Whats the difference between

 

1. I add a bad disk to unraid array without preclear.

2. I add a good disk to unraid with preclear. Later, the disk turn bad.

 

Whats the difference between those 2 situation? I mean, if later I found out that a disk is bad in unraid(with or without preclear), i just replace with a new disk and everything will be fine. No data loss, even if im not preclearing.

 

What happen if i add a bad disk to unraid? Will it results to data loss? Disk will eventually be bad. Its just a matter of time.

 

Im trying to find justification for preclearing.

 

Thanks.

Google "bathtub curve". Most electronics either fail early, or don't fail for a long time. After they get past "infant mortality", some will become obsolete before actually failing. An extensive test of a drive when it's new will help get past infant mortality. Many use preclear for this extensive test.

 

What you want to avoid is having more bad disks than you have parity drives. Parity by itself doesn't allow a disk to be rebuilt. All bits of all disks are needed to calculate the data for the rebuild. If you have a drive failure, you want to be confident that all the remaining drives are going to be OK while you rebuild the failed disk.

 

You can definitely lose data if you have multiple failures at once.

It makes sense to test your hardware before you put it into service. Pre-clear is a convenient tool for testing disks. You don't have to use it, though. Some people use badblocks, some use the manufacturer's own utility but pre-clear is very convenient and does a good job.

 

It's peace of mind knowing with certainty that the disk you are putting in is ok, versus not knowing at all. That does not mean the disk you just pre cleared might not fail in an hour, a day, a month or six months, but it means at that moment you have verified its ok. It means it shouldn't fail anytime soon and jeopardize your data. Remember with one parity disk, when a data disk fails, you are ok, but you need to replace that failed disk, if you don't and another data disk fails, you lose data. So anytime a data disk fails, your data is at risk, less so with two parity drives, but you get my point. Better to do what you can and pre clear your disks then put in a disk you 'think' is ok, heck its brand new it should be fine? People have had brand new drives die almost immediately, it can happen, better to catch it before if you can.