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Disk fail in X-RAID2, after sync half my files are gone!
Hi, I'm on a ReadyNAS NV+ v2, with RAIDiator 5.3.11. I had it configured for X-RAID2 with 3x 2GB drives and one older 512GB drive. A few days ago the 512GB drive failed, leaving the array unprote...
rabidh wrote:
It seems particularly unlucky that the replacement drive I put in was faulty. Having just read into it a bit, I wasn't aware that in most RAID systems if one copy of the data becomes corrupt then even though it is duplicated ...
In your case your NAS is using RAID-5. RAID-5 doesn't duplicate your data. Rather it uses parity blocks that allow it to reconstruct data when something is missing.
Putting this in mathematical terms: Imagine a 4-disk RAID-5 array. If disks 1,2, and 3 have A, B, and C data blocks at sector N, then the fourth disk would have P=A+B+C in that sector. (It doesn't use normal addition, but does something else that has the same effect). Then if the disk 3 is replaced, the NAS reconstructs C using P-A-B.
This only works if the remaining disks can all be read (and when all have the correct data). If a disk can't be read duiring reconstruction, then the reconstruction fails (and the NAS knows that). If a disk is read, but gives the wrong data, then the reconstruction gives the wrong result (and the NAS has no way to detect that). Similarly, if the wrong data was somehow written to one of the disks in the first place (or if a disk write was lost), then the reconstruction will fail (and there is no way to detect that).
rabidh wrote:
it'll probably still cause corruption ... and probably the more high end systems have options in place to work around that.
Once corruption happens, then there is risk of data loss - that's just as true in high-end enterprise/cloud systems as it is in home NAS.
High-end systems have some features which can reduce the chance of corruption happening in the first place. For instance
- Error-correcting RAM
- Dual Power Supplies to help ensure that a PSU failure doesn't result in lost writes.
- UPS protection
BTW, UPS protection is something I always recommend (for all NAS). Often data corruption occurs with unexpected power loss.
Also if you have more disks in the NAS, there are some advanced RAID modes that can handle more than one failed disk. There is a price for that (both reduction in capacity and lower performance). And they don't help if the wrong data is on one or more disks. They only help if the disk can't be read.
Newer OS-6 ReadyNAS (at all price points) do have some features that are relevant here. They have more scheduled maintenance functions, that can detect issues sooner. They also use a newer file system called BTRFS, which supports built-in checksums that can detect corruption. That also gives those NAS some more sophisticated options for reconstruction.
But for all storage (enterprise and home) the primary defense against data loss/corruption is to have independent backups - full copies of the data on other devices,
It looks like there were some smart errors on the disk that I put in as a repacement, so I pulled that, restarted with a filesystem check, and some of my files are back (not all). Is there anything else I can do short of sending the nas off, or pulling drives and spending $200 on ReclaiMe like you suggested?
Luckily I do have some of the more important files backed up elsewhere because I lost some confidence in my ReadyNAS a while back, but I have still lost data that I needed.
Is it Netgear's official position that you can lose data each time you swap a drive on a ReadyNAS? If so it seems like I should probably reconsider my storage choices.
rabidh wrote:
Is it Netgear's official position that you can lose data each time you swap a drive on a ReadyNAS? If so it seems like I should probably reconsider my storage choices.
Netgear's OS 6 software manual (page 231 of http://www.downloads.netgear.com/files/GDC/READYNAS-100/READYNAS_OS_6_SM_EN.pdf) says
If your data is important enough to store, it is important enough to back up. Data can be lost due to a number of events, including natural disaster (for example, fire or flood), theft, improper data deletion, and hard drive failure. If you regularly back up your data, you can recover your data if any of these situations occur.
If someone else tells you something different, and you actually believe them ... then I have a bridge to sell you :smileywink:
Data that is not backed up is always at risk. And that risk is higher when the RAID array is being resynced due to a disk replacement.
Businesses spend e.g. tens of thousands of dollars on servers and still recognise the need to backup any data that's important to them. WIth a few hundred dollar NAS we don't have some magic fix to prevent any and every possible problem.
When a disk fails in a single-redundant volume you're in a situation of heightened risk till the RAID has been rebuilt.
Using an unhealthy disk as the replacement disk isn't going to help either.
When a disk fails it's important to check that the disks still in the NAS look good, looking at the SMART stats. If other disks are going bad as well then that does affect what the best way forward is.
Some users test new disks using SeaTools for SeaGate or WD Data LifeGuard Diagnostics for WD disks before adding them. We do run a quick check of disks before adding them but that's not going to pick up any and every problem and sometimes it's quite subjective as to whether you want to use a disk or not. The disk manufacturer's tool can be used to run lengthy disk checks.Our software based data recovery services involve remotely connecting to NAS units. If the disks have to be sent off that adds considerably to the cost.
If you're wanting to use a data recovery service or data recovery software it's important not to keep on making changes to the data volume. Every change you make has the potential to reduce the chances of further attempts being successful.Backing up data can be significantly cheaper than data recovery attempts which may ultimately prove completely unsuccessful.
Thanks for the in-depth reply.
It seems particularly unlucky that the replacement drive I put in was faulty. Having just read into it a bit, I wasn't aware that in most RAID systems if one copy of the data becomes corrupt then even though it is duplicated, it'll probably still cause corruption. I guess that's what happened in this case, and probably the more high end systems have options in place to work around that.
Do you have a link to where the NetGear recovery service is? In my.netgear.com and 'purchase service contract' for my device I just see 'There are currently no service contracts available for this product' - I assumed because it was too old (6 years).
rabidh wrote:
So even though the data is duplicated in RAID, if one copy of the data becomes corrupt then the ReadyNAS struggles to detect which copy is the correct one?
No, but if there's a problem with another disk in the array adding a failing disk is going to increase the chance you'll have problems. The failing disk may fail to integrate into the array properly and the rebuilds do put heavy stress on the other disks.
rabidh wrote:
I guess as you say that's something that a more expensive business offering might sort out...
That's not what I said at all. What I was saying was that the NAS is not going to be magically not run into problems that devices that cost say a hundred times as much can still run into. RAID is great, but it's not a magic fix for any and every possible problem.
rabidh wrote:
Do you have a link to where the NetGear recovery service is? In my.netgear.com and 'purchase service contract' for my device I just see 'There are currently no service contracts available for this product' - I assumed because it was too old (6 years).
Hmmm. We should still offer data recovery contracts (non-refundable and may be completely unsuccessful).
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