BTRFS Raid 1 vs. MDADM Raid 1

The current status of btrfs can be found on the btrfs wiki's “Status” article. btrfs RAID1 has been stable for a while, and has several useful features that mdraid does not (e.g., mirror is per-file, so you can have unmirrored files too if you want—though tools to do this aren't readily available). Also, adding a third drive (when you need more space) is much easier with btrfs than mdraid.

mdraid's main advantage is it's far more mature. So mdraid+ext4 is probably a little bit safer. Documentation & experts are easier to find. But if you're running btrfs on top of it anyway (because, e.g., you want snapshots)….

Personally, I have a set of external drives I use for one layer of backups; they use btrfs. I started with one drive (back in 2014), then later added a second (larger) drive for redundancy. Later the first drive failed, so I replaced it with another larger drive. Then I was going to run out of space, so I added a third drive, twice as big as the other two. Sometime in the next year or so, I'll have to add another. All of this would be much more painful (or at least time-consuming) to do with mdraid than it was with btrfs RAID1.

So, for your application, I'd suggest btrfs RAID1 is a better choice. If you were running a mission-critical darn-near-100% uptime required server, it'd be different. And of course, as always, keep backups (of your media files, at least if you care if they're lost — you don't want to know how many copies of my music collection I have...)

The main tradeoff is pretty simple to understand:

• BTRFS has better data safety, because the checksumming lets it ID which copy of a block is wrong when only one is wrong, and means it can tell if both copies are bad.
• MD RAID has better performance, because it does a better job of parallelizing writes and striping reads.

Realistically, raid1 mode is the most well tested of the various multi-device configurations provided by BTRFS, by virtue of the facts that it's one of the oldest supported configurations, and it's also one of the most widely used. Most of the developers consider it stable enough (if you're not using complicated features) to be a realistic option in production usage, provided of course that you stay up to date.

One advantage of mdadm raid is that it is well understood and stable. Also grub was previously not able to boot from multi device btrfs filesystems. There may be other software bugs with such setups. You also could use mdadm to create a raid10 instead of a raid1. Using the offset layout with a largeish chunk size gets you much of the performance of a raid0 with the redundancy of a raid1.

One advantage of the native btrfs method is that you can decide to convert from raid1 to raid0 later. Another theoretical advantage is that you can mix and match and put some files in raid0 and some in raid1, but the last time I checked at least, the tools weren't up to that yet.

checksum

First, md-raid do have checksum and parity checks.

# echo check > /sys/block/md127/md/sync_action
# cat /proc/mdstat 
Personalities : [raid1] 
md127 : active raid1 sdc1[0] sdb1[1]
      1855337472 blocks super 1.2 [2/2] [UU]
      [=================>...]  check = 87.8% (1629808384/1855337472) finish=18.7min speed=200062K/sec
      bitmap: 0/14 pages [0KB], 65536KB chunk

unused devices: <none>
#

So parity validation is not an advantage of either, as both have it. What btrfs have is an advantage over plain ext4 et al. Because it stores duplicate values (profile: DUP) and can (very rarely) recover even with a single disk. But that is irrelevant for raid1 discussion.

Administration

PRO md-raid: mdadm have integrated monitoring. You can add EMAIL or PROGRAM to your mdadm.conf and have it email/trigger a script on any event. Disk failed? progress of rebuild? progress of check? result of important changes? etc.

It is also more explicit on how you assemble it. You must assemble the md device and then you mount whatever is in the md device. With btrfs it will create N blockdevs with the same UUID, and the first time you mount them you must pass mount -o device=/dev/other /dev/main /mnt/point

layout flexibility

Both can be created and new devices added on most combinations. Directly on entire disk (/dev/sda) without partitions is OK. Both can be under a partition (/dev/sda1). Both can be under LVM. Both can be under dm-crypt (cryptsetup, luks). Both can be under dm-parity (but you don't wait it). etc.

PRO md-raid: You can host block devices underneath it. for example, luks or other filesystems.

CON md-raid: it is close to impossible to resize the thing hosting it.

PRO btrfs: you don't have to deal with several layers, mostly because it is all btrfs. Easy to resize (grow) partitions.

CON btrfs: if you plan to use dm-crypt/luks, if must be over btrfs, and I have no idea what will happen if you have corruption. With md-raid you can have luks underneath, and then it is predictable that the corruption will be fixed.

Both are the same in terms of adding more devices and spares.

I won't mention snapshots etc because you can use brtfs under md-raid and also have it.

RAID1 profiles

md-raid is only full copy. It might be better if you care for full reliability. It might be worse if you care for performance.

brtfs guarantee 2 copies at minimum, but not 1 copy on all devices! For example, if you have three disks, md-raid will have three copies of the data. btrfs will still have only two.

you can force more copies if you have 3 or more devices using the new (untested?) raid1c3 and raid1c4 profiles.

I won't mark which one have a pro, since md-raid is a PRO for redundancy and btrfs is a PRO for performance.

rough edges

mdadm et al will have very weird commands and syntax. But it is generally considered stable.

btrfs as everyone mentioned, even in 2025 have some rough edges: for example, you get some scary messages from the tooling:

# btrfs filesystem resize max /mnt/point
Resize device id 1 (/dev/sdb2) from 3.64TiB to max
WARNING: the new size 0 (0.00B) is < 256MiB, this may be rejected by kernel

which is some weird logic handling the word "max" but if you look at the logs it is mostly fine:

BTRFS info (device sdb2): resize device /dev/sdb2 (devid 1) from 4000000770048 to 8100829442048

(Also, keep in mind you must expand each "device" in the RAID, so run for each one btrfs filesystem resize 2:max /mnt/point increasing the number before :max... and afterwards, remember to btrfs rebalance /mnt/point)

So with mdadm you will be constantly looking up commands and conf file syntax. With btrfs you will be constantly looking up error messages to see if they are relevant :)