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You can run periodic scans to look for bitrot. I think this is the standard way of handling this, my ReadyNAS before this had the same functionality.
by pfarnsworth 11y ago
You can run periodic scans to look for bitrot. I think this is the standard way of handling this, my ReadyNAS before this had the same functionality.
- usefulcat 11y agoHow can it look for bitrot without any checksum info?
- rakoo 11y agoInstead of running some homemade script, you better use something like SnapRaid (http://www.snapraid.it/ http://www.snapraid.it/)
- pfarnsworth 11y agoRAID has redundant information so they use that.
- Moru 11y agoDepending on what RAID you are running, you will only know you have bitrot, you can't fix it since you don't know what harddrive has the correct information.
- cmurf 11y agoThis doesn't unambiguously detect mismatches though. If it's a mirror, you have two copies of data, which one is bad? All RAID 1 knows is that they're different. If it's RAID456, again, all it knows is that there's a mismatch, it doesn't know if the data strips are wrong, or if the parity strips are wrong. The way ZFS and Btrfs deal with this is data is checksummed, and the fs metadata which includes the parity strips, are checksummed. So there's a completely independent way of knowing what's incorrect, rather than merely that there's a difference.
- baruch 11y agoIn RAID6 you can find which of the different combinations shows a mismatch and which combination doesn't. Run through all combinations and find the one that shows no mismatch, the dropped drive is the one with the bad data, rewrite it and go on with your life. This can only detect one bad drive, if you have two you are toasted.
- Ded7xSEoPKYNsDd 11y agoFWIW Linux software RAID doesn't do that. I think the argument was that differences like this were mostly related to power loss where some disks have the new data and others the old data. And at the block layer, it's impossible to tell which is which and so the code just picks a winner basically at random. I'm not 100% convinced myself that a 'majority wins' strategy like you described wouldn't be superior, but I can see why they decided otherwise.
- SirMaster 11y agoExcept Synology uses MDRAID (Linux Kernel RAID) and even in RAWID 6 mode it doesn't do this. It just overwrites the corrupt sector with a new value to make the parity data consistent. It doesn't know which is right or wrong even though technically with RAID6 it would be possible to determine.
- usefulcat 11y agoTheoretically for RAID6, but not for anything less than that. And in any case I'm pretty sure Linux RAID doesn't implement that. Though, as the owner of a Synology NAS I'd love to be wrong about that.
- ksec 11y agoI am not aware Synology offer this at the moment.