Just a question about parity

[Kegle]

I am looking at building a unraid server,  my question is that if u loose lets say 2 drives in a 25 drive setup,  do you need to copy the data from the other 23 drives before you can put them back into a array or can you just add 2 new drives and start a new array. I know you loose the data on those two drives who is lost,

 

I know you can do this with flexraid, but cant find any info about this on unraid

[vl1969]

I am sure more experienced unRaid users will chime in soon

but from what I heard you should be able to use the new config option to build new array  and rebuild the parity using existing  good drives without loosing data.

just make sure you know what your parity drive is.

[CaptainSpalding]

But if the parity has errors on those 23 disks, the errors cannot be fixed.

[trurl]

But if the parity has errors on those 23 disks, the errors cannot be fixed.

Not sure what you mean here. 2 disks are already lost in the scenario so no errors can be fixed. A new config will do a parity recalculation.

 

[CaptainSpalding]

Correct me if I'm wrong, but If some file(s) is corrupted on those 23 disks it cannot be repaired when new parity is calculated.

[trurl]

Correct me if I'm wrong, but If some file(s) is corrupted on those 23 disks it cannot be repaired when new parity is calculated.

With 2 disks already failed nothing can be repaired from parity.

 

Parity only allows one disk to be rebuilt. To do so it must have the parity plus all of the other disks. If parity or any of the other data disks cannot be read then rebuild of the one failed disk is not possible.

 

New config will just rebuild parity on the existing disks in the array and if any of them have bad data then that bad data will just become part of the new parity.

[CaptainSpalding]

Sorry, in short what I meant was that the array is unprotected during the new parity calculation. 

[JonathanM]

Correct me if I'm wrong, but If some file(s) is corrupted on those 23 disks it cannot be repaired when new parity is calculated.

Unraid does not do ANY file level correction, EVER. It does not calculate parity on files. There is no protection against file corruption. Parity in unraid only works with a drive as a whole, it does not care or check individual files.

 

Please read this for a more in depth explanation.

 

http://lime-technology.com/wiki/index.php/Parity

[superloopy]

Correct me if I'm wrong, but If some file(s) is corrupted on those 23 disks it cannot be repaired when new parity is calculated.

Unraid does not do ANY file level correction, EVER. It does not calculate parity on files. There is no protection against file corruption. Parity in unraid only works with a drive as a whole, it does not care or check individual files.

 

Please read this for a more in depth explanation.

 

http://lime-technology.com/wiki/index.php/Parity

 

That is when checksums (made when the data IS known to be good) come in handy. These could then be checked for consistency BEFORE remaking reduced parity with any errors being able to be corrected from backup.

[dgaschk]

Sorry, in short what I meant was that the array is unprotected during the new parity calculation. 

 

The array is considered unprotected after a single drive fails because a second drive failure will cause data loss. The array is unprotected during new parity calculation.

[Kegle]

What i ment is if u loose 2 drives. Can u insert new disks and

Make new parity of the new array. So you loose just the data on the 2 broken drives. Or do u need to copy all data off. Then make a new array and copy the data back since you cant add a disk with data on

[trurl]

What i ment is if u loose 2 drives. Can u insert new disks and

Make new parity of the new array. So you loose just the data on the 2 broken drives. Or do u need to copy all data off. Then make a new array and copy the data back since you cant add a disk with data on

Sorry we got sidetracked on the details.

 

You don't need to copy all of the data from your good disks.

 

The way I would probably proceed in this situation is just new config with the existing data drives and let parity rebuild, then add each new (precleared) data drive, then copy the data back. There are probably other ways to get there but in the end you are going to have to rebuild parity at some point. In any case, you shouldn't copy the data back until you have rebuilt parity.

 

You asked this as purely hypothetical. If you are really in this situation then ask further so we can guide you through each step.

 

[garycase]

... make a new array and copy the data back since you cant add a disk with data on

 

... This quote explains your confusion.

 

As already noted, you do NOT need to copy any data off -- you simply create a new config and all will be well.    Your comment "... you can't add a disk with data on ..." only applies to adding a disk to a parity-protected array.    When you are first creating the array, that's not a restriction -- ALL of the disks can have data on them.

 

The clearing of disk before you can add it is only required so fault-tolerance can be preserved when you add the disk.    Since the new disk is cleared (all zeroes), there is no change to the parity, so everything is still protected.    But when you do a "New Config", the array isn't protected until parity is computed, so it doesn't matter what the contents of all of the data disks are.

 

[Kegle]

The reason that i asked is because i have a highpoint 24 port raid card,  but if u loose 2 drives in a raid 5 you loose everything.  I have a license on flexraid that i have used long time on my windows computer.  But i wanna find a good solution for my archive box. And i have read much positive about unraid, and i like the fact that you dont loose everything.

 

So if i understand correct.  That you can just rebuild after a 2 or 3 drives crash.  I have backup of everything on usb drives so i'm not afraid to loose a drive or two, lol,   

 

Lets say u loose 2 drives,  Make a new parity of the existing drives, then add new drives and copy data back from my usb drives.

Did i understand correct.

 

If so i think this is the safest way to go without loosing everything.

 

[redlaws]

yes that is correct with new system for parity.

I have done this (because 2 disks had Reiserfsck --rebuild-tree errors),

I ended up pulling those 2 out to rebuild a good parity.

this was mainly because I could not find much info about Reiserfsck --rebuild-tree at the time.

 

then on a basic system rebuilt the Reiserfsck --rebuild-tree to see what I could get back.

 

I think I would had been better to rebuild them in the 14 drive system,

but if another drive went faulty then 3 disk of data could had been lost.

 

[garycase]

Yes, that's correct.  If you lose one drive, you simply do a rebuild.  If you lose two, you can just do a "New Config" with all the others;  then rebuild parity; then add any additional drives you want (up to the 23 data drive max); and then just copy the missing data back from your backups.

 

[Kegle]

Thank's for all help.