I have an array of disk with RAID 6 and 16 Drives. Days ago three disk failed and the Array was marked as Degraded. I cannot access the data and I cannot boot into the Operative System. I need access to the data but I cannot do anything. Any advice? How can I recover or access the data? Could I use a Live cd to boot an OS? I'm using SAS Disk. Thanks in advance
As said before, if more than two disks in a RAID-6 array die, the array is unrecoverable.
However, three simultaneous disk failures are quite an unlikely event: it might very well be a case of a faulty enclosure, backplane and/or controller.
You should try removing and re-inserting the disks, replacing the controller and/or the enclosure, and even putting the disks in a different server with the same controller (if you have one available).
As stated in the comment, RAID6 can sustain up to two disk failures; if a third disk fails, your array is toast.
The most obvious thing is to restore from backup. If this is not possible and at least one of the failed disk is still readable (albeit with read errors), you can try to do a block-level copy of each failed disk on another, healty disk (eg: via
ddrescue <failed_disk> <new_disk>) and to re-start the array using these copies (plus the other good disks).
You will end with a punctured arrays where some original data can be lost/corrupted; however, with any luck, the greatest part of data should be accessible.
If you have no backup and none of the failed disk is readable, you need to contact a data rescue service.
You probably don't have a software RAID, no matter what the tag says. You cannot boot OS from a software RAID6.
3 disks out of 16 failing together are quite rare occurence, except when you drop the server on the floor. It is either 3 disks failing one by one over a large timespan and no one noticing or a failed controller, failed cable, failed power supply, failed backplane or a firmware bug kicking in. It is important to determine which case you have, because the recovery strategy is different. There may be BIOS or RAID controller logs accessible.
In either case, you start by backing up every single disk on another media, using a different, known to work controller. In the process, you will see how many of the disks are actually broken and how much.
Most (probably all) hardware RAID controllers are crap. I learned the hard way. A "disk failed" condition may actually be a single bad sector and most (or even all) data could be recoverable.
A "degraded" array is an array that still has all the data accessible. What you describe is a "failed" or "offline" array, rather than "degraded". If you are not experienced in these matters, call someone who IS.
Starting from a recovery/live CD may or may not be a part of the process. If you don't know how to mount a filesystem in read-only mode, call someone who knows. It is possible to kill a perfectly recoverable data by such a mistake.
After a lot of sleepless nights I design my servers in such a way that everything stops working when the FIRST disk fails. THIS is the only error message that no one ignores.
RAID 6 can only survive two failed hard drives. If you do not have any backups and need the data, I would recommend hiring a hard drive recovery company. I would not try and recover the data on your own because the more you work the hard drives, the higher the chances are the data will not be recoverable.