Selecting RAID type for expanding from 2x4TB to more drives

[dimfil]

Being a long time user of 2-bay NAS devices, I plan to move to a new one with more bays. DS920+ and DS1522+ are my candidates for this upgrade.
I now run 2x4TB disks on a DS220+, obviously SHR raid type which at the moment gives redundancy for my 4TB total available space. I try to understand what are the exact differences from a user's perspective when adding more drives on the new device.
If I add 2 more 4TB drives, SHR gives me 12TB of total space and 4TB of protection, 1 drive tolerance. SHR-2 gives 8TB of total space and 8TB of protection, 2 drive tolerance.
My questions are:

• In case of SHR, which are the 4TB that are protected from the total 12TB? How is this decided? In this scenario, 1 drive tolerance means that the system can go on if ANY of the drives fails or somehoe is decided which disk is protected?
• In case of SHR-2 it is easier for me to understand that all drives and all data will be protected. How it compares to SHR? Should I prefer to use this raid type in my scenario sucrifising 4TB of total space that SHR gives?

Thank you in advance for any information...

 

[Rusty]

In case of SHR, which are the 4TB that are protected from the total 12TB? How is this decided? In this scenario, 1 drive tolerance means that the system can go on if ANY of the drives fails or somehoe is decided which disk is protected?

Correct, any drive can fail and the array will recover. The reason is that it is how RAID5 parity (that SHR with 3 or more drives) works. All drives are protected in this case just as they are in SHR2 (RAID6), the only difference is that RAID6/SHR2 will have a 2 disk parity scenario, doubling the number of drives that can fail, and lowering the usable space of the array by 50%.

 

[dimfil]

Correct, any drive can fail and the array will recover. The reason is that it is how RAID5 parity (that SHR with 3 or more drives) works. All drives are protected in this case just as they are in SHR2 (RAID6), the only difference is that RAID6/SHR2 will have a 2 disk parity scenario, doubling the number of drives that can fail, and lowering the usable space of the array by 50%.

View attachment 11563

So, in this case, I understand that it is quite safe if I continue to use SHR and gain that extra space. SHR-2 sounds more safe, but I believe that maybe is an overkill for non business environments...

 

[Rusty]

So, in this case, I understand that it is quite safe if I continue to use SHR and gain that extra space. SHR-2 sounds more safe, but I believe that maybe is an overkill for non business environments...

I would say so myself as well. Just keep in mind that RAID is not backup, so one drive redundancy just means that your array will continue to work if any drive fails. Backup should still be in effect, just to be certain that if multiple drives fail your data will be safe. I'm sure you are aware of that, so hopefully you won't take this the wrong way of me "educating" you, I just wanted to say that RAID5 while giving more usable space is still a bit "scary" in terms of data protection.

As long as you are aware of risks and have a backup, it will be more then fine for SOHO setup.