Which is quickest, recheck parity or preclear ?

[MartynK]

I am currently building up a new Unraid server, its currently got 4x16tb and 1x16tb parity drives.

Once I have copied data from my old NAS on to this initial build, I will have a further 5x16tb drives to add to the array.

What is going to be best practice and also fastest, to just add the disks and let the system rebuild parity or preclear each disk first ?

I am also thinking of using one of the additional disks as a second parity disks, so would the rebuild be quickest ?

[bmartino1]

I have tried to use preclear before and it bit me.

I will also way recommend unbraids praity check and wait to guarantee operations use.

I have heard that preclear is faster, but may invite instability.

[itimpi]

6 hours ago, MartynK said:

What is going to be best practice and also fastest, to just add the disks and let the system rebuild parity or preclear each disk first ?

Unraid does not rebuild parity when you add new disks to a parity protected array - instead it Clears them (sets them to be all zeroes) so that parity is unaffected and only allows you to format and start using the drives once they have been Cleared.   If they have been pre-cleared then this Clear stage is skipped and they are immediately available to be formatted and used when added.   

 

The Unraid Clear is functionally equivalent to the write phase of a pre-clear and takes about the same time as that would take, but is not as thorough a test of the drive.  Normally with pre-clear one tends to run some read phases as well to check the disk better which is why pre-clear typically takes longer.

 

5 hours ago, bmartino1 said:

I have heard that preclear is faster, but may invite instability.

Preclear is normally slower if you run the read phases as well as the write phase so no idea where you heard this?    Perhaps that running multiple pre-clears in parallel can put more stress on the server?

[MartynK]

Hi,

Thanks for the reply, but I am still not 100% sure what I am best to do.

 

So I am going to add 5 new 16tb drives, one will be the second Parity drive.

 

Do I just add them and let Unraid deal with them ?

 

Thanks

 

[itimpi]

34 minutes ago, MartynK said:

So I am going to add 5 new 16tb drives, one will be the second Parity drive.

 

Do I just add them and let Unraid deal with them ?

Unraid will not let you add a new parity drive and new data drives in a single step.  You have to split it into 2 operations - one for parity and the other for the data drives (and you can do them in either order).

[MartynK]

OK, but once again, what is the quicker option, preclear the disks (does Unraid do it a disk at a time) or just add them and let it rebuild Parity ?

 

Adding the second Parity disk is realy "belts and braces"

[Terebi]

Adding the parity drive first will be faster. If you add parity second, it will have to read all of the new drives to calculate its parity.  If you add the drives second, they can be individually zeroed without affecting parity. 

[itimpi]

3 hours ago, Terebi said:

Adding the parity drive first will be faster. If you add parity second, it will have to read all of the new drives to calculate its parity.  If you add the drives second, they can be individually zeroed without affecting parity. 

Not sure it would make any difference.    When building parity all the data drives are read in parallel so the time is almost completely determined by the size of the parity drive - not by the number of data drives.

[MartynK]

Maybe I should turn this on its head.

 

If I add 4 new drives now to the array without preclearing them first, I believe it will mean the existing parity disk will become invalid.

 

So once the new disks are added, I could then add the second parity drive and let Unraid rebuild both at the same time.

Either way I need to rebuild parity for the second disk, just will it do both parity disks at the same time ?

[itimpi]

4 hours ago, MartynK said:

If I add 4 new drives now to the array without preclearing them first, I believe it will mean the existing parity disk will become invalid.

No.   The disks will be Cleared so that parity is unaffected.