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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.
I tried another experiment and got a positive result. This one is not as interesting as the original, but it is worth letting the community know about in case this ever helps someone in the future.
I created an XRAID2 volume using wildly different sized drives (40GB, 300GB, and 500GB). XRAID, predictably, decided to create several different layers. It ended up doing the following:
1 40GB layer that ran across all three drives (RAID 5)
1 260GB layer shared between the two largest drives (RAID 1)
That decision left 200GB of unused space on the largest disk in the array. I have been repeatedly told that you cannot replace a disk with a smaller disk, but in this situation it would seem silly not to allow a smaller disk to fill in for the 1/3 empty 500GB drive.
I pulled out the 500 after the array finished bulding and hot-swapped for a 320 disk (which was 20GB larger than it needed to be to accommodate all of the data on the 500). Although I had just swapped a larger disk for a smaller disk, the ReadyNAS unit accepted it and rebuilt the array onto the smaller disk. I don't know how often this will be helpful, but it is good to know that the drive looks for the necessary capacity, rather than an exact replacement. Makes sense, but it's good to have confirmation.
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