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Comment by XelNika | Hacker News Reader

> Keep in mind that RAID5 isn’t feasible with multi-TB disks (the probability of failed blocks when rebuilding the array is far too high).

What makes you say that? I've seen plenty of people make this claim based on URE rates, but I've also not seen any evidence that it is a real problem for a 3-4 drive setup. Modern drives are specced at 1 URE per 10^15 bits read (or better), so less than 1 URE in 125 TB read. Even if a rebuild did fail, you could just start over from a backup. Sure, if the array is mission critical and you have the money, use something with more redundancy, but I don't think RAID5 is infeasible in general.

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Last time I checked (a few years ago I must say), a 10^15 URE was only for enteprise-grade drives and not for consumer-level, where most drives have a 10^14 URE. Which means your build is almost guaranteed to fail on a large-ish raid setup. So yeah, RAID is still feasible with multi-TB disks if you have the money to buy disks with the appropriate reliability. For the common folk, raid is effectively dead with today's disk sizes.

Theoretically, if you have a good RAID5, without serious wire-hole and similar issues, then it is strictly better than no RAID and worse than RAID5 and RAID1.

* All localized error are correctable, unless they overlap on different disks, or result in drive ejection. This precisely fixes UREs of non-raid drives.

* If a complete drive fails, then you have a chance of losing some data from the UREs / localized errors. This is approximately the same as if you used no RAID.

As for URE incidence rate - people use multi-TB drives without RAID, yet data loss does not seem prevalent. I'd say it depends .. a lot.

If you use a crappy RAID5, that ejects a drive on a drive partial/transient/read failure, then yes, it's bad, even worse than no RAID.

That being said, I have no idea whether a good RAID5 implementation is available, one that is well interfaced or integrated into filesystem.

I have a couple of Seagate IronWolf drives that are rated at 1 URE per 10^15 bits read and, sure, depending on the capacity you want (basically 8 TB and smaller desktop drives are super cheap), they do cost up to 40% more than their Barracuda cousins, but we're still well within the realm of cheap SATA storage.

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