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X-RAID 2 Operation and Downsizing
I've tested answers in other posts on my NAS and found some of them to be incorrect, so please cite when you can to support your response. Questions: (1) If I lose a disk in a RAID-5 and don't h...
Learning2NAS wrote:
Hey Stephen,
Thanks for answering my questions. You addressed everything in my original post, but I have a clarification to your response to #2 (quoted below). Using your hypo, if I have a base layer of 1TB that is always there and my current array is 1x1TB (original disk that hasn't died yet) and 2x3TB, can I replace the 1x1TB with another 1TB? Additionally, can I replace the 3TB with a 1TB if it fails? It sounds like the answer to that second question is no, because there might be data stored somewhere in the 2TB layer, but I just want to verify this.You can replace the 1 TB drive with another 1 TB drive. But you can't replace the 3 TB drive with a 1 TB drive.
Learning2NAS wrote:
If that's true, that brings me full circle back to what I'm having such a hard time understanding: when you have a failure in RAID 5 there is enough redundancy to rebuild an effected file from the remaining drives (that's the point, right?). With this being so, I don't understand why the RAID can't rebuild the missing files on any size drive I stick in, so long as it is at least large enough to support the amount of data that needs to be striped to the new drive.
The RAID software sits between the physical disks and the file system - and has no idea about how the file system is organized (e.g., where files are stored, and where there is free space; or even what the file system is). When you replace a drive, the RAID software reconstructs all the disk blocks on the drive from the blocks on the other drives (whether there is data in them or not). Likewise, a resync always takes the same amount of time, no matter if the file system is full or empty. So RAID treats all data blocks on the drives identically (used or not).
There potentially are some advantages to fully integrating RAID-like redundancy into the file system itself - then features like the ones you are envisioning would be easier to do. The BTRFS team is working on a design like that, but it is still experimental - so Netgear (wisely) isn't using it. If they were to switch to it, it would likely require a factory reset.
Regarding Flexraid: on OS6 you can change from xraid to flexraid and back. So that may be a route to test something.
However it seems that a volume that has been expanded using flexraid can NOT be switched back to xraid.
Thanks for the link. If I can find some time, I'd like to try to change an OS6 raid6 volume 'back' to raid5 and use the remaining drive for space.
With bitrot protection enabled, raid6 maybe is overkill.
IcyK,
I agree with Stephen, RAID 6 and Bitrot protection are for different kinds of failures. There are certainly situations where it makes sense to have a RAID 6 with Bitrot protection turned on, but it depends on your use case. Unless you're taxing your unit heavily, I would advise you to leave Bitrot protection on because it's a good insurance policy for your data.
However, with regard to your question about RAID 5 or RAID 6, I have another link to give you. Take a look at the "Ideal Uses" section under each RAID level. http://www.prepressure.com/library/technology/raid If you're trying to warehouse data for as little money as possible and don't have an enormous array (10+ disks) or high capacity disks that take a long time to rebuild (3TB +), RAID 5 is probably the right choice.
However, if you're trying to store mission critical information where downtime is unacceptable or where data loss is unacceptable, RAID 6 is the best option. I've seen a lot of people use RAID 6 for volumes that have their operating system installed on them, for example, and RAID 5 for their storage volumes because it provides a more efficient use of their disk space. That said, you just have to determine how much risk you're willing to accept. Less risk? RAID 6. Some risk? RAID 5 and get a bit of bonus storage for your trouble.
Note that this discussion is off topic for this thread, and you should post elsewhere if you have additional questions.
Take care
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