Difference between JBOD, Spanning, and RAID 0

[lishaohua]

I just got a My Cloud Mirror and don't plan on using the mirroring function. I went to the device configuration and saw that there were four options: JBOD, Spanning, RAID 0, and RAID 1. I know that the RAID 1 is the mirroring option, but I'm not sure which one of the other three to select.

Someone had previously tried to explain to me the difference between JBOD, spanning, and RAID 0 - but I couldn't quite grasp it. If anyone here could clarify to me the difference between the three - so I know which one to select - I'd very much appreciate it.

 

[Sakkura]

RAID0 is striping, so when your system needs to read 2GB of data it can read 1GB from each drive simultaneously, doubling the effective speed.

Spanning just makes the independent drives show up like a single drive in the OS, but there's no mirroring or striping going on - they're still functioning mostly as separate drives. And JBOD can either refer to spanning or, more likely in this case, simply separate drives.

 

[lishaohua]

RAID0 is striping, so when your system needs to read 2GB of data it can read 1GB from each drive simultaneously, doubling the effective speed.

Spanning just makes the independent drives show up like a single drive in the OS, but there's no mirroring or striping going on - they're still functioning mostly as separate drives. And JBOD can either refer to spanning or, more likely in this case, simply separate drives.

I think I get the difference between RAID 0 and spanning now. RAID 0 writes a file across both disks so it's faster, while spanning would put the file entirely on one disk, so it's slower. I think that also means that in RAID 0, if one disk goes bad, the other disk becomes useless, since files are written across both disks; but in spanning, if one disk goes bad, the files on the other disk can still be used. Is that correct?

Regarding spanning and JBOD, is there a difference between the two? When I had searched on-line, I saw somewhere that someone was asking a similar question, and also couldn't understand why there seemed to be two options that were seemingly the same. Here is what the options look like on the My Cloud Mirror:

 

[lishaohua]

Okay, I think I understand now. Spanning is having the drives act as one drive, and JBOD is treating the drives as individual drives.

Let me try to use a simple example to see if I'm understanding the three options correctly - and anyone is free to correct me if I'm mistaken on anything.

Let's say there are two drives, drive A and drive B, each with 5 "units" of capacity (A1, A2, A3, A4, A5, B1, B2, B3, B4, and B5), onto which 3 files - file X, file Y, and file Z - each of them 2 "units" in size, will be stored.

For RAID 0, file X will be stored on A1 and B1, file Y will be stored on A2 and B2, and file Z will be stored on A3 and B3. If either drive goes bad, then all the data is lost. If a backup is desired at some point, either two drives of 5 "units" capacity or one drive of 10 "units" capacity will be needed.

For spanning, file X will be stored on A1 and A2, file Y will be stored on A3 and A4, and file Z will be stored on A5 and B1. If drive A goes bad, all three files will be lost. If drive B goes bad, file Z will be lost but files X and Y will still be okay. If a backup is desired at some point, both drives will need to be duplicated (either with two drives of 5 "units" capacity or with one drive of 10 "units" capacity) in order to backup file Z, but only drive A needs to be duplicated in order to backup files X and Y.

For JBOD, file X will be stored on A1 and A2, file Y will be stored on A3 and A4, but since the two drives are considered separate, file Z won't be able to fit on drive A and will have to be stored on drive B - in B1 and B2. If drive A goes bad, files X and Y will be lost but file Z will be okay. If drive B goes bad, file Z will be lost but files X and Y will be okay. If a backup is desired at some point, drive A will need to be duplicated in order to backup files X and Y, and drive B will need to be duplicated in order to backup file Z.

I'm sure this is an oversimplification of things, but I hope I am understanding the basics correctly. Please do correct me if I'm mistaken on something or missing something.

 

[Sakkura]

Yeah you pretty much have it. Though I'm not sure a failed drive will lose all data with spanning; if the second drive fails, it might still be possible to access the files that are on the first drive.

 

[lishaohua]

I'm glad I finally figured it out. I chose your initial response as the "solution." Thanks, Sakkura.

 

[Sakkura]

Cheers.