Confused about Raid

[gravity]

Hi All
Whilst I know a bit about computers im a little new to Raid and X Raid.
I've been doing some reading but am a little confused so excuse me if I get something wrong!

I've got a ReadyNAs Pro:
Hostname: nas-EA-81-29

Model: ReadyNAS Pro Business Edition [X-RAID2]
Serial:
Firmware: RAIDiator 4.2.19
Memory: 1024 MB [6-6-6-18 DDR2]

IPv4 address: 1: 192.168.0.11
IPv4 address: 2: Not Connected

Volume C: Online, X-RAID2, 4 disks, 0% of 5560 GB used

The disks im using are: 2x2tb and 2x3tb

What I wanted to do with my system was load on all of my video files and then access this through my home network on my tv's via netgear media players. Whilst having a backup of each.
So basically what I wanted to do was use 1x2tb and 1x3tb for all my files and then i was under the assumption that the other disks would copy these files and if 1 failed i simply remove the failed drive and load a new one and it will copy over. Therefore giving me 5tb of storage with a backup.

How do I do this in the settings or is there a better way to do this and is that what Xraid2 is all about? Ive watched the Xraid2 video showing capacities etc. but i dont understand. Im under the assumption that to have backups of disks effectively your at 50/50 eg 1tb and 1tb backup. Now using raid 2 i get the impression that your largest disk size, in my case 3tb you would be needed again for redundancy which i assume is another term for back up. so if i use my 2x3tb and 2x2tb i would get 7tb of storage. what I dont get is how can it fix a failed drive. eg a 3tb drive goes down and i put a new 3tb drive in will i still have all 7tb of data safe? (what im getting at i dont understand how (if im correct in understanding) it can back up 7tb of data on just a 3tb drive)
im not sure if im really confused or am right or have a poor understanding of this all which i apologise for.

Any help, advice, links or guidance would be appreciated.

[mdgm]

Volume C: Online, X-RAID2, 4 disks, 0% of 5560 GB used

The disks im using are: 2x2tb and 2x3tb

Ah, you should have two layers. A 4x2TB RAID-5 layer and a 2x1TB RAID-1 layer. If any one disk fails data remains intact. It looks like you currently have the 4x2TB RAID-5 layer and after a reboot or two your volume will expand to add the 2x1TB RAID-1 layer.

What I wanted to do with my system was load on all of my video files and then access this through my home network on my tv's via netgear media players. Whilst having a backup of each.

RAID is not a backup. It provides redundancy or high availability. There are some problems RAID cannot protect you against which indicates it's not a backup. See Preventing Catastrophic Data Loss

So basically what I wanted to do was use 1x2tb and 1x3tb for all my files and then i was under the assumption that the other disks would copy these files and if 1 failed i simply remove the failed drive and load a new one and it will copy over. Therefore giving me 5tb of storage with a backup.

Still not a backup, and with X-RAID2 if any one disk fails you replace it and a resync will occur to restore the redundancy.

redundancy which i assume is another term for back up.

No redundancy or high-availability is very useful, but it refers to the ability to replace a failed component without needing to resort to restoring from backup.

so if i use my 2x3tb and 2x2tb i would get 7tb of storage. what I dont get is how can it fix a failed drive. eg a 3tb drive goes down and i put a new 3tb drive in will i still have all 7tb of data safe?

Yes.

(what im getting at i dont understand how (if im correct in understanding) it can back up 7tb of data on just a 3tb drive)

Simple answer it's not backing up the data. The data is spread across the disks so that if any one disk fails data remains intact.

Let's call the 2TB disks, disk A and B and the 3TB disks disks C and D

Now for the RAID-5 layer, we have:
A = 1/3 B + 1/3 * 2/3 C + 1/3 * 2/3 D
B = 1/3 A + 1/3 * 2/3 C + 1/3 * 2/3 D
2/3 C = 1/3 A + 1/3 * B + 1/3 * 2/3 D
2/3 D = 1/3 A + 1/3 * B + 1/3 * 2/3 C

For the RAID-1 layer we have
1/3 C = 1/3 D

im not sure if im really confused or am right or have a poor understanding of this all which i apologise for.

A lot of people are confused about this. Glad you asked.

Any help, advice, links or guidance would be appreciated.

You're welcome.

[gravity]

Hi mdgm
THanks for your quick and detailed reply

"Ah, you should have two layers. A 4x2TB RAID-5 layer and a 2x1TB RAID-1 layer. If any one disk fails data remains intact. It looks like you currently have the 4x2TB RAID-5 layer and after a reboot or two your volume will expand to add the 2x1TB RAID-1 layer."
Yep your right! the readynas is busy and atm is:
Volume C: Online, X-RAID2, 4 disks, 0% of 6471 GB use

Thanks for the link to the great article. There will be some important files and I was going to use the readynas vault backup service for those or something similar. I did realise it wasnt the best backup i was more thinking of just having that 2nd copy should a hard drive fail. the files are only movies and tv shows so should there be a catastrophic failure it wont be the end of the world!

"No redundancy or high-availability is very useful, but it refers to the ability to replace a failed component without needing to resort to restoring from backup."
in regards to this im trying to get my head around it. this may seem odd but if you catch my drift, one way i thought of would it be like this one harddrive would have lets say part of a file that reads 1+2= and another harddrive have say the answer and if the hardrive with the answer failed when i load a new harddrive it could figure it out because 1+2=3 (i know thats not how files etc work but just trying to dumb it down so I can get my head around it when my forte isn't raid and files etc!)

So summing it up to ensure i fully understand this and have got my head around it! with my 4 hardrrives of 2x2tb and 2x3tb I will have approx 6.5-7tb of storgae and if one drive fails I can replace it and still have my data intact. however it obviously wont survive catastrophic failure eg fire, theft or 2 hard drive failures at once. Therefore critical files should be backedup via readynas vault or similar?

[mdgm]

So summing it up to ensure i fully understand this and have got my head around it! with my 4 hardrrives of 2x2tb and 2x3tb I will have approx 6.5-7tb of storgae and if one drive fails I can replace it and still have my data intact. however it obviously wont survive catastrophic failure eg fire, theft or 2 hard drive failures at once. Therefore critical files should be backedup via readynas vault or similar?

Yes. Or backed up some place else. You might find you have some non-critical data you would backup e.g. to a USB disk that you take offsite.

[vaultservices]

Hi Gravity. I'm Aaron from ReadyNAS Vault. It sounds like you may have some questions about the Vault. If you'd like further assistance please feel free to contact me at aaron@vaultservices.net.

Best Regards,

Aaron Belsh
Business Development
ReadyNAS Vault Services