X-RAID vs. 2xRAID-1

The configuration you linked to is sometimes called "jbod". You have two completely separate volumes of 2 TB each. You can use them anyway you want, and there is no failure protection. You can if you wish use backup jobs to regularly copy data from one to the other.

x-raid on an RN102 mirrors the two drives at a low level. All disk writes are automatically done twice, once to each drive. It is not identical to a backup job, as the files are not being read - just written twice. This cuts the amount of space in half (only one 2 TB volume). One benefit of x-raid is that you can expand the drives seamlessly. That is, if you buy 2 4TB drives in the future, you can insert one, wait to the NAS to resync, then insert the other. After the second resync the NAS will have grown to 4 TB. Your data can be accessed throughout this process, which is quite convenient. You can also replace a failed disk, without losing your ability to access the data.

If you are using your NAS only for backups, then either jbod or xraid will do. If you are using your NAS for primary storage, then xraid is better for most users.

Either way you need a backup on a different device if you truly want to protect your data from loss. Don't imagine that RAID ensures that your data safe - it does not.

Thanks for explanation but still have a couple of questions.

StephenB wrote:
The configuration you linked to is sometimes called "jbod". You have two completely separate volumes of 2 TB each. You can use them anyway you want, and there is no failure protection. You can if you wish use backup jobs to regularly copy data from one to the other.

Failure of what? Disks or data? If I create a backup job and will have the same data on both disks, isn't it enough to protect against disk failure or data loss?

StephenB wrote:
If you are using your NAS only for backups, then either jbod or xraid will do. If you are using your NAS for primary storage, then xraid is better for most users. Either way you need a backup on a different device if you truly want to protect your data from loss. Don't imagine that RAID ensures that your data safe - it does not.

The point is I'd like to use it for both: a Lightroom folder with photos accesible from different computers and a backup of users' home catalogues. My thinking was to move a Lightroom folder with photos to vol 1 and make a backup home of calogues also on vol 1. Then make a copy of vol 1 on vol 2 and this way I'd kill two birds with one stone. But you're saying it's not enough. What happens to a drive/data when the other drive fails (in 2xRAID-1 configuration)? I thought it stays ok, data is safe and I can insert a new drive and manually sync it with the old one.

Jarkod wrote:
...If I create a backup job and will have the same data on both disks, isn't it enough to protect against disk failure or data loss?

Disks-perhaps. Though identical disks in the same chassis can fail at close to the same time. Data loss - definitely not. There are plenty of scenarios that destroy both volumes. Power spikes and software bugs that corrupt the file system are two. Then there's disaster (flood, fire, theft, etc).

Jarkod wrote:

The point is I'd like to use it for both: a Lightroom folder with photos accesible from different computers and a backup of users' home catalogues. My thinking was to move a Lightroom folder with photos to vol 1 and make a backup home of calogues also on vol 1. Then make a copy of vol 1 on vol 2 and this way I'd kill two birds with one stone. But you're saying it's not enough. What happens to a drive/data when the other drive fails (in 2xRAID-1 configuration)? I thought it stays ok, data is safe and I can insert a new drive and manually sync it with the old one.

Again, drives are not the only thing that can fail. You will find regular posts here from people who assumed otherwise, and are desperately trying to recover their data. Such recovery might be a bit tricky with the new OS6 systems, since btrfs isn't as widely used at present as ext.

Most users are using the NAS for both purposes. The safest approach is to combine xraid with backup to another device. Your alternative to xraid is workable, and gives you some more space (since you only replicate the primary storage). But in my view it would be better if you backed up to a different system - or at least a usb/esata drive - that you removed after the backup was finished.

Personally I am conservative on backups - losing data years ago made me that way.

BTW, with jbod there is no syncing. You'd insert a new drive, re-create the shares, and restore them from backup.

StephenB wrote:
Disks-perhaps. Though identical disks in the same chassis can fail at close to the same time. Data loss - definitely not. There are plenty of scenarios that destroy both volumes. Power spikes and software bugs that corrupt the file system are two. Then there's disaster (flood, fire, theft, etc).

But scenarios you mentioned (and other you didn't) apply to disks regardless of the NAS configuration. They're not an argument for or against xraid or 2xRAID-1. When spike happens I can loose the whole NAS, be it xraid or raid-1, and - I agree - the additional backup is the only remedy.

StephenB wrote:
Most users are using the NAS for both purposes. The safest approach is to combine xraid with backup to another device. Your alternative to xraid is workable, and gives you some more space (since you only replicate the primary storage). But in my view it would be better if you backed up to a different system - or at least a usb/esata drive - that you removed after the backup was finished.

I was planning to do it anyway. The problem I have is which NAS configuration to choose. I was thinking 2xraid-1 could be a bit safier because when one drive fails, I'll have another one with data (other scenarios aside). At the end of a day I'll have data on vol. 1, 1:1 copy on vol. 2 and a backup on a usb drive. Or am I completely mistaken?
What does xraid do to the second drive, how does this protection work? How is data stored on it; can I retrieve it from the second driveit e.g. by connecting it in a PC and mounting in Linux?

StephenB wrote:
Personally I am conservative on backups - losing data years ago made me that way. BTW, with jbod there is no syncing. You'd insert a new drive, re-create the shares, and restore them from backup.

The whole thing with NAS and backup is because I lost all my data due to a disk failure (yes, I'm one of those that start doing backup after a disaster). I've still got it and hope one day I'll be able to recover it (it's extremely expensive here in Poland). At the same time I'm trying to protect againt another accident.
By syncing I ment what you wrote, it was only shorthand for all those operations.

Jarkod wrote:

...I was thinking 2xraid-1 could be a bit safier because when one drive fails, I'll have another one with data (other scenarios aside). At the end of a day I'll have data on vol. 1, 1:1 copy on vol. 2 and a backup on a usb drive. Or am I completely mistaken? What does xraid do to the second drive, how does this protection work? How is data stored on it; can I retrieve it from the second driveit e.g. by connecting it in a PC and mounting in Linux?...

I thought I answered most of this.

(a) xraid with 2 disks is mirrored. Every write to the volume is literally written to both drives in parallel. Since the disks are written together, the data on them would be precisely the same up to the point of failure. If a drive fails, all your data remains accessible through the NAS. You can in principle mount either disk in another linux system, though it would need to support the btrfs file system.

So this maximizes data availability, and enables volume expansion even while you access your data, and allows data access to continue as usual if a disk is being replaced. In all these cases, the users don't know the expansion, etc. is happening - it is invisible to them.

(b) Your scheme by contrast is not invisible to the users, and does not support volume expansion. It is more economical on space, and for the shares you are copying you get a little more protection from user mistake - since you can access the older copy if you want to. You have some insulation against any glitches that corrupt the file system itself, since you have a unsynced copy. But if there is a failure, you would lose any work that was done after the last backup. Like (a) you can in principle mount either drive in a linux system that understands btrfs.

That's the trade-off. Both will work. I wouldn't say your method is safer than xraid - it has some advantages, and some disadvantages.

Once more, thanks for explanation. Somehow I couldn't get a grip on how x-raid works even though it's not that complicated. Now it's clear.