Missing disk -> array running -> parity still valid?

[lixe]

Couldn't find any information about this. If I start the array with a missing disk (emulated) and copy files on an other array drive, will the parity still be valid after adding the missing disk again? If that's not the case, what exactly do I have to do to get a valid parity disk again?

[itimpi]

I assume you mean 'rebuild' the missing disk?   If so then parity will be valid after the rebuild.   Not surprising when you consider that parity is used during the rebuild process.

[lixe]

No actually I had to remove a drive temporarily. I was able to start the array nevertheless. I can see the content of the missing disk, even if I can not "use" any of it.

Edited February 22, 2017 by lixe

[JorgeB]

If there were no writes to that disk and as long as you are not using reconstruct write you could do a new config and parity would remain valid (although it would probably find a few sync errors on the next check), but it's recommend to just rebuild the missing disk.

 

I

Edited February 22, 2017 by johnnie.black

[trurl]

If the disk is being emulated, then not only can you write to other disks, but you can write to the emulated disk. Parity will remain valid and you will be able to rebuild the emulated data, but you are unprotected. And since all disks are being used to emulate, performance will be affected.

 

Best to use as little as necessary until rebuild gets you back to a protected state.

 

What exactly are you wanting to do? There is no need to copy the data from the emulated disk if you are going to rebuild it.

 

If you don't want to rebuild it then it might be better to copy the emulated data to another system and thus avoid writing to your unprotected system. Then you could rebuild parity without the disk and copy the data back to your protected system.

[JonathanM]

1 hour ago, lixe said:

I can see the content of the missing disk, even if I can not "use" any of it

What do you mean by this? You should be able to "use" it just as if the disk was there, that's the point of parity protection.

[lixe]

I accidentally deleted some files on one of my disks. So I removed it from the array to rescue the deleted data, which went fine. I didn't do any changes to the disk itself though.

 

Since I wanted to use my unraid server while restoring the data, I startet it with a missing disk (I have a cache drive, a parity drive and two array drives). Now having the data restored on an other disk (not part of my unraid system) I want to add the removed disk to my array again.

 

But before I add the disk and start the array, I'm curious what's going to happen?

 

I thought that unraid will remember what the missing disk contains and just work without it meanwhile. When adding the disk again I can access the files and add new data to the drive.

 

Or should I add the drive and rebuild the parity disk?

[garycase]

First, it's not at all clear why you removed the disk in the first place.   But given that you did, what will happen is that UnRAID will rebuild the disk back onto itself when you restore it to the array.    This will include any changes you might have made to the emulated disk (if you didn't write anything to the emulated disk, then the disk should be exactly as it was when you removed it).   Note that if you had any cached data for a share that includes the removed disk, there may have been files written to the emulated disk as a result of the mover process.

 

In any event, simply re-assigning the drive and letting UnRAID rebuild the disk is most likely what you want to do.

 

The other option -- if you're confident the disk is good -- is to do a New Config and let parity be rebuilt, but this would put your system "at risk" (not fault tolerant) during the parity sync.

 

[garycase]

On ‎2‎/‎22‎/‎2017 at 7:55 AM, lixe said:

I can see the content of the missing disk, even if I can not "use" any of it.

 

Actually you CAN use it -- you can read it; delete it; write new files to it; etc.   When you eventually replace the disk UnRAID will do a disk rebuild and the disk will then match the state that was being emulated.

 

[JorgeB]

Just be aware that a rebuild will reconstruct the disk as it was when it was disable, any undeleted files or data added to that disk since it was out of the array will be deleted again, in case you didn't copy them elsewhere.

[lixe]

I did remove it from the array so that no new data would be written to it. I wanted to use Raise Data Recovery on a windows computer to restore my files to an external drive. But I had some problems using my windows computer so that I used a windows VM on my unRAID system. Not a single byte should be changed on the drive since I only restored files to an other drive.

 

And I didn't see an other option to restore the deleted files...

 

Now after restoring I want to "readd" the drive to my array, but as it seems it shouldn't be a problem, because unRAID will restore the drive to the old status in case some bytes changed anyway.

[trurl]

17 minutes ago, lixe said:

unRAID will restore the drive to the old status in case some bytes changed anyway.

It will restore the drive to the emulated data, which isn't necessarily the same as the old data since it is possible to write to the emulated disk.

[garycase]

9 hours ago, lixe said:

Now after restoring I want to "readd" the drive to my array, but as it seems it shouldn't be a problem, because unRAID will restore the drive to the old status in case some bytes changed anyway.

 

That will indeed be the case vis-à-vis any inadvertent changes you might have made to the drive when it was removed; but as trurl noted, any changes that might have been made to the emulated drive will be part of the reconstructed drive.   Assuming you didn't do any writes to the emulated drive (including any that might have been done by a Docker or VM), then it will indeed be exactly as it was when you removed it.

 

If you ever need to do this again, it would by FAR be a better idea to shut down the UnRAID system;  remove your drive and do what you need to with it -- being CERTAIN you don't write anything to it -- and then replace it before you reboot UnRAID.    But an even better option would be to do the recovery from within UnRAID -- which it sounds like you did anyway.   This would retain full parity protection even if you made changes to the drive.

 

[SSD]

If you were writing to the disk when it was kicked from the array, there is a chance that one block of the file you wrote as the drive failed will be corrupted on the rebuilt disk. If you know what file it is (or one of a relatively small set that you still have on some other media), you might want to run MD5 checksums on both sets and confirm all was copied accurately.