Consider a scenario with a degraded RAID 1 array comprised of two 1.6 TB disks capable of transferring data at a sustained rate of 6 Gbps: you should be able to recover from a single disk failure in just over half an hour.
Repeat the same scenario with 32 TB members, now we’re looking at a twelve hour recovery - twelve hours of intensive activity that could push either of your drives over the edge. Increasing data density actually increases the risk of data loss.
The speed and method you use recover from data loss is not relevant to the discussion of how to handle drive failure. That varies wildly depending on your specific setup.
Finally, we say you shouldn’t think of RAID as a backup because the entire array could fail, not for the excruciatingly literal reasons you are attempting to convey. If you lose half of a two disk mirror set, you haven’t lost any data.
My premise is that reducing the number of drives reduces the risk of drive failure which could lead to data loss. RAID is not a backup, because it literally isn’t. If you have two drives in RAID1 you have 1 set of your data. If you have 4 drives in RAID6 you have 1 set of your data. In both examples you have a single very durable drive, but you do not have a backup. A backup prevents data loss, RAID does not.
Think of it this way. You have a single very large drive, and you explicitly only use 1/2 of it. The other 1/2 of the drive becomes broken and you cannot read or write to it. The first 1/2 work perfectly fine, and fits all your data. Would you consider this drive functional, or failed? A RAID degradation is a warning to the user that a portion of the single drive is broken, and needs to be repaired. A RAID block device should always be treated as a single physical drive, with varying levels of durability and warning signs depending upon its configuration. It can’t be a backup, because all its doing is delaying the eventual failure. Delaying a failure does not prevent the failure from happening, and does not help you when a failure occurs.
Oh thanks for the tip! I’ve edited my comment to reflect the minimum of 4 drives for a RAID6 array.
I’ve not used RAID6 for a small array like that before so I didn’t know it had a conventional lower limit. From the technical sense it doesn’t have to have 4 drives, it just wouldn’t make any sense to use it that way so I see why software wouldn’t support such a use case.
Drive 1: A, Drive 2: 1/2 A, Drive 3: 2/2 A. Drive 2 + Drive 3 = Drive 1. Hmm that would only be one set of the party though. So you could also add 1/2 of A to Drive 1, and 2/2 to Drive 2 so that the parity on Drive 1 + Drive 2 = Drive 3. Which is extremely silly, and doesn’t make a lot of sense to use in the real world.
The speed and method you use recover from data loss is not relevant to the discussion of how to handle drive failure. That varies wildly depending on your specific setup.
My premise is that reducing the number of drives reduces the risk of drive failure which could lead to data loss. RAID is not a backup, because it literally isn’t. If you have two drives in RAID1 you have 1 set of your data. If you have 4 drives in RAID6 you have 1 set of your data. In both examples you have a single very durable drive, but you do not have a backup. A backup prevents data loss, RAID does not.
Think of it this way. You have a single very large drive, and you explicitly only use 1/2 of it. The other 1/2 of the drive becomes broken and you cannot read or write to it. The first 1/2 work perfectly fine, and fits all your data. Would you consider this drive functional, or failed? A RAID degradation is a warning to the user that a portion of the single drive is broken, and needs to be repaired. A RAID block device should always be treated as a single physical drive, with varying levels of durability and warning signs depending upon its configuration. It can’t be a backup, because all its doing is delaying the eventual failure. Delaying a failure does not prevent the failure from happening, and does not help you when a failure occurs.
You can’t have a three drive RAID 6 array.
Please just stop.
Oh thanks for the tip! I’ve edited my comment to reflect the minimum of 4 drives for a RAID6 array.
I’ve not used RAID6 for a small array like that before so I didn’t know it had a conventional lower limit. From the technical sense it doesn’t have to have 4 drives, it just wouldn’t make any sense to use it that way so I see why software wouldn’t support such a use case.
Please explain how you think you can distribute two sets of parity data across a three drive array?
Drive 1: A, Drive 2: 1/2 A, Drive 3: 2/2 A. Drive 2 + Drive 3 = Drive 1. Hmm that would only be one set of the party though. So you could also add 1/2 of A to Drive 1, and 2/2 to Drive 2 so that the parity on Drive 1 + Drive 2 = Drive 3. Which is extremely silly, and doesn’t make a lot of sense to use in the real world.