SOLVED Another newbie in town with a question

[kyletak]

Hey guys,
New to FreeNAS and I'm loving it so far. Got everything all setup on my new server that I have spent months lurking various forums to gather information.

List of Parts:
CPU - i34370
Board - SM Ultra ATX LGA 1150 X10SL7-F-O
Memory - 16GB DDR3 1600 ECC UDIMM (thinking of getting another 16GB)
Supply - SeaSonic 400W 80 PLUS Platinum Fanless
Air Movers - 4x 140mm Premium Quiet Quality PWM Fan
Case - Fractal Design Define R4

Lastly but most importantly as far as my question is concerned....

Storage - 8x 3TB WD Red

My question is thus. I have 8x 3TB drives totaling to an amount of 24TB raw storage space. I'm using a RaidZ2, which according to every forum post, FreeNAS document I have read, I should have # of drives minus 2 for total storage capacity. I'm sitting at what appears to be 15TB of storage space.

Now don't get me wrong, I'm happy with that! I just thought going into this build that I would have closer to 18TB. I know there is a serious lapse of judgement going on somewhere (most certainly on my end). I'm just wondering what it is I am overlooking to have misplaced 3TB of potential storage space. Please enlighten me, I have been hesitant put much data on so that in the event that I did set something up wrong (highly likely) I can simply start from square one. Thank you in advance!!


Please see pictures below for where my question comes..

 

[demon]

Notice that your raw capacity for the zpool (the top line in the second screenshot, the first one that says "My_Volume") says 21.7 TiB? That's because TB != TiB. Drive manufacturers quote sizes in TB (or GB with smaller drives). 1 TB is 10^12 bytes, whereas 1 TiB is 2^40 bytes. (Also, keep in mind that's unformatted capacity). Many OSes will mention MB, GB or TB, but typically the math they're using is multiples of 2^10, not 10^3, so they're actually using MiB, GiB or TiB.

The following bc input/output shows you what I mean. This is the math for your 18 TB:

Code:

18 * (10 ^ 12) / (2 ^ 40)
16.370

So you'd have about 16.3 TiB of unformatted capacity after factoring out RAIDZ2 overhead. There's a fair amount going to filesystem overhead, of course. Anyway, it's a bit of math, but basically 15-16 TiB net total capacity, after considering the TB/TiB math, and RAIDZ2 overhead, and filesystem overhead and such, is about right. I have about 10.5 TiB net capacity on my RAIDZ2, which has 6 WD30EFRX (WD Red 3 TB) drives in it, so that's not really out of line.

That said, I would turn off compression before you load up a bunch of data, but that may just be me. I'm sure someone will tell me I'm wrong. :)

 

[SweetAndLow]

Notice that your raw capacity for the zpool (the top line in the second screenshot, the first one that says "My_Volume") says 21.7 TiB? That's because TB != TiB. Drive manufacturers quote sizes in TB (or GB with smaller drives). 1 TB is 10^12 bytes, whereas 1 TiB is 2^40 bytes. (Also, keep in mind that's unformatted capacity). Many OSes will mention MB, GB or TB, but typically the math they're using is multiples of 2^10, not 10^3, so they're actually using MiB, GiB or TiB.

The following bc input/output shows you what I mean. This is the math for your 18 TB:

Code:

18 * (10 ^ 12) / (2 ^ 40)
16.370

So you'd have about 16.3 TiB of unformatted capacity after factoring out RAIDZ2 overhead. There's a fair amount going to filesystem overhead, of course. Anyway, it's a bit of math, but basically 15-16 TiB net total capacity, after considering the TB/TiB math, and RAIDZ2 overhead, and filesystem overhead and such, is about right. I have about 10.5 TiB net capacity on my RAIDZ2, which has 6 WD30EFRX (WD Red 3 TB) drives in it, so that's not really out of line.

That said, I would turn off compression before you load up a bunch of data, but that may just be me. I'm sure someone will tell me I'm wrong. :)

great explanation about TB vs TiB!

and you probably shouldn't turn off compression there is a reason it is on by default. There are very few reason to turn it off and there is little to no cpu performance penalty and it will drastically increase your read and write speeds because you will read/write less off/on disk and get more data than you read/write.

 

[Ericloewe]

I wasn't planning on using compression, but it was changed into being the default shortly before I built my system. I just let it be and have had no complaints.

 

[kyletak]

Thank you for the replies! This makes a lot more sense now. I knew I was missing something... I really appreciate the help!