untypical HDD demages. Is RAID 1 is immune?

[bartek158]

Hello
I'am not HDD specjalist and I would like to know opinion of Raid 1 recovery specjalist.
I was talking with sombody who work in recovery field and I'am in a state of confusion.
As this specjalist explain it can occour that one disc can have some untypical problem but Qnap will not recognize it. Qnap will be able to read data from second disc. But in case that after some time this second disc are damage and user thing that it is not a problem becouse he has the first one disc but this is wrong. Some part of the first one will be damage also ( only some small part) and you can't read 100% of data but only ex 99%. He was talking something about PLIST. IS this true that some untypical problems can be disguise by RAID 1 ? This specjalist sad that for some reasons one bay NAS has adventage then RAID 1 becouse in some untypical problems user will be seen immediately that there is problem. In raid 1 some problems can be hidden. Of course thise one bay solution will be with online backup.
As I anderstand whot this specjalist sad He was talking about some plist problems. How offetn it can occour ? It is very interesting and I spend the third evening to find some information about this kind of damage ? Could somebody say something more about this ? I know that Raid 1 is not 100% safety becouse theft , fire , raid array problems but I mean problem with HDD here

[DR-Kiev]

Reliability Raid1 from one server to raid1 from another is different. It is matter how controller's firmware deal with the problem when 1 drive die.

[data-medics]

Well, I don't think the PList is any concern as that's generated at the factory when the drive is made and never changes after that. Perhaps it was the GList that your friend referred to, which is the grown list of defective sectors which need to be reallocated by the drive to another sector. I've never really seen or heard of that becoming a real issue with a RAID 1 though. More often the issue I've seen is that one drive will develop minor issues such as S.M.A.R.T. errors, and be taken out of the array. For a typical motherboard RAID 1 the only indication of this will be a message at boot up which many people tend to just ignore. Then a year or two later the second drive fails completely and it reverts back to the first disk which has been offline for some time and is now out of date.

In fact I just finished up a recovery today that was that exact case and we had to recover the drive which had totally failed.

[DR-Kiev]

The major problems on RAID 1 apart from someone stealing your drives and or a fire would be other physical issues that would damage the 2 drives at the same time like a nasty power surge, or something like a ransomware virus that would encrypt your data.

RAID 1 protects against one drive failing but will not protect you in case something is "erased" or re-written by software/os as those changes will be made on the 2 disks at the same time.

Also if one drive is dropped out of the array you should replace it otherwise you will no longer have RAID 1.

I disagree. The major problems on RAID 1 when one drive died long ago , and once another drive get first hard bad sector , server crash , and after reboot, automatic or force rebuild process makes migration from older drive to a last one.

[lcoughey]

I just did a RAID 1 recovery where one drive failed 2 years ago. The second drive just recently failed. End user or tech figured that the non booting system was "fixable" and forced things back online where failed drive from 2 years ago overwrote recently failed drive...with the exception of a few bad sectors.

[rogfanther]

Well, as a matter of good practice, paying attention to the controller alerts about errors and the status of the drives would help a good amount in preventing this problems, or not ?

[data-medics]

Well, as a matter of good practice, paying attention to the controller alerts about errors and the status of the drives would help a good amount in preventing this problems, or not ?

That's putting a lot of confidence in the end user. And it still doesn't protect against ransomware, sabotage, accidental deletion, etc. RAID is not backup - EVER! RAID is for uptime when you can't afford long periods of downtime. It's not meant to be a substitute for a backup of important data.

[Matth79]

If you have a partially failed RAID, then you don't have RAID any more!

Allowing a RAID 1 to run with a drive failed, tripped out of the set for any longer than it takes to provide a replacement is folly.

PS. I managed to nearly wreck a RAID 5 installation on a Microvax - the controller / drive bay unit had 3 PSUs (2 required, 1 as redundant) and I'd only switched on one of them! - and I was the one that specified the extra PSU, still it proved the system would have died if it lost one of two.

One of the drives corrupted and dropped out, leaving the set in degraded mode, fortunately after correcting the power, forcing re-initialize of the drive brought it back to life