I currently have a HGST 4TB NAS HDD as my parity drive. The other drives in my array are 2TB or less.

Here are my questions:

1. My HGST 4TB NAS drive has a formatted capacity less than 4TB, its actually 3.91TB. If I want to add another brand of drive to my array, say a Seagate 4TB that has an actual formatted capacity larger than my parity drive (i.e. >3.91TB) will this cause an issue as the parity drive needs to be the largest drive in the system?

2. I want to purchase another parity drive in preparation of the upcoming 6.2 release so that I can have a second parity drive. As Lime Tech suggests in their wiki, it is a good idea to have drives of different types to reduce the risk of a drive specific failure issue or otherwise down the road. If I get a seagate or other brand 4TB for my second parity drive and it has a less than or greater than actual formatted capacity other than 3.91TB like my HGST, will that create an issue?

Thank you for the help.

Typically all 4TB drives will have the same useable space unless you have HPA (Host Protected Area) on one of them.

 

If you go to the Main page and click on your 4TB drive and go to the Identity section for that drive, what does it say for User capacity? Mine says

User capacity:	4,000,787,030,016 bytes [4.00 TB]

I was looking at the partition capacity and not the user capacity. My number is the same as yours. Thank you!

You need to compare in bytes, not in TB. Because your 3.91 TB is the base 2 number typically used by OS e.g. Windows. 1 TB is which case is 1024GB.

 

The 4TB quoted by manufacturer is base 10. So 1TB is 1000GB.

You need to compare in bytes, not in TB. Because your 3.91 TB is the base 2 number typically used by OS e.g. Windows. 1 TB is which case is 1024GB.

 

The 4TB quoted by manufacturer is base 10. So 1TB is 1000GB.

 

I understand that I was simply rounding the number for simplicity sake. 3,907,018,532 KB

I understand that I was simply rounding the number for simplicity sake. 3,907,018,532 KB

That's worse, you are mixing it all up which will make things even more confusing.

 

So there are 2 ways to intepret the kilo - mega - giga - tera.

 

There's the standard metric way. kilo = 10^3, mega = 10^6, giga = 10^9, tera = 10^12.

So 4,000,787,030,016 Bytes = 4.00... * 10^12 = rounded to 4 terabytes (TB) or 4000 gigabytes (GB).

 

Then there's the computer way (which is commonly used in various OS). kilo = 2^10 (=1024), mega = 2^20 (=1024^2), giga = 2^30 (=1024^3), tera = 2^40 (=1024^4).

So 4,000,787,030,016 Bytes = 4,000,787,030,016 / 1024 KB = 3,907,018,532 KB <-- what you said.

 

However, 3,907,018,532 KB is NOT 3.91 TB.

3,907,018,532 KB = 3,907,018,532 KB / 1024^3 TB = 3,907,018,532 KB / 1,073,741,824 = 3.64 TB

 

I know it sounds pedantic but mixing it up is kinda similar to how some satellites failed to launch some years back because American used inches and European used cm.

 

That's why I meant you need to compare in bytes. One the K/M/G/T comes into play, things will get confusing.

 

And not just that.

For example, depending on which Western Digital Red version you have, it can be 2,000,421,444,608 bytes or 2,000,398,934,016 bytes (or even more variations that I do not know of).

So both sizes are "2TB". But if you put the latter as a parity, you won't be able to add the former on the array.

 

 

 

 

I understand what you are saying I just wanted to get the question out there and get a response before i bought a drive so my rushing was the cause of the mixup. Back to the original question though. I do understand that most 4TB drives are the same size (4,000,787,030,016) but what if there are different? You explained that for the array i could not add even the slightest larger of a drive, but what about for a second parity once that becomes available?

 

FWIW, the two parity disks don't have to be the same size, but neither can be smaller than the largest data disk.

I've been doing this stuff for well over 50 years, and it never ceases to amaze me that folks STILL don't fully recognize the difference between "computer-ese" and "disk-drive-maker-ese" in measuring disk capacities.

 

Nevertheless, you still see posts on many forums about folks who buy an xTB drive and complain that it doesn't have the full capacity.  e.g. a 4TB drive will show a 3.63TB capacity in Windows (but doing a "Properties" on the drive will show well over 4,000,000,000,000 bytes).

 

In any event, the only thing that matters vis-à-vis using another drive is that it have at least as many bytes as your largest data drive (as already mentioned).  FWIW, MOST drives of the same stated capacity will have exactly the same # of bytes -- THAT has at least been very well standardized in the industry.    There are a few exceptions -- notably in drives that sold as external units in USB enclosures, which sometimes have small HPA's that slightly reduce the # of available bytes ... not by enough to matter unless you're "shucking" drives from these to use in UnRAID and try to use it in a manner where the size matters (e.g. as parity).

 

As has already been explained above, the fundamental difference is that in "computer-ese" 1Kb = 1024 bytes;  in "disk-drive-maker-ese" 1Kb - 1000 bytes.    And it just grows from there ... e.g. 1Mb = 1024 x 1024 vs. 1000 x 1000; etc.