need Parody clarity + direction w/ drive(s) setup

solstormedia · Aug 27th 2021

If I had four 2TB drives, identicals, three of them in a pool, one outside of pool as parity. How can a total of 2TB be able to compensate for/ recall for potentially six TB of data.

Then if a 2TB chunk of the 6TB fails (single drive), how can the parody fill in that 2TB chunk? Maybe I am thinking of it too much like Raid redundancy. I messed with Raid when it was first being marketed. Moving on.

60GB SSD - OMV System

1TB SSHD - "workspace" "download, writing space. etc

2TB Black

2TB Green

1TB Green

What to do? Based on many readable sources, 2TB Green as parody, Both 2TB Blacks + 1TB Green in pool. Use of raidsnap, merger, unionfilesystem. At least those are where the trails lead me.

Are these pool+parody combos another way of achieving similar goals as RAID, but different? Have to be one or the other?

Because I am concerned a little bit of the performance of the green drives. I want 1TB SSHD on its own . striped 2TB Black total of 4TB for archiving, and the green drives to mirror/redundancy/backup. BUT that is only 1TB+ 2 for combined of only 3TB.

My demands are not as high or complicated as I sometimes try to convince myself. Bored, challenge, what not.

Those are my cards to play. Please suggest something simple that makes the most of these hardware. Thank you!

added: I don't even know if striping the Black drives would do a gain that is worthy of the effort. They were marketed as performance drives. This is where experienced and knowledge can answer.

Adoby · Aug 27th 2021

If you have several drives you can look at a certain bit in all these drives. And check to see if it is a 1 or a 0. Ones or zeros.

If you count all the ones at the same position on all drives, you get either an odd or an even number of ones. This is the parity. Using this information, if one single drive is missing, you can figure out if it was a one or a zero.

For instance, if the old parity was even and the new parity, with one drive missing, now is odd, the missing drive had to have held a 1 at that location.

https://en.m.wikipedia.org/wiki/Parity_bit

RAID use parity this way, in real time. This has some benefits and some drawbacks.

Snapraid also use parity in this way, but not in real time. This has some other benefits and some other drawbacks.

I'd use snapraid. One 2TB hdd drive for snapraid parity. The rest of the hdd drives as a mergerfs pool. If you expand, in the future, you need to use a parity drive that is as large as the largest drive in the pool. So you need to buy at least two bigger drives, and use one of them for parity.

If one drive in the pool break, you will need a replacement, and then restore the contents.

As you build it it could be a great start to begin with one 2TB drive spare. And pretend that the 2TB drive in the pool broke. And restore to the spare drive. Experiment before things get critical... Afterwards you can add both drives to the pool and snapraid. (Format one of the drives to avoid duplicates.)

Participate now!