Re: What exactly is BTRFS Raid 10?

[Phillip Susi <phill@thesusis.net>]

"George Shammas" <btrfs@shamm.as> writes:

> Hello,
>
>  I've been searching and reading docs for a few days now, and btrfs raid 10 is a mystery to me. 
>
> This is mostly a documentation question, as many places reference it
> but nothing actually describes it and the real question is how does it
> differ from btrfs raid1.
>
> Both BTRFS Raid1 and Raid10 
>  - Allows arbitrary number of drives (>=2), including odd numbers. 
>  - Will write duplicate blocks across disks.

Btrfs raid10 requires an even number of drives with a minimum of 4.
It's pretty much raid 1+0.


> https://btrfs.readthedocs.io/en/latest/mkfs.btrfs.html?highlight=raid10#profile-layout
>
> The Raid 1 example there also likely needs a bit of explanation or
> validation, as all the blocks are written to one device. In that raid
> one example three devices could be lost as long as it is not one of
> them is the first device. It also cannot be accurate once the amount
> stored is above 1 full drive.

It is meant to show a *possible* layout, not every potential layout.
The data may be stored like than and then yes, you could lose multiple
drives and still recover as long as the lost drives were 2, 3, and 4.