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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.
Learning2NAS wrote:
How do you know that "prosumer" products running OS6 are using block level RAID, rather than a software based file system RAID?
Software raid on linux is block-level. ZFS and BTRFS are the only file systems I know of that have attempted to integrate raid features into the file system itself (there could be others, but I don't know of them).
BTRFS's RAID integration isn't mature/stable. ReadyNAS definitely runs mdadm (https://en.wikipedia.org/wiki/Mdadm). You can see the layering partitions in the downloaded logs.
Test results are in. I found a useful blog post about shrinking linux software RAID sizes here (http://www.starcoder.com/wordpress/2011/08/shrinking-a-linux-software-raid-volume/). Those instructions allow you to easily shrink a partition, to remove a disk from the array, or both. However, that process only works smoothly with a manually configured RAID array. Since Netgear devices default to XRAID2 and create multiple layers on each drive, it is more complicated than those instructions. It is still theoretically possible to reduce the size of an XRAID2 and you would do it by repeating the steps in that blog post, however it is a time intensive process and there is no guarantee that it will work (in my case, I had a small stripe that was created by XRAID2 that was 100% full, and spread across several drives, so I could not resize that small stripe whatsoever. This proved fatal to my shrink operation, and made it fail.).
In the end, I was not able to resize my XRAID2 array without destroying the volume and having the device rebuild it. If this function is important to users, they should be mindful to build a Flex-RAID array when commissioning their devices (however, this is more complicated and should be reserved for somewhat experienced users).
I also tested the automatic functions of XRAID to confirm that it was unable to shrink a drive on its own. When you insert a drive smaller than the one that was there previously, the system just throws and error and continues to operate in degraded mode. It will not shrink a volume, automatically or otherwise.
I appreciate your time, Stephen. Hopefully this is helpful to someone in the future.
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 wrote:
With bitrot protection enabled, raid6 maybe is overkill.
RAID-6 gives you more availability on drive replacements (e.g., the volume stays up even if you get two disk failures).
But you still need backups, which is what keeps the data safe. I've chosen RAID-5 for the main NAS. If I were running a business using it, I might use RAID-6. But with my current usage, losing access to the volume for a couple days would be an inconvenience, not a disaster.
Bitrot protection doesn't provide the same protection features as RAID-6 (and vice versa). It can't protect against a double-disk failure. It is designed to recover a file that gets corrupted somehow on the volume.
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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