Hi all. I am building a new Unraid. A few years back it seemed that preclear was a fairly important step, which of course took a very long time with multiple 3TB drives. Is this still import? I see very few threads about it in the forum. Thanks.

The latest version of unRAID will clear a drive added to a new data slot without taking the array offline. Many people still use preclear for testing though. You really should test any new drive thoroughly before trusting it in your array, whether with preclear or some other method. Preclear gives it a good workout to help eliminate infant mortality, and can help make sure all bits can be read and written. All bits of all disks must be trustworthy since all bits of all disks are needed to rebuild a failed disk.

OK thanks. I will do so with new drives.

Agree you should always test a new drive to help eliminate infant mortality issues.

 

Specifically, "preclear" is no longer an important function -- the primary motivation for JoeL when he wrote that utility was to eliminate the long downtime in an array while it was clearing a new drive.  The PreClear utility provided a way to clear a drive BEFORE adding it to the array; and in conjunction with LimeTech a special "cleared" signature on the drive allowed it to not require clearing when added, since UnRAID "knew" that had already been done.    As trurl noted, the newest version of UnRAID no longer disables array access when you're adding a new drive -- it will clear the drive BEFORE incorporating it into the array; and then automatically add it.  So the PreClear function is no longer needed.

 

But Joe included a fairly thorough bit of testing in the process -- reading every bit to confirm all sectors could be successfully read;  zeroing (clearing) the drive; and then post-reading to confirm everything had been written correctly and could be successfully read back; and included a good bit of seek testing in the process.    Running a few cycles of this became somewhat of a defacto "test" for new drives to confirm all was good before adding them to the array.

 

This testing, however, can just as easily be done using various 3rd party disk utilities or the manufacturer's diagnostics, so if you'd prefer to test new drives on another system (e.g. Windows, Mac, or another Linux box), that is just as good.    The important thing is that you DO test your new drives before using them.

 

Personally, I test all new drives using WD's Data Lifeguard -- I run a short test; long test; then a full write zeroes; and then repeat the short and long tests.  I do this regardless of which system the drive is destined for -- one of my desktops;  an HTPCD; or one of my UnRAID servers.

 

Note that if you do use some other way to test the disk, unRAID may still need to clear it. If you are using it to replace a drive it doesn't need to be clear, and if you are adding it to an array without parity it doesn't need to be clear, but if you are adding it to a new data slot in a parity array, it will have to be clear so parity will remain valid.

UnRAID will clear it ... but on the new version that's done without losing the ability to use the array.  THAT was the big issue before.  With the new version, clearing is no big deal, since the array is still fully useable while the new dirve(s) is/are being cleared.

 

Thanks all. I will use the plugin for this. Is there any way to somehow preclear multiple drives at once? I assume the plugin only allows for one at a time. With 4 3TB drives, this will take a week or more. Thanks.

You can preclear as many drives at once as you want => subject to your system resources.    Most systems will easily do 3 or 4 at a time.

 

question what if you have 24 drives to preclear?

Best have multiple systems to test the 24 drives, or deal with testing 4 to 6 drives at a time before moving onto the next set.

If you have 24 drives to pre-clear, it sounds like you're building a new system from scratch.    If you add all of these drives to the initial configuration, no clearing is required.

If, however, you want to test the drives first, then you can do that on other systems; or you can, as Brit suggested simply pre-clear 4-6 drives at once until you've got them all done;  THEN do the initial configuration of the system (which won't need to clear anything if it's the initial config).