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Ask HN: Does RAID 1 of 3 HDD offer any protection against bitflip on 1 of drives

1 points by kokojumbo ↗ HN
Theoretically if 3 HDD are set up in RAID 1 - meaning every disk has same content and bit flip happens to the bit on one of them 2 other disks can be used to "vote" which value of bit being read should be "chosen as correct one by majority of votes". Does RAID 1 of 3 HDD make use of that fact in any way?

Of course bit flips can happen for same bit on 2 HDD at the same time but such cases potentially could be very very rare.

2 comments

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WantonQuantum
No, in a RAID 1 configuration (of any number of drives) writes are written to all drives and each read comes from only 1 drive.

Depending on the implementation, reads can be spread across drives but no parity is checked.

db48x
Yea, this was one of my disappointments about raid as well.

ZFS, on the other hand, stores the hash of every block separately from the block itself, and then uses those hashes to check every read. So it only reads the block from a single disk, but it hashes the read block and compares it against the expected hash. If there is an error of any kind the hashes won’t match and ZFS will increment the error counter for the drive and pool. It then examines the other mirrors to find the correct data. Once it has the correct data it overwrites the incorrect block.