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I have a single external HDD that I'd like to use as storage for data that's not read/written very often. I'd like to make this storage resilient against faults that might occur.

ZFS apparently supports copies=2, where each bit of information is stored twice on the disk, preventing bad sectors from leading to data loss.

Question 1: Is it worth it to use this type of single disk redundancy? What are the relative risks of partial drive failure to total drive failure?

Question 2: Two copies seems excessive for my use case. Is there a way to have, say 30% redundancy, like Parchive does but on the filesystem level? Ideally this redundancy would be a lazy process, where the filesystem returns as soon as it has written a single copy, and writes the redundancy bits when it's idle.

Mark
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  • Which 30% of your files do you want to be redundant? – Michael Hampton Jun 23 '20 at 23:29
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    All of them, but less than a full copy. For example, for 50% redundancy I'd like a file that consists of 2 blocks to have a 3rd block on disk that serves as a parity block. – Mark Jun 23 '20 at 23:32
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    raidz does that. But of course you'll need three vdevs. You could put them all on one disk, but that would be extremely slow, and really, disks are cheap and your data is expensive. – Michael Hampton Jun 23 '20 at 23:39
  • Magnetic medium failure is certainly a concern when considering durability, but there are plenty of other failure conditions that affect entire disks. The model you're describing would produce an enormous amount of overhead, but more importantly it will trade storage medium durability for read element durability, more rapidly wearing a significant single point of failure. Overall, this single-disk redundancy model will probably reduce overall durability non-trivially. Disks are so cheap and redundancy solutions are so available that multiple disks are likely the least expensive option. – Spooler Jun 24 '20 at 00:11
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    @Mark what did you end up doing? I'm looking for file system level redundancy on an external drive, and was also looking for a parity block type 2/3 storage capacity. – Redsandro Feb 17 '23 at 12:10

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