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
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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:
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- 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
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