RAID Mirror as Backup

[rcrampton]

I'm running 2 HDDs configured as a mirror RAID. If I take one off-site, bringing it back on-site, say weekly, to refresh it, that should suffice for a consumer grade backup, yes? Since hot swapping is not yet supported I have to power down my NAS which is inconvenient, but it seems to work.

Aside from the (in)convenience factor and the occasional co-location of main and backup disks it seems like a reasonable solution and lets me use ZFS with snapshots in a simple way.

Does that seem reasonable?

 

[cyberjock]

Uh.. arrgg.. uhh.. aargg.. Well, you make a good case for your idea. But ZFS really relies on parity/checksums/mirrors to correct errors such as bitrot. You'd basically be deliberately be giving up the reliability that makes ZFS so great.

So my recommendation would be to find out if a 3-way mirror is possible(I'm not sure if it is to be honest). Then you have 2 mirrors in the working system plus the backup mirror.. I'm guessing that the GUI doesn't support it but the CLI does.

The only real drawback I see is that if someone deletes a file you don't have a bunch of historical backups to go back and find a file from 2 weeks ago that was deleted. Of course, you might have that issue solved with snapshots.

You know, I wanted to dismiss your whole idea as "omg this guy is a moron", but aside from the fact that you don't have any data protection for the server typically since you have no mirrors/checksums/parity, I'd say it's definitely "workable".

I'm really interested to know if you can do a 3 way mirror with ZFS now. Anybody with more experience with ZFS able tocomment about ZFS supporting 3 drive mirrors?

 

[rcrampton]

You know, I wanted to dismiss your whole idea as "omg this guy is a moron", but

So funny, when I wrote this post I kept thinking "this is the kind of crap some hack moron comes up with..." :)

I thought about just running an ext3 share and rsync'ing it to another etx3 share and carry the copy off-site. But ZFS's snapshot ability sure made this seem like a super-simple way to keep a relatively up-to-date backup of my data plus snapshots. No cron jobs, no complicated scripts.

I thought a ZFS mirror of 2 HDDs resulted in two identical disks, bit-by-bit. So there would be no loss of parity like you get with say RAID 3.

 

[cyberjock]

I thought a ZFS mirror of 2 HDDs resulted in two identical disks, bit-by-bit. So there would be no loss of parity like you get with say RAID 3.

You are correct. Except you are removing the mirror and deliberately keeping it out of sync with the working drive. So you basically have zero redundancy from a disk error or bitrot. But you are protected from a drive failure(remove the previously working drive and plug in the backup).

I'd try to see about doing a 3 way mirror but my virtual machine is currently running a chkdsk that I schedule once a month. Takes about 30-45 mins to run.

 

[cyberjock]

Well, I was able to create a 3 drive mirror through the GUI. Impressive. So.. my recommendation would be to go with a 3 way mirror so that you can have redundancy for the server when online, but the extra drive for offsite backups. :)

With that configuration you'd have the best of both worlds.

 

[Stephens]

[deleted because noobsauce80 tested and replied before I saved my reply]

 

[rcrampton]

Well, I was able to create a 3 drive mirror through the GUI. Impressive. So.. my recommendation would be to go with a 3 way mirror so that you can have redundancy for the server when online, but the extra drive for offsite backups. :)

With that configuration you'd have the best of both worlds.

Thanks noobsauce80, a beer on me when I get the chance!

Is it easy for me to add a third disk and convert my existing volume to a three way mirror?

Also, could I pull a ZFS mirror disk and mount it on something like an Ubuntu distro and copy the data from it?

 

[cyberjock]

You should be able to mount it on any system that has ZFS support. You'd have to consult the Ubuntu experts to figure out how to use the ZFS drive on Linux. I do know you 'can' use ZFS in Linux, but I'm not sure how good/bad it is.

 

[rcrampton]

Ubuntu does have ZFS support, I've read it's solid but haven't tried it.

I did a lot of searching to see if there's a way to convert a two-way mirror to a three-way mirror. I suspect it can be done but I'm not sure how. Does anyone know?

Thanks again.

 

[paleoN]

This idea is a bit dumb. You are not the first person to think of this. The problem is you are over thinking it or haven't thought enough. In trying to keep things simple you are over complicating them. You want two pools not one. This way you will have real backups and still get to use ZFS snapshots.

Buy your additional disk and create a second pool with it. Then zfs send/receive to the backup pool, zpool export and take it offsite.

 

[rcrampton]

Good idea, thanks.

 

[Agent1]

rcrampton, Did you have any success?

Noobsauce80, Would you idea of a 3 way mirror also improve performance due to the added spindles?

Just curious.

 

[cyberjock]

Honestly, I'm not the guy with a solid answer. It's a yes, but no, but yes. Overall, as you add more spindles the theoretical performance can increase. You might increase the theoretical read and write rates, you are also paying a small penalty in terms of latency(unless you are using SSDs). ZFS does try to obtain data from multiple drives simultaneously in mirrors.

In reality, if you are really trying to increase performance you're better off looking at more RAM, faster CPU, or an Intel NIC depending on your situation.

 

[rcrampton]

I ended up just making 3x 2TB UFS file systems. One is my main data disk and I rsync to the others via overnight cron jobs. I rotate the 2 backup disks offsite.

I was able to make ZFS work quite well via the CLI (that was a lot of fun actually, what a great filesystem) but not through the GUI. I couldn't get all of my snapshots covered under one recursion to replicate, it would do say 5 of 9 of them for some reason. I had clones that showed up in the GUI but could not be removed/destroyed. It got confused a couple of times and I had to clear the PULL replications and start over to keep it going. There were a handful of little odds and ends like that. I spend some weeks trying to get it all to work and bailed and got the UFS/rsync setup running in a trivially small amount of time.

I love the concept of ZFS and will return to try it again down the road but it's not for me today.

Oh, after reading up on RAID mirrors more I saw that, using a disk that's part of a mirror as a backup just doesn't make sense. There are some articles on the web that talk about the pitfalls. I wouldn't go that route, it's the wrong tool for the job. But - I learned a lot in the process of reading and trying.