Allocator Hints for Btrfs
5 comments
·February 7, 2025jeroenhd
ThatPlayer
RAID1 has been fine for years, but hardly anyone is running that and RAID5/6 are still considered unstable even by the devs: https://btrfs.readthedocs.io/en/latest/Status.html#block-gro...
I ran into Btrfs RAID6 corruption myself about ten years ago. Though to btrfs's credit, I didn't lose any data: the filesystem only locked up read-only on me. But this time I'll take their word on unstable.
bjoli
Btrfs raid 1 works great. I have 3 10tb drives in a raid1c2 for a total of 15tb of space. Btrfs raid1 allows you to use any combination of drives and sizes. If I would have one 12tb drive instead of one 10tb I would have had 16tb of usable space.
For raid5/6 I dont use the built in raid, but mdraid. I stopped waiting for btrfs raid56 to stabilise.
ThatPlayer
The article mentions bcache, but not bcachefs which you can also do this on. I also like bcachefs more for mixed drive type filesystems because of the bcache feature of using the SSDs as write caches for data that gets moved to the HDD in the background. Also read cache.
This btrfs feature only supports having the data chunks prioritize the HDDs.
Whether or not that matters depends on your use case of course: it's not worse than a RAID setup of only HDD. I'm using bcachefs on budget gaming PCs built from old spare parts, so having a small SSD and a bunch of HDDs for games is great.
Cumpiler69
[dead]
Are these tweaks exclusively beneficial to mixed-media RAID setups, or do they also pose an advantage to single-device BTRFS setups?
I know BTRFS is the default FS for a bunch of distros now, but I haven't heard much about it in the RAID space. Most people seem to use ZFS, despite the massive memory pressure it adds, because of fear of bugs and reports of filesystem corruption years ago. Perhaps someone who uses BTRFS for RAID can comment on how well BTRFS RAID works these days?