I can do either of the following:

 

1. Add a 2nd parity drive.

2. Add a 2nd cache SSD.

 

My setup is a 7 disk, 10TB unRAID array with a single 480GB cache SSD. I have a couple VM's on the cache drive and appdata. I'm using the array for personal data and mostly media.

 

Thoughts? What would you do?

2nd parity if you don't have an urgent need for more cache, which, with what you've described, you don't.

If it were me, I'd:

[*]Make sure I had a solid backup strategy in place.  That's more important than either a 2nd parity or cache drive.

[*]Begin migrating in the direction of managing a smaller number of larger disks.  For instance, you could have a 12TB array with 3 6TB disks.  Dual parity is probably overkill in that situation.

[*]Add the 2nd cache drive.

 

If it were me, I'd:

[*]Make sure I had a solid backup strategy in place.  That's more important than either a 2nd parity or cache drive.

[*]Begin migrating in the direction of managing a smaller number of larger disks.  For instance, you could have a 12TB array with 3 6TB disks.  Dual parity is probably overkill in that situation.

[*]Add the 2nd cache drive.

 

I had a lot of spare drives, so I went with the "add drives" approach but I understand using less, larger drives. That will be my go-to as I move forward.

 

For backup, I plan to use Crashplan. I've been experimenting with it and Backblaze on my personal computer, and both have their merits. Since I can do Crashplan through a docker and backup parts of my unRAID that way, that's the plan.

I had a lot of spare drives, so I went with the "add drives" approach but I understand using less, larger drives. That will be my go-to as I move forward.

 

For backup, I plan to use Crashplan. I've been experimenting with it and Backblaze on my personal computer, and both have their merits. Since I can do Crashplan through a docker and backup parts of my unRAID that way, that's the plan.

Makes sense.  I mention those things because I'm guessing that you have an array of 1TB and 2TB drives.  I just don't think $100 for another 2TB drive is a great purchase.  Another SSD is a good purchase, as are 3,4, and 6TB drives.  So is an 8-10TB drive in a USB enclosure to act as a backup.

Drives are 1 and 2 TB as you guessed.  Parity, however is 4 TB. Another 4 TB is an investment as they will eventually replace the 2's most likely.

 

If I can back up the cache (VMs and appdata) to the array or cloud then it's protected enough for me.  Having the extra parity protection seems like a good idea.

 

...and I'm sitting outside the computer store.

For small array like yours, double parity is not really cost effective. You might be better off buying a high capacity and use it as parity, and use current parity as data. Or buy an external USB 3.0 8TB as backup.

Well, both were such a good deal at a local store that I might now have to figure out what to do with the 2nd 4TB drive. Clearly the 2nd 480GB SSD is going into the cache pool. 

 

What is the true value of the 2nd parity drive, then? (Maybe this 2nd 4TB needs to become storage.)

Given the choice you originally outlined, I'd go with the 2nd cache drive, as this will give you cache fault-tolerance.

 

But doing BOTH is even better 

 

r.e. "... What is the true value of the 2nd parity drive ..." ==> It's simply added insurance.  It gives you significantly better fault tolerance, as you can then have two drives fail with no data loss.    You shouldn't, of course, wait for a 2nd drive to fail before replacing a failed drive; but with dual parity if a 2nd drive was to fail while you were rebuilding another failed drive, the rebuild would still be successful -- with single parity if that happened you'd lose the data from both of the failed drives.

 

If you have a good backup strategy and all of your data is backed up, then the value of the 2nd parity is lower, since if you did have a failure as I just outlined, you could simply replace both failed drives, and copy the data that was on them back to the array from your backups.    But dual parity is certainly more convenient.

 

If you decide to add another cache drive for a cache pool, the default btrfs raid1 will give a mirror so you get redundancy, but with only 2 drives it will only be as large as the smaller of the 2, so it is best to have them the same size.

 

See the unRAID 6 FAQ sticky at the top of this subforum for more about cache pools.

All good points.

 

FYI: The 2nd SSD is the same size as the original cache SSD, so I'm covered there. Now the waffling on what to do with the 2nd 4TB drive.

 

How easy is it to pull out the 2nd parity drive and go back to single parity once it's been activated? i.e. I put in 2nd parity until my initial backups are complete, then yank parity 2 and turn it into storage space.

 

 

How easy is it to pull out the 2nd parity drive and go back to single parity once it's been activated? i.e. I put in 2nd parity until my initial backups are complete, then yank parity 2 and turn it into storage space.

You would just set a New Config without parity2, and check the box that says parity is valid, since it would be.

How easy is it to pull out the 2nd parity drive and go back to single parity once it's been activated? i.e. I put in 2nd parity until my initial backups are complete, then yank parity 2 and turn it into storage space.

You would just set a New Config without parity2, and check the box that says parity is valid, since it would be.

 

So the two parity drives are independent of each other, right? Either one is a valid parity of the entire system?

How easy is it to pull out the 2nd parity drive and go back to single parity once it's been activated? i.e. I put in 2nd parity until my initial backups are complete, then yank parity 2 and turn it into storage space.

You would just set a New Config without parity2, and check the box that says parity is valid, since it would be.

Note that you would add the old parity2 to a new data slot, or use it to replace an existing, smaller data disk, after you had already set the New Config, not at the same time.

So the two parity drives are independent of each other, right? Either one is a valid parity of the entire system?

That is true, but parity2 is a different calculation that depends on drive order, so if you change drive order, parity would still be valid, but parity2 would not.

 

If you do remove a parity drive, best to remove parity2, or everyone is going to get confused.

If you do remove a parity drive, best to remove parity2, or everyone is going to get confused.

 

This is very good to know. Is this described somewhere? I couldn't find much on Parity 2. (Or I missed it.)

If you do remove a parity drive, best to remove parity2, or everyone is going to get confused.

 

This is very good to know. Is this described somewhere? I couldn't find much on Parity 2. (Or I missed it.)

When I say "everyone", I mostly mean anyone who looks at your syslog. Parity is assigned as disk0 and is the first in the list when the drive assignments are shown, but parity2 appears as disk29, way down the list after all the others, including all the unassigned ones.

 

As far as I know, there is no problem with having parity2 as your only parity disk, except as noted about the disk order. Also there have been reports of performance issues with the parity2 calculation on older, lower end processors.

New config works but it's not needed for this, just stop the array, unassign parity2, start the array and you're done.

How about when adding in Parity 2 to an already working array? I stopped the array, assigned the new disk as Parity 2, then restarted the array and it's rebuilding the parity on that drive. Is that all good? I don't need a new config for that?

 

Thanks again.

Yes, that's the way to do it, no need for new config.