Yoh-Dah XRaid 1-2 space calculator

The rule of thumb that most of us use is that about 7% of the "nominal" Gigabyte disk volume is lost to the 1000/1024 conversion and overhead, which we generally just call overhead. The 1000/1024 conversion is needed because of the fact that when a disk manufacturer lists the capacity of a hard drive 1K Bytes=1000 bytes, but in reality because of the binary calculation, 1K bytes=1024 bytes. On a Terrabyte disk, when calculating terrabytes, it is about 9% because there is one additional 1000/1024 calculation. (1 for Kilobyte volumes, 2 for Megabyte volumes, 3 for Gigabyte volumes and 4 for Terrabyte volumes).

Thus a 500 GB drive will yield about 465GB but 4GB is used for the RAID overhead so it will be reported as 461GB allocated. A 1TB drive will yield about 931GB less the 4GB so the volume will be reported as around 927GB allocated. Some specific drives may be slightly more or less depending on the make and model.

For most drives those calculation will be very close. My 1TB Seagates all report 931GB with 927GB allocated to the array. My Hitachi 3TB drives all report 2794GB with 2790GB allocated to the array.

Keep in mind that on the sparc based ReadyNAS (current models the Duo and NV+), the amount allocated from each drive will match the smallest drive in the array. Thus if you have a 500GB drive and 3x2TB drives, your volume will only be 1.3TB (461GB x 3). In the array for all Raid levels except 0, one drive is dedicated to being the parity/mirror disk. On the x-86 based models with X-RAID2 with four or more drives and single redundancy, a second layer will be built in the array once it has redundancy. Thus my NVX array with 2x1TB and 2x3TB drives yields a volume of 4620 reported in Frontview (added up 2790+927+927=4645 but you have to allow space for the operating system and 1x3TB drive provides the redundancy). The front panel reports 4.5TB (4620GB/1024=4.511TB).

It can get complex. If you have a specific model in mind and a specific disk arrangement in mind, list it and we will estimate it for you.

I sorta get it but it's still a little confusing. I just got a Pro 6 and I stuck 6x Seagate ST33000650NS 3TB drives in and each drive is showing 2794GB. The total space is showing 13TB. Does this look right? I was expecting somewhere closer to 15TG because I thought 1 drive is used for redundancy which should give me (aprox 3TB X 5= arpox 15TB)

The 2794GB per drive is the correct number (3000x.931). Your volume would then be 2790 x 5 drives (4GB for RAID overhead) = 13,950. From that you wold subtract around 25GB for the operating system.

The exact number should be shown on the bottom line of the Frontview home page. It should say: "Volume C: Online, X-RAID2, 6 Disks, x% of 13xxxGB used".

Great thanks!! It just says 13TB on the frontview. I'm guessing it's just rounding.

It may be programmed to go to TB once it gets to 5 digits if it was using GB. And if TB, it may just use two digits. Since none of mine have gotten that high, I can't say for sure.

Thanks again for clearing this up.