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D-Link FAQ => ShareCenter FAQs => Topic started by: JavaLawyer on May 02, 2012, 05:31:44 AM

Title: DNS ShareCenter - Total Usable Storage Space
Post by: JavaLawyer on May 02, 2012, 05:31:44 AM
The amount of usable storage space on a DNS ShareCenter is primarily dependent on the following factors: number of installed HDDs; size of each HDD; HDD formatting overhead; and HDD configuration (Standard, RAID, JBOD, etc).

HDD Size, Marketed versus Actual

The actual available usable space for a non-RAID formatted HDD is an artifact of how the HDD industry defines what a TB really is. It's all about the math. From a computer science perspective (and in engineering terms), 1 TB = 1024 GB, 1 GB = 1024 MB, and 1 MB = 1024 KB.

Industry standard practice (for HDD manufacturers) is to round-down and call 1 TB = 1000 GB, 1 GB = 1000 MB, and 1 MB = 1000 KB. This simplified math results in a compounded storage loss of roughly 2.3% for each level of conversion as you scale up from the KB to TB range.

For each advertised TB of storage, roughly 70 GB is lost due to this industry standard definition.  A HDD advertised as 3 TB, will therefore lose roughly 210 GB of space, resulting in a HDD with a usable storage capacity of approximately 2.8 TB or 2790 GB. The actual space is further diminished after factoring in the overhead for indexing/addressing and reserved memory to compensate for bad blocks.

In all, a HDD configured in Standard Mode will have a usable storage space roughly 93% of the HDD's advertised capacity (or 7% loss).

The difference between advertised and usable storage was a sticking point around 20 years ago when storage was very expensive and consumers felt cheated by this creative marketing.

Lost Storage Space Due to Overhead

As described above, the actual space is somewhat diminished after factoring in the overhead for indexing/addressing and reserved memory to compensate for bad blocks.

HDD Configuration

HDD configuration often has the greatest impact on usable storage space. These calculations do not consider space reserved for redundancy as "usable" storage space. Depending on the HDD configuration, and under ideal circumstances, the amount of usable storage can vary from 100% to as low as 50% of the usable HDD space. Other variables, such as disparate HDD sizes, can significantly reduce this percentage even further.

Furthermore, RAID arrays format HDDs based on the size of the smallest HDD in the array. Consider the following two Examples

Therefore, using HDDs of the same size for RAID is critical to maximizing the total percentage of available storage space (although there are methods to recover unused space using more complex formatting techniques).

Calculating RAID-5 Storage Capacity

The storage capacity of a RAID 5 array is equal to (N-1) * (size of smallest HDD), where N is the number of HDDs in the array.