I am trying to install debain 9 and have a question about partitioning. I install Debian via uefi and use software RAID which distributes my root filesystem and swap partitions to my hard disks. However, I don't know if I have to create an efi partition or a biosgrub partition. As far as I know, the efi partition contains the biosgrub data. However, the Debian website says that it is not yet possible to distribute an efi partition with RAID on several disks. So do I have to create an efi partition on all 3 disks? And what if a hard disk fails, is one of the other 2 efi partitions used? And do I have to activate "boot flag" on the efi partitions?
Do I have to create an efi partition on all 3 disks?
No, you can simply create the EFI partition on one of the disks.
However, this may impact how well RAID can utilize you hard disks because in some RAID configurations storage is allocated in equal amounts on each disk. In such a case, the amount of space utilized by the EFI partition will not be utilized in the other disks.
An alternative is to add a disk only for EFI, or for EFI and non-RAID partitions.
If a hard disk fails, is one of the other 2 EFI partitions used?
I don't know.
And do I have to activate "boot flag" on the EFI partitions?
No, the EFI partition is marked as such by the partition type ef00, not by the boot flag.