5D where have my terabytes gone and why?

I’ve got a 5D with 5 4TB HD’s. That’s 20 TB. Now one drive is my insurance against a disk failure. Good idea. That’s why I bought the Drobo. That leaves 16 TB. However, the calculator and the Drobo Dashboard tell me I only have 14.48 TB. It seems that the Drobo needs about 10% of my disk space for its own purposes. I presume this is on top of the file management systems directory structure. I also presume it’s needed to make the Drobo and its data protection work, but 10% is a lot. Can anyone explain just that this is used for?

On top of the above, the Drobo warns me if I’m using more that 85% of my available space. What this means is that I must pay a penalty of some sort if I use more than 75% of my raw disk space = 12.31 TB out of 16. I understand that nearly full disks have performance issues related to file fragmentation and directory structure problems and this is not just for Drobo. However, I’d like to learn what kinds of performance problems I’ll have as I do beyond 85%. Is it a gradual change? Do things get a lot worse as I approach 95%? Until I bought my second 5D earlier this week I was at about 92% full.

nope, completely wrong, sorry

the disks you purchased will be 4,000,000,000,000 bytes

as im sure you know you know a kb is 1,024 bytes

a mb is 1,024 kb, which is 1,048,576 bytes (NOT 1,000,000 bytes)

by the time you get to talking in TB (1024x1024x1024x1024) … the 3 tb disk you buy… is 3,000,000,000,000 bytes, which is actually 2.7TB

and a 4,000,000,000,000 byte drive is actually just over 3.6TB !!! so 3.6TB x 4disks = 14.4TB

its nothing to do with drobo, its to do with drive manufacturers screwing you over by claiming that their drives are a certain capacity and counting it in decimal

incidentally the newer versions of mac OS x also now count in decimal… so they would tell you that your 4TB disk IS 4TB … but windows would tell you that its 3.6TB… when the drive has the exact same capacity (in terms of number of bytes) … they are just using different definitions of a TB

Yes, I had just worked that out for myself. A binary TB is 10% larger than a decimal TB. Sound like comparing metic tons and English tons. I’ve been working with computers since 1963 and Macs since 1985 and I just realized today that OSX 10.9.5 was reporting size in decimal rather than binary. That turned on the light for me.

Now can anyone explain what kind of degradation I’ll face when my Drobo goes over 85%?

hi read/writes will be slower, though im not too sure of the ratio of slowdown etc across the thresholds