I have been  using unRAID for few months now. I was just curious , if a drive fails with uncompressed audio and video files on it and you install a new drive and it is reconstructed by the un RAID server , is the data bit perfect compared to the old drive. Whats the error rate while reconstructing the new drive?

 

thanks

mani

Yes, it will be bit perfect.  Unraid doesn't use a lossy algorithm like MP3 or jpeg, it is a bit-by-bit recovery method using the data on the remaining good drives plus the parity drive to figure out exactly what is missing.

 

The only flaw in the approach is that you could have a second drive failure while the first is rebuilding, but the odds of that happening are many hundreds (if not thousands) to one.  To minimize this possibility, I recommend that you shut down your unraid as soon as you find the problem, go buy the drive, then replace it immediately.  As power issues are famous for causing drive issues (spikes, brownouts, and outages), you should also run your unraid on a UPS.

 

 

Bill

thanks

 

Drives can literally break and be dead as a doornail.  This is what we commonly think of as broken.

 

Far more common (at least in my experience) is that a sector on the disk fails, and the drive gets a read error on just that one sector.  Modern drives have SMART features.  SMART will, when a marginal sector is encountered, remap that bad sector to a reserve sector.  No one is the wiser.  But if a sector is not accessed for a very long time, the sector can pass the marginal state and become completely unreadable.  (New drives are also susectable to a couple bad sectors)  When this happens, and someone tries to access that sector, it will be returned as a read error.  SMART "remembers" that this is a bad sector and will fix it on the next write, but the ability to perfectly access the data from that sector is lost.

 

Why do I tell you all this?  If you try and reconstruct a drive based on a bunch of drives in your array that have not been read for a very long time, your risk of having a sector error is significant.  I am pretty sure that unRAID would continue to reconstruct the missing disk, but a bad sector will be computed as all binary zeros, and you will have some corruption somewhere on the reconstructed disk.

 

To prevent this, I recommend everyone run a parity check periodically on the array.  The purpose is not so much to check parity, but to read each and every sector on all the disks and let SMART do its thing.  You should do this just before replacing a disk (growing a disk) to reduce the risk of encountering a bad sector during reconstruction.  Obviously you don't have that chance with a failed drive.