RAIDZ space calculations

[weecho]

I'm a little confused as to how to calculate volume size. As I play around on my final configuration which was going to be 8-4TB RAIDz2 stripped with 8-4TBRaidz2. Technically, all stripped I should get 32TB. With the configuration I laid out, I THOUGHT I should get 24TB+24TB for a total of 48TB.

But it looks like I'm actually getting 20TB. Why is this? How do I actually calculate available space on drives under RAIDz,RAIDz2 or RAIDz3 for a various number of drives? Is it because I'm not using the ideal 10 drive size for RAIDz2?

Thanks

 

[danb35]

Part of what's going is the difference between TB (10^12 bytes) and TiB (2^40 bytes). Hard drives are sold in TB, while FreeNAS and some other operating systems count in TiB. But that doesn't account for what you're seeing--it looks like you've just got 8 disks in there, rather than 16. Post the output of zpool status in code tags so we can see what's going on.

 

[weecho]

What is a TiB? Old school you had TB (Tera Byte) and Tb (Terabit). 8 bits in a byte. Why does Freenas use TiB?

I only have 8 of the 16 drives connected so far. I wanted to confirm things before I continue. Why does the system show 27TiB and 20TiB? Very confusing to me showing 2 different numbers.

Doing the conversion, 27TiB is 30.45TB. 19TiB is 21TB. My simple math, 8-4TB RAIDz2 gives me 2 used for parity and 6 for storage leaving me 6*4=24TB (21.8TiB). 8*4=32TB (29.1TiB).

I just answered my own question. I can now see the trees through the forest.

 

[jgreco]

TiB is a true terabyte, base 2 based. At some point, the morons in a hard drive manufacturer's marketing department realized they could exaggerate if they redefined "GB" to not-mean 2^30.

So a NAS that reports REAL GB or TB appears to be not giving users all the space they think they bought. But the real problem is that the hard drive manufacturers shortchanged them.

 

[Robert Smith]

IEC multipliers:
tera (T) 10^3^4 (Decimal: 1000000000000, or one trillion on the short scale)
tebi (Ti) 2^10^4 (Decimal: 1099511627776)

 

[danb35]

What is a TiB?

I told you in my post--2^40 bytes.