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I used to have RAID10 btrfs setup with 4 disks. Now I added sdf a new disk (its doing the balancing):

Total devices 5 FS bytes used 5.73TiB
devid    1 size 3.64TiB used 2.78TiB path /dev/sdd
devid    2 size 3.64TiB used 2.78TiB path /dev/sdc
devid    3 size 3.64TiB used 2.78TiB path /dev/sdb
devid    4 size 3.64TiB used 2.78TiB path /dev/sde
devid    5 size 3.64TiB used 336.00GiB path /dev/sdf

What I don't understand is that how is disk 5 inserted into this array?

http://www.easynas.org/wp-content/uploads/2014/05/Raid10.png

By looking at the standard raid10 I would say it got into as a 3rd mirror to the right on the picture (storing the A2 A4 A6 blocks) but I'm sure it's done differently for BTRFS.

Could someone explain it please?

  • Link don't work.. – Drakonoved Mar 5 '18 at 6:10
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Btrfs doesn't mirror entire disks. It mirrors individual blocks of data ("chunks") and spreads them around the disks.

One chunk will be mirrored between disk 1 & 2, another between 3 & 4, another between 2 & 5, and so on.

So every chunk of data will have a backup copy on another disk, but not all on the same disk.

There are two good aspects of this:

  • Your disks don't need to be all the same size. It will spread chunks around on whatever space is available, on any two drives.
  • When a disk fails, one drive does not take all the stress of redundancy and recovery. All other disks with needed chunks are used, speeding up access.

It's a very flexible system. I like it especially because the disks don't need to be the same size.

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