Kasper Dupont wrote: > > Derek Vadala wrote: > > > > RAID-1 --------> RAID-5 (D0,D1,D2,D3,P0) > > |--> RAID-5 (D0,D1,D2,D3,P0) > > (four disks used for data, only one from each RAID-5 can fail) > > Wrong, any three disks can fail. If the one RAID has only > one faulty disk, the other RAID can have any number of > faulty disks without loosing data. > This is a bit excessive, you waste more than half your disks for 3-disk safety. Consider raid-5 on top of raid-5. You're still safe from any-3 failure, and the overhead in percent can be made arbitrarily small. Using nxm disks, you get (n-1)(m-1) data disks, and (n+m-1) parity disks. (The overhead (n+m-1)/((n-1)(m-1)) approach 0 as noth n and m grows towards infinity. Using many few-disk arrays to build the big array gives good chances to survive 4-disk failures too, as few of the many 4-disk combinations take out two small arrays simultaneously. I'm not sure about the write performance for such a beast, but it should be fine for reading. I.e. a safe archive. Helge Hafting - To unsubscribe from this list: send the line "unsubscribe linux-raid" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html
