Re: What exactly is BTRFS Raid 10?

["George Shammas" <btrfs@shamm.as>]

On Fri, Aug 19, 2022, at 6:18 PM, Chris Murphy wrote:
> man mkfs.btrfs explains some of this. Minimum devices 2.

My first mail included a link the the man page of mkfs.btrfs. It is devoid of information of raid10 other than it is an option.

> And keep in mind all btrfs raid is at the chunk level. Not block device 
> level. So there's no such thing as a mirrored device, but rather 
> mirrored chunks (two copies of a block group on separate block devices).
>
> And yes, you can only lose one device with btrfs raid10. 

And this is exactly why I am asking this question. Given that 
- both raid1 and raid10 can only tolerate a single disk failure
- chucks are placed evenly across drives, effectively making files striped even if the chucks themselves are not striped. 

It seems that both "raid1" and "raid10" are functionally equivalent in btrfs. Or there is a nuance that I'm missing and is not documented. 

These gotchas are not obvious to me, even after 12 years of working with traditional raid setups. 

Perhaps raid1 does not require that chucks are placed evenly, allowing for hotspots? In which case raid10 is almost always preferable unless you have disks of unequal size?

There needs to be some authoritative text on the differences and pros/cons  btrfs raid1 and btrfs raid10, especially since raid5/6 are not recommended.

--George