RN214, query on a 2nd vertical expansion

berillio wrote:

Wouldn’t that be faster and simpler than doing the 3rd vertical expansion as I described earlier?

The number of RAID groups isn't really relevant to the speed, as every RAID group needs to be resynced when you replace a disk.  I usually size the process by totalling up the disk reads/writes.

Just hot-swapping the disks in the NAS requires reading or writing every sector in the volume at every step (including the parity blocks).  So

1. 32 TB when you swap the first disk
2. 40 TB when you swap the second
3. 44 TB when you swap the third
4. 48 TB when you swap the fourth

164 TB of total disk activity, which will take quite a while.  Note each step takes more time than the one before it.

As far as the RAID is concerned, your process takes

1. 36 TB to sync the initial 3x12TB volume
2. 48 TB to sync and expand to 4x12TB

So 84 TB for that bit. 

But copying the data to the 2x12 disks at the beginning requires 42 TB of disk I/O (21 TB of reads and 21 TB of writes), the second transfer to the 8 TB drives requires another ~20 TB of copying (assuming about 10 TB of data on the source 12 TB drive).  And 42 more TB to transfer the data back to the NAS.  A total of 114 TB of disk I/O for the copying.

So your process requires more disk I/O than hot-swapping, not less.  Though this is distributed across two devices, so in practice the wall clock time is likely similar.

That said, copying the data to the PC first is safer, because you have no RAID protection during the hot-swap process. If one of the 8 TB drive fails during the hot-swap process you lose all your data.  Personally I think that maintaining data safety is more important than the amount of time the process takes.

What you could maybe consider is leaving two of the 12 TB drives in the PC.  Then you'd stop the NAS expansion at 2x12TB + 2x8 TB.  You'd gain only 4 TB of storage, but that would still give you 25% free space.  This would look like

1. insert 2x12 TB in the PC and copy the data from the NAS (42 TB of disk I/O)
2. hot swap one 12 TB drive and and resync (32 TB)
3. hot-swap the second 12 TB drive and resync (40 TB)
4. add at least one of the 8 TB disks to the PC to give more free space there, and back up the NAS to the PC on schedule

Total disk I/O is 114 TB, so less than either of the two methods above.  Later on, you can continue the expansion by hot-swapping two more 12 TB drives to the NAS (and maintaining enough disk space in the PC to keep the files backed up).

FWIW, I am ignoring the disk test time, since you'd be doing that no matter what path you pick.  Personally I run the extended read test, followed by a full (single-pass) write test on each disk before I put it into my NAS.  That would take more than 15 hours with a 12 TB drive.  I have had some out-of-the-box drives that pass one of these tests but fail the other.

Hopefully, you've not unboxed the 12TB drives, because your plan is very inefficient cost wise.  Updating all 4 8TB to 12TB only adds 12TB of storage at the price of 4 x 12TB drives.  For less money (and fewer re-syncs), you can swap out two for 24TB drives and get a 16TB increase with the option of later swapping another, and then later another, for yet another 24TB for an additional 16TB from each.  I believe 24TB to be the best size-to-cost ratio available today.  On Amazon, a 24TB Ironwolf Pro is the same price as a 20TB.

The only reasons to be doing what you are is if you suspect all the 8TB drives are in need of replacement or if you are using FlexRAID (which requires all drives be of equal size).  But if you are using FlexRAID, the incremental syncs StephenB​ listed are not what you will see.