I want to set up a BTRFS RAID1 setup with two WD Red 4TB disks, which possess 4K sectors. Since I am keen to get it right the first time before copying data to it (yes it will be backed up), my question is whether it needs any special considerations regarding sector-alignment and size for Advanced Format disk.
In the past I know I had to take care of this, when setting up MDADM+LVM and ZFS (ashift=12) for example, but I cannot find any information for BTRFS regarding this (apart from SSD-specific stuff, e.g. here). I had planned to use the whole disks without partitions in the following way:
$ mkfs.btrfs -m raid1 -d raid1 /dev/sdX /dev/sdY
So, considering I couldn't find anything, can I trust BTRFS to just 'do the right thing'? How can I control what it does? I checked the result in GParted and the disk information shows
First sector: 0
for both drives, which seems wrong, since normally GParted aligns the first sector to MB, which would have a first sector of 2048, as I have seen for a BTRFS partition I created on another disk (but I am not sure).
Perhaps I should just create a disk-spanning BTRFS-partitions on both drives, that is aligned and then use those in RAID1? BTW: Is there any pros/cons of using whole disks vs. partitions I should consider?
Hope someone with some experience can help me out.
Best Answer
From my experience, letting btrfs take over a whole disk with no partitions will not result in proper logical/physical sector alignment. I've tried this with 3.14.1; latter versions may do this differently (current release is 3.18.x).
My suggestion is to use properly aligned GPT partitions and tell BTRFS to use a 4K sector size with mkfs.btrfs --sectorsize.