Hi all.

 

I have been trying to find out, how storage management works on Unraid and ended up quite confused. I was hoping someone could assist in my understanding.

 

I am migrating away from a Synology, where I have 2x6TB disks running essentially raid 1 (mirror). To avoid data loss, I have procured 2x6TB drives so the data could be migrated 1:1 over my local LAN.

 

Here comes the bit I don't quite understand. If I choose array, then it looks like all data will be stored on one drive, with the option to enable a parity drive. This is probably safe, but seems to require immediate intervention for when the time comes that the drive will fail. It occurs to me, that data loss might be close to inevitable in this use case.

If I choose storage pool, then how the data is stored, doesn't seem that obvious to me.

 

Any advice as to how I can manage to have 2 drives mirrored to reflect 6TB of capacity?

 

Thank you in advance.

Best regards

Andreas

2 hours ago, antaeusdk said:

If I choose storage pool, then how the data is stored,

With two drives it defaults to a mirror (raid1)

Most use parity array for large storage and pools for fast storage. 

 

You can have many array disks protected by one or two parity. 

 

Data loss is not inevitable at all. Actual disk failures are more rare than other causes of data loss, including user error. 

 

Parity and even mirrors are not a substitute for backups. You must always have another copy of anything important and irreplaceable. 

8 hours ago, antaeusdk said:

This is probably safe, but seems to require immediate intervention for when the time comes that the drive will fail.

For any single redundant system (including mirrors) this is always the case. While you can continue to access your data (in both mirror and parity scenarios) it is no longer protected and the next device failure results in data loss.

 

8 hours ago, antaeusdk said:

It occurs to me, that data loss might be close to inevitable in this use case.

This is not the case. In fact technically speaking (the way unraid does it) a 2 device parity protected array (1Ddata 1Parity) ends up being essentially a mirror. If your data drive dies you are able to continue to access your data off the emulated device calculated from the parity drive. 

Thank you all for your feedback. I would wish this was described in the documentation, or I have to have new spectacles prescribed

 

I have reached  another issue, but in terms of keeping the forum clean, I will post another thread tomorrow.

 

Again, thank you!

Just wanted to add something to the conversation a lot of people assume incorrectly and that mirrors and parity are not, and cannot be, considered a "backup." It does nothing to prevent data loss. It only limits your downtime. Losing a single disk does not mean the data is lost as it can be rebuilt from parity. 

 

HOWEVER...

 

If a file is corrupted, deleted, overwritten, parity is immediately updated to reflect the current state of the entire array including the corruption, deletion, and the overwrite. There will be nothing to go back to or to recover from. Parity does not have a memory of previous files. Only a calculation of the very current state.

 

 

On 3/26/2025 at 5:57 PM, MowMdown said:

Just wanted to add something to the conversation a lot of people assume incorrectly and that mirrors and parity are not, and cannot be, considered a "backup." It does nothing to prevent data loss. It only limits your downtime. Losing a single disk does not mean the data is lost as it can be rebuilt from parity. 

 

HOWEVER...

 

If a file is corrupted, deleted, overwritten, parity is immediately updated to reflect the current state of the entire array including the corruption, deletion, and the overwrite. There will be nothing to go back to or to recover from. Parity does not have a memory of previous files. Only a calculation of the very current state.

 

 

So, having 2 disks, in mirror, does not prevent data loss in the case where one of them dies? That goes beyond anything I have ever read or heard. Not replacing the defective drive, will in the end cause data loss, I know. I also know that it is not a backup. It is the 2 in the 3-2-1 rule.

43 minutes ago, antaeusdk said:

So, having 2 disks, in mirror, does not prevent data loss in the case where one of them dies?

It does, it doesn't if a file is deleted, overwritten, you are infected by ransomware, etc.

6 hours ago, antaeusdk said:

So, having 2 disks, in mirror, does not prevent data loss in the case where one of them dies? That goes beyond anything I have ever read or heard. Not replacing the defective drive, will in the end cause data loss, I know. I also know that it is not a backup. It is the 2 in the 3-2-1 rule.

In a mirror, you have two identical copies of data. You lose the data on the dead disk, you still have a redundant copy on the good disk. If both disks die, your data is permanently lost.

 

however if the data on the mirror is altered in any way that data is no good without a backup to restore it from. For example if you delete all your data, it gets deleted from the mirrored disk as well. 

Edited March 28, 20251 yr by MowMdown