September 3, 20187 yr Hi everyone, I'm thinking to switch from Synology to a custom NAS with unRAID. I have a question about RAID systems. Let's say I have 4x 4TB disks, one for parity (unRAID, RAID 5 is striped). In RAID 5, if a disk fails, I have to rebuild parity, so read every bit of the 3 other disks (12TB). If the disks have a URE (unrecoverable read error) of 10^14 (WD Red), or once every 12.5TB, that means that I have around 62% chance (see calculation below) that the rebuild of the parity will fail losing all the data. In unRAID, if the parity disk fails, it would be exactly the same as with RAID 5, right? So again around 62% of failure? With the benefit that the disks are less stressed in daily use because unRAID is not striped but JBOD. And how would RAID 6 be? I assume better but could not quantify how much. RAID 10 would be better, having only around 27% chance of failure (you only copy the failed disk, so only 4TB of data). Maybe I'm doing the math wrong, but I wanted to ask since I couldn't find any straightforward answer anywhere. Failure probability: 1 - ( (10^14-1)/10^14 )^(8*12*10^12) = 0.617 = 61.7% (Probability that one URE does not happen in 8*12^12 bits) Edited September 3, 20187 yr by andyn33 Added more details I found
September 4, 20187 yr Community Expert 2 hours ago, andyn33 said: In unRAID, if the parity disk fails, it would be exactly the same as with RAID 5, right? So again around 62% of failure? With the benefit that the disks are less stressed in daily use because unRAID is not striped but JBOD. If parity fails no data is lost because all of the data disks in the array are separate disks with independent filesystems. Each disk can be read by itself.
September 4, 20187 yr Should be noted that a URE does not constitute a drive failure in unRaid. Additionally, when running a Raid 5/6 (or any traditional RAID system), once you've exceeded the redundancy, you will have lost ALL of your data. With unRaid you will only have lost a small portion under the same scenario. That was my reasoning for buying unRaid. Worst case scenario I'd rather lose some, not all. Sent from my SM-T560NU using Tapatalk
September 4, 20187 yr Author 7 hours ago, trurl said: If parity fails no data is lost because all of the data disks in the array are separate disks with independent filesystems. Each disk can be read by itself. Yes sure, I expressed myself bad. If parity fails in unRAID with 12TB of usable space, the chance of losing data while rebuilding the parity disk is still around 62%. Even though you can lose only one disk (compared to the whole array in RAID 5), you still could lose some data, right? 6 hours ago, Squid said: Should be noted that a URE does not constitute a drive failure in unRaid. Additionally, when running a Raid 5/6 (or any traditional RAID system), once you've exceeded the redundancy, you will have lost ALL of your data. With unRaid you will only have lost a small portion under the same scenario. That was my reasoning for buying unRaid. Worst case scenario I'd rather lose some, not all. Ok, so what you are saying is that with unRAID, if a URE happens even during parity rebuild, you would not get a drive failure, but you would probably only get a corrupted file where the sector fails? That would be a big improvement over RAID 5.
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