Varying hard drive capacities and RAID

Question

Hard drives with equal capacity specs are not really equal in size, e.g. I have two 4TB disks with the following capacities:

Disk 1: 4,000,752,599,040 bytes
Disk 2: 4,000,786,153,472 bytes

Afaik a 4 TB drive has at least 4,000,000,000,000 bytes.

I read that one should make a RAID partition 100MB smaller than the disks full capacity to ensure that future disks will still be functioning in the same RAID even if they are slightly smaller.

I also read that someone had 1GB more space available than specified (a difference of more than 100MB compared to my drives).

How do you handle those situations, do you ensure that a RAID partition is never larger than the hard drive capacity specification?

100MB made sense back when drive capacity was measured in tens of gigabytes. — Ignacio Vazquez-Abrams, Nov 04 '17 at 16:24

score 1 · Accepted Answer · answered Nov 05 '17 at 10:24

A 4 TB disk will always have a capacity of at least 4,000,000,000,000 bytes, so if you sacrifice the "bonus" capacity you'll be on the safe side.

Usually, you buy disks of the same type and replace them with the same type. When using "what's around" you have to be careful. Especially late-market and after-market disks have a tendency to be somewhat larger (in order to always fit) and if you start with those you can run into trouble - when you replace a fully-used disk in a RAID and the replacement is only one sector short it won't work.

Ok, thank you, then I go for "the safe side". – Toxiro Nov 06 '17 at 08:57 — Toxiro, Nov 06 '17 at 08:57

Varying hard drive capacities and RAID

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