RAID 5 Concern

I am aware that if two of the drives failed on a RAID 5 we will loss the data. But, I have a weird issue where we have a RAID 5 setup with 4 disks. Two of the drives went missing as shown on "VDM before reseating the controller" image below.
VDM before reseating the controller
And, after reseating the controllers and the drives it detected all drives except one as shown on "VDM after reseating the controller" image below.
VDM after reseating the controller
Can we still rebuild the RAID 5? Any help will be much appreciated!
RebootincAsked:
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Paul MacDonaldDirector, Information SystemsCommented:
As long as the array recognizes 3 of the 4 original drives as good, you can rebuild the array.  

I would replace the currently bad drive, rebuild, then replace the drive that failed-and-then-came-back, then rebuild again.  That second drive might be flaky.
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rindiCommented:
What caused you to reseat the controller? Did you have a defective disk before you did that or not? After reseating did you boot up further or did you just go to the controller's settings?

The disk that is missing now, was it the one that was defective before reseating?
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DavidPresidentCommented:
Well, technically your statement, "I am aware that if two of the drives failed on a RAID 5 we will loss the data." is much too optimistic.

You can easily lose data with just one drive failure.  All it takes is inconsistent parity or an unreadable block on any of the surviving disks and you lose data.

That is why it is vital to do weekly data consistency (XOR) checks/repairs.  

You need to do a data consistency check NOW.  You very well could have this very problem now. You might even have unrecoverable data loss and not know it.
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RebootincAuthor Commented:
hi rindi,

What caused you to reseat the controller? Did you have a defective disk before you did that or not? After reseating did you boot up further or did you just go to the controller's settings?
- Yes, we only had one defective disk before this happened. We actually replaced the controller as our onsite tech thought it could be a bad controller. After changing the controller, putting same drives and importing foreign configurations; the server would not boot further.

 The disk that is missing now, was it the one that was defective before reseating?
- Yes.
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RebootincAuthor Commented:
hi paulmacd,

we tried replacing the bad drive but controller detects the new drive as missing still. we are at a point of looking for the exact brand and model of the missing drive.
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Paul MacDonaldDirector, Information SystemsCommented:
"...but controller detects the new drive as missing still."
Then it could be that the drive itself is good, but that port on the controller is bad.  Do you have an unused port on your controller you can plug the "bad" drive into to see if it starts working?  You might try changing cables, too.
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skullnobrainsCommented:
@dlethe

You can easily lose data with just one drive failure.  All it takes is inconsistent parity or an unreadable block on any of the surviving disks and you lose data.

actually this may well happen with ZERO drive failure and is a plague to many old raid5 systems running on poor drives that do not preserve consistency. if the parity goes bad, you'll hardly be aware in many implementations ( at least not when reading ) and even if you're notified, the raid can't determine which drive has the proper data.

---

you might want to allow more time for the disks to start spinning and try a number of soft restarts ( reset without power down ) so 3 of the drives are detected.

and if that happens, you'd better backup before you attempt to rebuild the raid. rebuilding/backuping might destroy what is left of the data if the drives are dying.

backuping the filesystem rather than the disk blocks won't allow you to boot but should put much less strain on the system and making the system bootable afterwards should only be a matter of running an automatic repair
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DavidPresidentCommented:
There is no way to confirm whether or not a rebuild/reconstruction will be successful based on the information available.
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skullnobrainsCommented:
agreed

@dlethe : just noticed i accidentally revived a very old thread. sorry about that. see you around
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skullnobrainsCommented:
simple question, simple answer.
sorry all for accidentally reviving this thread after 6 months.
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