Consider your self very lucky, i have seen over the years pretty much all, faulty hardware, buggy firmware, multiple disk failures causing raid arrays to brake down, and thats not because our customers buy shitty hardware even A-brand hardware vendors like Dell and HP have their share in storage issues.m.novelli wrote: RAID6 write penalty is too high IMHO for a storage dedicated to backups
In 15+ years of IT I've never lost a RAID5... All my servers and storage always have firmware updated (both raid card and disk firmware). I use only Dell PowerEdge servers. In the last years I've prepared many Veeam Backup servers with 2 - 3 - 4 and now 6 TB NLSAS disks, so far so good. I always use an hot spare. On a recent Dell PowerEdge T630 where failed a 6TB disk, the automatic hot spare coverage took about 8 hours to complete the rebuild (it was a RAID5 made of nine 6 TB disks)
I can't see why a disk that get intensive writes during backups should have an higher failure rate due to intensives reads in case of array rebuild...
Raid as we know and use is in my opinion outdated and we desperately need something better, which can handle those multi TB disks and their issues. One of them is the infamous unrecoverable read error, if a disk has unrecoverable read error rate of 10^14 it means that statistically every 12 TB read from this disk it will encounter a unrecoverable read error. Your 42 TB effective storage would statistically hit 3,5 of those errors in a single read pass. If such error so happens during a rebuild, the controller will halt the rebuild and offline the array, because there is no more parity available to recover from this error. The very lucky admin, can with assistance from the hardware vendor get the array glued back together and get a successful rebuild done, the less lucky ones can get the array online to evacuate as much data as possible and the unfortunate ones get the message that the data is considered lost. Because of this pretty much all vendors doesnt recommend using raid5 especially for business critical data or systems.