Storage is a really weak area of mine, but it is important, and I do not see much technical discussion here anyway, so I am going to expose myself: How does integrity protection work in practice? I know that the bits on the HDD itself do not map one to one to bits you can actually use and that it uses that to protect us from flipped bits. I know that beyond that RAID/RAIDZ is supposed to help. I guess redundancy alon…
Checksums.
If you get big enough checksums (aka: Reed-Solomon Codes), you can not only detect errors but correct them as well. See https://en.wikipedia.org/wiki/Reed%E2%80%93Solomon_error_cor... for the math.
Now that you have this "error-correcting checksum", where do you put it? Raid5 means you place the error-correcting checksum on different disks.
If you have three disks: A, B, and C. You'll put the data on A & B, then the checksum on C. This is RAID4 (which is never used).
RAID5 is much like RAID4, except you also cycle between the drives. So the checksum information is stored on A, B, or C. Sometimes the data is on A&B, sometimes is B&C, and sometimes its on A&C.
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> Could I put a FreeNas in a corner a room and leave it there? Would it just blink when I need to put a new drive in or would it need more maintenance? Of course talking about worst case scenarios here, so say I want this to live for 10 years sitting in the corner.
Maybe, maybe not. If all the drives fail in those 10 years, of course not (Hard Drive arms may lose lubricant. If the arms stop moving, you won't be able to read the data).
"Good practice" means that you want to boot up the ZFS box and run a "scrub" on it every few months, to ensure all the hard drives are actually working. If one fails, you replace it and rebuild all the checksums (or the data from the checksums).
RAID / ZFS isn't a magic bullet. They just buy you additional time: time where your rig begins to break but is still in a repairable position.
ZFS has more checksums everywhere to check for a few more cases than simple RAID5. But otherwise, the fundamentals remain the same. You need to regularly check for broken hard drives and then replace them before too many hard drives break.
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This also means that no single box can protect you from a natural disaster: fires, floods, earthquakes... these can destroy your backup all at once. If all hard drives fail at the same time, you lose your data.