February 19, 20251 yr Hi all, New to unraid. So far it's been working great, but I wanted to consult folks to understand how parity works. I've read some stuff on the forum and some guides and that only got me more confused. So, I have 2 14 tb parity drives and 4 14 tb data drives currently in my array. I've started moving data onto my array, but everything seems to written to only data drive 1 and not 2-4. I've selected Highwater as the allocation method, and I read that it'll try to fill the first disk to ~50% before writing to another disk. Currently I have about 2 tb worth of data written to the array, and they all seem to be on drive 1. What would happen if drive 1 fails now? Would the 2 parity drives be enough to recover the data or will the data on drive 1 be lost? The reason I ask is because I'm planning to add 2 2 tb ssds as a pool. One of the ssds currently has all of the data that's been written to disk 1 on it. I'm trying to figure out if I need to buy another 2 tb ssd for the cache or if it's safe enough to use the one that I already have and lose that data on the ssd. Thanks!
February 19, 20251 yr Community Expert Solution 5 minutes ago, misosoup7 said: What would happen if drive 1 fails now? Would the 2 parity drives be enough to recover the data or will the data on drive 1 be lost? 1 parity drive would be enough to recover 1 disk failure. But, there are plenty of much more common ways to lose data than a disk failure. Parity is not a substitute for backups. You must always have another copy of anything important and irreplaceable.
February 19, 20251 yr Author 5 minutes ago, trurl said: You must always have another copy of anything important and irreplaceable. Understood. I do have a copy at my parents’ house, but recovering from there is a huge pain now that we live hundreds of miles apart. I just wanted to understand if uneven storage between the disks will result in difficulties in the event one drive fails. I bought these drives refurbished. They show 0 hours power on smart so I worry because of the recent reporting about drives that had their smart data wiped. It’s not a seagate so I don’t have farm data to check either… extended smart test seems fine, but I wanted to understand what I might be in more if one does fail. Thanks!
February 19, 20251 yr Community Expert 5 hours ago, misosoup7 said: Currently I have about 2 tb worth of data written to the array, and they all seem to be on drive 1. What would happen if drive 1 fails now? Would the 2 parity drives be enough to recover the data or will the data on drive 1 be lost? Parity works at the raw sector level and has no concept of the data or file system on a drive - it just tries to ensure a drive has the bit pattern it thinks should be there and is not aware that a higher level in the Unraid system has so far written most of the data to a particular drive. The number of simultaneous drives failures that can be handled is always equal to the number of parity drives you have.
February 19, 20251 yr Community Expert https://docs.unraid.net/unraid-os/overview/nas/#parity-protected-array
February 19, 20251 yr Community Expert 9 hours ago, misosoup7 said: hey show 0 hours power on smart so I worry because of the recent reporting about drives that had their smart data wiped FWIW- if it was refurbished properly and stamped on the drive 'refurbished' or whatever, then the factory will have wiped SMART data, and FARM data as well, to reset the drive to zero
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