On Tue, Feb 14, 2023 at 3:27 PM Heinz Mauelshagen <heinzm@xxxxxxxxxx> wrote: > > > > ...which is RAID1 plus a parity disk which seems superfluous as you achieve (N-1) > resilience against single device failures already without the later. > > What would you need such parity disk for? > > Heinz > I thought that at first too, but threw that idea out as it did not make much sense. What he appears to want is 8 linear non-striped data disks + a parity disk. Such that you can lose any one data disk and parity can rebuild that disk. And if you lose several data diskis, then you have intact non-striped data for the remaining disks. It would almost seem that you would need to put a separate filesystem on each data disk/section (or have a filesystem that is redundant enough to survive) otherwise losing an entire data disk would leave the filesystem in a mess.. So N filesystems + a parity disk for the data on the N separate filesystems. And each write needs you to read the data from the disk you are writing to, and the parity and recalculate the new parity and write out the data and new parity. If the parity disk was an SSD it would be fast enough, but if parity was an SSD I would expect it to get used up/burned out from all of parity being re-written for each write on each disk unless you bought an expensive high-write ssd. The only advantage of the setup is that if you lose too many disks you still have some data. It is not clear to me that it would be any cheaper if parity needs to be a normal ssd's (since ssds are about 4x the price/gb and high-write ones are even more) than a classic bunch of mirrors, or even say a 4 disks raid6 where you can lose any 2 and still have data.
