3 minute readRaw Capacity vs Usable Capacity

Wondering how much space you have remaining on your storage infrastructure? There is a big difference between raw capacity and usable capacity but we’re going to clear it up by unpacking what these terms mean, and how they are used. 

Let’s start with the purchase of thirty (30) new 2TB drives for our storage array. 30 x 2TB = 60TB right? Well, not really. At most, we’d be redlining our equipment at 80% or 48TB. Typically the sweet spot is only around 65% or near 40TB.  Sure, there is 60TB of raw capacity but we can’t use all of it, and here’s why.

RAW CAPACITY VS. USABLE CAPACITY

Hard disks are horribly inefficient because the overhead to manage them eats into the available space. The manufacturer takes a chunk of space after the disk is formatted. The RAID configuration reserves a bit more. The type of drive and type of RAID configuration in our pooling structure will play a big role in determining the recommended and maximum usable capacity thresholds. And that could put a damper on expectations. 

When we exceed these thresholds, our equipment defaults to survival mode and everything slows down as it attempts to preserve our data until we add more physical storage to its overall capacity. When we are monitoring our usage properly and set milestone triggers to keep us aware of how fast we are using the available space, we can proactively upgrade the right amount of raw capacity with the expectation of only ever using 65% of it. But if we aren’t paying attention, we’ll run out of space and our systems will limp along, at risk of failure, until we fast-track an unexpected purchase. In that case, we recommend you talk with an experienced storage engineer who can help you plan ahead.  

We can also allocate capacity to host specific applications. This ensures we’ve accounted for the need and are able to more accurately budget usable capacity. Allocated capacity is considered used even if not in the moment because it’s been set aside for when the application it’s dedicated to needs it. 

When purchasing and managing our storage infrastructure, it’s important to know in advance what our usable thresholds are, and the variables that dictate them. Misinterpreting raw capacity as usable capacity or trying to hedge our thresholds will cause serious operational complications. 

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