases and much dissatisfaction in the user community.

Quota applicability. Quotas are most commonly used with e-mail systems and file servers. Most organizations do not put their databases (production or development), web environments, or other critical customer applications under quotas. When looking for quota benefits, these environments and their growth rates need to be excluded from the equation.

Clean-up alternatives to quotas employed by end users. The concept behind quotas is that of forcing users to better manage their server-based storage. I've seen users implement a number of alternatives to get around quotas, such as:

  • Printing email documents and other documents that they want to keep. They then keep the wanted documents in drawers and filing cabinets. I think electronic storage technology was long ago less expensive than printing and filing techniques.
  • Moving data to their entity's PCs. If the data can't be on shared, entity-managed storage, then users can put it on their hard drives. Where the entity backs-up these PCs, there is probably little difference in the management cost between server-based storage costs and PC-based storage costs. Where the entity does not backup PCs, there is a danger that potentially valuable data could be lost or stolen.
  • Move needed data to removable media. This probably comes out of office supply or local budgets but is more expensive than server storage. It will cost the entity more.

All these alternatives utilize the time of people who should be doing their assigned job for the entity.

  • Hierarchical Storage Management-- moves unused or large files to less expensive storage devices. Leaves a stub or indicator so that the file can still be accessed. The file moves back to the original storage if it is accessed. Some HSM packages can be set up to work on thresholds so that there is always space for new file creation. Such HSM packages use policies to control their actions.
  • Storage Pooling -- can separate different types of users and data. Ensures that space will be available for critical applications.
  • Storage Resource Management software -- can identify inappropriate or over-use of storage resources. Specific users can then be targeted for education on the use of storage resources. Large files can be identified for special clean-up actions. Here too, policies can trigger the education or clean-up activities.
  • Chargeback or cost awareness processes -- charge groups and users for the use of storage resources. It provides a method for IT organizations to recover their costs.
  • Selective file cleanup -- deletes certain types of files such as temporary files, or deletes files of know types that are not allowed on a system such as game files.
  • Archiving -- moves files to a less expensive storage media and deletes the original file.
  • Quotas -- which the article by Mr. Backa addresses.

Considerations for implementing quotas

Every alternative provides solutions, but also has its own problems. Since the original article dealt with quotas, what are some of the considerations around quotas that storage administrators might want to think about before they commit to this alternative?

  • Storage management costs associated with quotas
  • Quota sizes
  • Quota applicability
  • Clean-up alternatives to quotas employed by end users
(For more information on these types of quota considerations, see the sidebar to this story.) Another very important consideration involves calculating the potential costs or benefits to your organization for implementing quotas. This area is discussed in more detail below.

Identifying the costs and benefits involved in implementing quotas

There are costs involved with any storage management alternative, just as there are also costs involved with a status quo approach. Some of the costs involved with quotas include:

  • User cost. Quotas force users to clean up files and manage a fixed amount of storage space. Users are employed to perform services for an organization. The time spent managing storage space is time that they are not performing their intended function. There is a cost to the organization for this time. There may also be a cost to recovering or re-creating data that a lack of storage space has forced users to delete. This is much harder to measure or estimate.
  • Administrative cost. There is a cost to administering quotas. There are always exceptions and requests for more space. Usage may have to be evaluated and quota sizes changed.
  • Software cost. Some e-mail systems include quota management. File servers generally need to have additional software and maintenance purchased.
  • Education and customer relations. The article stresses the need for user education and suggests a number of alternatives. Quotas probably have a greater requirement for these activities than do other storage management techniques.

On the benefits side of the equation, quotas provide a mechanism to limit growth in portions of the storage subsystem. The savings accrue between the growth prior to quota implementation and the growth after quota implementation.

There may also be some administrative savings achieved through no longer having to do manual management of storage, reduced call-outs for out-of-space conditions or easier chargeback.

How to calculate quota cost/benefit ratios

When trying to decide whether or not to implement a storage quota system, you might want to consider calculating the potential cost and benefits involved. The examples shown below are reasonably quick calculations to provide guidance. Every situation will be different.

Cost example

In reality, the costs don't scale directly with the storage added. Adding a tape drive or media to a tape library is less expensive than adding a new tape library. Utilizing part of another storage administrator is less expensive that adding a full storage administrator.

This example assumes a target environment of 200 users and 2T Bytes of disk storage.

Cost Cost Estimate
User Cleanup Cost - 200 users x .25 hours (15 minutes) per week x 52 weeks x $50 per hour $130,000
Software cost (Quota software) $5,000
Education/ User relations $4,000
Administration - 1 hour per day x 20 days per month x 12 months per year x $50 per hour $12,000

In this example, the total yearly cost of adding quotas to this organization would be $151,000.

Benefit example:

The benefits example assumes the savings to the organization will come from the avoidance of purchasing additional disk and the management costs associated with managing the disk.

To calculate the benefit side, a burdened cost of storage could be calculated. This burdened cost of storage should include the yearly people cost to manage the target environment, plus the yearly capital cost of the storage devices (disk, tape, optical), plus the yearly cost of storage software, plus the yearly cost of floor space and power for the target environment.

The burdened-cost of storage would then be divided by the number of gigabytes in the target environment. This would result in a burdened cost per gigabyte per year. The projected savings, in gigabytes, from the quota system would be multiplied by the burdened cost per gigabyte per year to determine the benefits.

Extending the costs example, let's assume that a particular target system serves 200 users, with 2T Bytes of disk storage, an average of 10G Bytes per user. The system managers are considering implementing a quota system and hope to avoid an increase of 2G Bytes per user, or 400G Bytes of total upgrade. The inputs to the burdened storage cost are as follows:

Burdened Storage Cost Cost Estimate
One Storage administrator - 8 hours/day x 20 days/month x 12 months = 1920 hours x $50 per hour burdened rate $96,000
2T Bytes of disk storage with a 3-year life purchased at $0.10 per M Byte or $100 per G Byte (reasonably expensive disk = (2000 x $100)/3 years ~$67,000
Automated Tape Library and drives with 3-year life ($200,000 / 3 years) ~$67,000
Storage software (backup/restore, utilities etc) $30,000
Floor space, including power for disk and tape = 30 sq ft @ $100 per foot $3,000

The yearly total would be $263,000 per year. The burdened cost of storage per year is $263,000 divided by 2000G Bytes or about $132 per G Byte per year.

The yearly cost of adding 400G Bytes for the purposes of this example is 400G Bytes x $132 per G Byte per year (or $52,800).

Evaluating quota costs and benefits together to make a decision

In the above two examples, the benefit of not adding the 400G Bytes to the organization is $52,800. The cost of the quota system to the organization is $151,000. In this case, the costs of implementing a quota system exceed the benefits by almost $100,000.

If a decision was based on organization-wide cost/benefits, a quota system could not be justified.

About the author: Paul Hilton is an independent consultant who has worked with mainframe, UNIX and Windows-based storage systems for the past 25 years. During that time, he's worked for customers and service providers as a storage manager, capacity planner, architect and consultant and has both performed pre/post sales support and product management functions. In the early 90's, Hilton was also involved with GUIDE, an IBM users group, where he participated in a number of storage-related projects. He can be reached via e-mail.

This was first published in March 2002

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