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by keeperofdakeys 1035 days ago
The BTRFS raid1 feature is very badly named, its best called two-copy. Skipping some details, two copies of data are written to the two drives with the most free space. So you can have multiple mismatched drives.

However you get no striping, and data is only read from one drive, so performance is limited to that of one drive for reads and writes. Plus with mismatched drives, smaller drives go unused unless you write enough data.
1 comments
wtallis 1035 days ago
> The BTRFS raid1 feature is very badly named,

You're going to have to support that claim a bit better. The core idea of RAID 1 is mirroring data, which BTRFS RAID 1 mode definitely does. Striping is not an essential part of RAID 1 (hence RAID 10), and reading data from two disks in parallel is an optional performance optimization that is not performed by all RAID 1 implementations (but could be implemented for BTRFS RAID 1: https://stackoverflow.com/questions/55408256/btrfs-raid-1-wh... ).

> Plus with mismatched drives, smaller drives go unused unless you write enough data.

Yes, the allocation is suboptimal from a performance perspective, as I've already said. But it is simple and straightforward and reasonably good at avoiding putting you into a situation where manually issuing a rebalance command is necessary. If you do need better performance, there's a RAID 10 mode. But since my NAS is currently a motley pile of SSDs, I don't need to to anything extra to have decent performance.

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keeperofdakeys 1035 days ago
RAID1 is about mirroring disks, BTRFS RAID1 mirrors block groups. Plus traditionally a RAID1 of 3 disks will mirror the same data on all disks, which is different to how the RAID1 mode on BTRFS acts. So the name leads to misunderstandings since it doesn't act like RAID1 at all.

It'd be way easier to talk about if it had a unique name, and you could say "It's like RAID1".

Despite all that I do like the mode, and use it in a few places.

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PlutoIsAPlanet 1035 days ago
> Plus traditionally a RAID1 of 3 disks will mirror the same data on all disks, which is different to how the RAID1 mode on BTRFS acts.

You have raid1c<drive count> mode in BTRFS which does this.

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