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The truth about utilization
"What's normal?" This is one of the most common questions I'm asked about storage utilization. Of course, normal isn't necessarily optimal, but it's a valid question, if only to provide the context for improvements. I normally see per-system file system utilization between 30% and 40%, but I've never been sure that this was true. Although I've heard the same average from many sources, I've never found a real source with concrete data. So, I pulled together as much real-world data as I could get to find out the truth.
I collected statistical data on more than 750 production AIX, HP-UX, Solaris, and Windows systems from more than a dozen large and small corporations. This data included the size and occupancy of file systems, the number and size of physical disks, and server hardware details. Of course, these numbers came from companies concerned enough about storage to bring in an external consultant to analyze their usage patterns.
And, lest you think these were lesser machines, the average system in the data set had six CPUs, 3.3GB of RAM, five FC ports and 325GB of storage. Plus, the file system utilization of this data set was actually much higher than we had expected. With this information, I can provide a more concrete answer to some of the questions I get, such as:
- What's the average file system utilization for a server? Across all four operating systems, the average file system utilization was 39%. Solaris and Windows tied for the lowest average at 27%, while HP boasted a 53% average with AIX leading the pack at an astonishing 60%.
- How much storage is left unused in volume managers? The average system that used a volume manager had 77GB of storage available in the volume manager, but unassigned to a logical volume. This is about 36% of the total storage contained in volume groups.
- How much storage is typically left unused in storage arrays? Of the 20 or so
arrays we looked at, we identified about 25% of the storage that wasn't assigned to a host.
The upshot of these measurements is this: The average site assigned just 75% of their storage to hosts, left 36% of that unavailable to applications and only used 39% of what was usable. So a typical host might have 500GB of external storage, 375GB in volume groups, 240GB in file systems and just 93GB used. And as mentioned earlier, these were production hosts at sites proactive enough to ask for a storage utilization analysis.
- Why the differences by operating systems? Windows administrators typically format their available storage on setup and leave it empty until it's needed. HP and AIX administrators have a history of using Volume Managers to provision storage only as needed. Surprisingly, no Windows host in our sample used a volume manager, while no AIX or HP storage was configured without one. Solaris systems resided somewhere in the middle: About 25% of Solaris storage was presented without a volume manager--even on systems using one for other file systems--with no visible storage left untouched.
- How do your utilization metrics compare? Let's expand this data set beyond the baseline established here. Write me at firstname.lastname@example.org, and I'll help you collect and summarize the utilization data for your environment. In a few months, I'll update these numbers to reflect the wider world of Storage readers, and we'll all achieve a higher level of understanding on this critical issue.
This was first published in April 2003