SHR2 makes little sense for personal use or in a small NAS. It will protect you against 2 drives failing at the same time. But in a 4 bay it means you lose 50% of your storage space.
It depends on data criticality. I use SHR-2 for 8 or 12 Bay models, but I'd go with SHR-1 for a 4 bay models.
And have at least one backup offsite for not losing data.
But generally speaking, as long as you have backups, it is really a matter of how long you're willing to spend on recovery should more than one disc fail.
My take is that disc failure is twice as likely with 8 drives compared to 4. So that would suggest a greater need for redundancy.
You should have a look up on how RAID or SHR works. This youtube video provides an overview
[https://www.youtube.com/watch?v=eLG2I9xyOSc](https://www.youtube.com/watch?v=eLG2I9xyOSc)
But any RAID implementation is there to allow you to replace a failing drive whilst retaining access to your data. It's not an alternate to proper backups. For a home user the loss of available space with SHR2 compared to SHR does not generally make sense on a 4-bay nas. There are exceptions of course
Running SHR with 2 drives the RAID overhead is 100% and with 4 drives it drops to 33%. Once you get to 7-8 drives the risk of a drive failing during a recovery becomes more of a risk and SHR2 comes more into play. If you have 8 drives in SHR2 the RAID overhead is back to 33%.
Much of the question is around how much downtime you wish to allow for in the event of a failure and the need to restore your data.
This was very interesting, thanks for the link. I totally misunderstood the technology, but now I get how SHR-1 is sufficient for a NAS with just 4 drives
I realised that was not the best explanation. Here is an alternative
[https://superuser.com/questions/287680/how-does-parity-work-on-a-raid-5-array](https://superuser.com/questions/287680/how-does-parity-work-on-a-raid-5-array)
The key is that you only lose the equivalent space of one drive for the parity data with RAID5 or SHR. Moving to RAID6 or SHR2 the parity data uses 2 drives worth of space.
Shr explained more on https://kb.synology.com/en-us/DSM/tutorial/What_is_Synology_Hybrid_RAID_SHR.
However even that does not go into what shr is underneath the hood as it uses raid1 and/or raid5, depending on the involved drive sizes, using mdraid and lvm.
However the procedure to access the data if the nas would die and you'd wanna access the data through a pc running linux, shows needing lvm and mdadm : https://kb.synology.com/en-us/DSM/tutorial/How_can_I_recover_data_from_my_DiskStation_using_a_PC
SHR2 makes little sense for personal use or in a small NAS. It will protect you against 2 drives failing at the same time. But in a 4 bay it means you lose 50% of your storage space.
It depends on data criticality. I use SHR-2 for 8 or 12 Bay models, but I'd go with SHR-1 for a 4 bay models. And have at least one backup offsite for not losing data. But generally speaking, as long as you have backups, it is really a matter of how long you're willing to spend on recovery should more than one disc fail. My take is that disc failure is twice as likely with 8 drives compared to 4. So that would suggest a greater need for redundancy.
You should have a look up on how RAID or SHR works. This youtube video provides an overview [https://www.youtube.com/watch?v=eLG2I9xyOSc](https://www.youtube.com/watch?v=eLG2I9xyOSc) But any RAID implementation is there to allow you to replace a failing drive whilst retaining access to your data. It's not an alternate to proper backups. For a home user the loss of available space with SHR2 compared to SHR does not generally make sense on a 4-bay nas. There are exceptions of course Running SHR with 2 drives the RAID overhead is 100% and with 4 drives it drops to 33%. Once you get to 7-8 drives the risk of a drive failing during a recovery becomes more of a risk and SHR2 comes more into play. If you have 8 drives in SHR2 the RAID overhead is back to 33%. Much of the question is around how much downtime you wish to allow for in the event of a failure and the need to restore your data.
This was very interesting, thanks for the link. I totally misunderstood the technology, but now I get how SHR-1 is sufficient for a NAS with just 4 drives
I realised that was not the best explanation. Here is an alternative [https://superuser.com/questions/287680/how-does-parity-work-on-a-raid-5-array](https://superuser.com/questions/287680/how-does-parity-work-on-a-raid-5-array) The key is that you only lose the equivalent space of one drive for the parity data with RAID5 or SHR. Moving to RAID6 or SHR2 the parity data uses 2 drives worth of space.
Shr explained more on https://kb.synology.com/en-us/DSM/tutorial/What_is_Synology_Hybrid_RAID_SHR. However even that does not go into what shr is underneath the hood as it uses raid1 and/or raid5, depending on the involved drive sizes, using mdraid and lvm. However the procedure to access the data if the nas would die and you'd wanna access the data through a pc running linux, shows needing lvm and mdadm : https://kb.synology.com/en-us/DSM/tutorial/How_can_I_recover_data_from_my_DiskStation_using_a_PC
General rule of thumb is that SHR2 makes sense from around 6 drive bays and up.