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DLink FAQ => ShareCenter FAQs => Topic started 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 nonRAID 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 rounddown 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.
 Standard Configuration: 100% available storage
 JBOD: 100% available storage
 RAID0: 100% available storage (Assumes 2 HDDs of equal size)
 RAID5: 75% available storage (Assumes 4 HDDs all of equal size)
 RAID1: 50% available storage (Assumes 2 HDDs of equal size)
 RAID10: 50% available storage (Assumes 4 HDDs of equal size)
Furthermore, RAID arrays format HDDs based on the size of the smallest HDD in the array. Consider the following two Examples
 Example 1: RAID5 Array with 3 HDDs, each with 3TB capacity and 1 HDD with 1 TB capacity
 Raw storage before formatting: 10 TB
 Available storage space after formatting: 3TB
 30% available storage
 Example 2: RAID5 Array with 4 HDDs, each with 2.5TB capacity
 Raw storage before formatting: 10 TB
 Available storage space after formatting: 7.5 TB
 75% available storage
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 RAID5 Storage Capacity
The storage capacity of a RAID 5 array is equal to (N1) * (size of smallest HDD), where N is the number of HDDs in the array.