On Wed, Dec 6, 2017 at 7:13 PM, Frank Steinmetzger <war...@gmx.de> wrote: > On Wed, Dec 06, 2017 at 06:35:10PM -0500, Rich Freeman wrote: >> >> IMO the cost savings for parity RAID trumps everything unless money >> just isn't a factor. > > Cost saving compared to what? In my four-bay-scenario, mirror and raidz2 > yield the same available space (I hope so). >
Sure, if you only have 4 drives and run raid6/z2 then it is no more efficient than mirroring. That said, it does provide more security because raidz2 can tolerate the failure of any two disks, while 2xraid1 or raid10 can tolerate only half of the combinations of two disks. The increased efficiency of parity raid comes as you scale up. They're equal at 4 disks. If you had 6 disks then raid6 holds 33% more. If you have 8 then it holds 50% more. That and it takes away the chance factor when you lose two disks. If you're really unlucky with 4xraid1 the loss of two disks could result in the loss of 25% of your data, while with an 8-disk raid6 the loss of two disks will never result in the loss of any data. (Granted, a 4xraid1 could tolerate the loss of 4 drives if you're very lucky - the luck factor is being eliminated and that cuts both ways.) If I had only 4 drives I probably wouldn't use raidz2. I might use raid5/raidz1, or two mirrors. With mdadm I'd probably use raid5 knowing that I can easily reshape the array if I want to expand it further. -- Rich
