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I would just skip RAID, add all disk to a single BTRFS and use the built in profiles for (meta)data redundancy.
Cache I don't know much tho.
https://btrfs.readthedocs.io/en/latest/btrfs-device.html
The man page at https://btrfs.readthedocs.io/en/latest/mkfs.btrfs.html says:
So those profiles have unknown, unspecified problems.
But btrfs is safe on top of md-based raid1/5/6. It also has the advantage that you only need to encrypt one volume.
Could you elaborate on btrfs on top of md raid?
This one seems the most likely solution for me.
Sure. First you set up a RAID5/6 array in mdadm. This is a purely software thing, which is built into the Linux kernel. It doesn't require any hardware RAID system. If you have 3-4 drives, RAID5 is probably best, and if you have 5+ drives RAID6 is probably best.
If your 3 blank drives are sdb1, sdc1, and sdd1, run this:
mdadm --create --verbose /dev/md0 --level=5 -n 3 /dev/sdb1 /dev/sdc1 /dev/sdd1This will create a block device called /dev/md0 that you can use as if it were a single large hard drive.
mkfs.btrfs /dev/md0That will make the filesystem on the block device.
This creates a mount point and mounts the filesystem.
To get it to mount every time you boot, add an entry for this filesystem in /etc/fstab
Do you need to do some maintenance to keep the data in the array intact?
I read of some btrfs scrub commands and md checks and such, but I am unsure how often to do them, and what they actually do.
In my system, the raid arrays seem to do periodic data scrubbing automatically. Maybe it's something that's part of Debian, or maybe it's just a default kernel setting. I don't think it helps much with data integrity -- I think it helps more just by ensuring the continued functionality of the drives.
When it's running, you can type
cat /proc/mdstatto see the progress.That command will also show you if there is a failing drive, so that you can replace it.
You should scrub your data regularly with btrfs. That's just a mean to verify the data is in-tact though; to detect corruption.
You cannot really do anything actively to keep the data in-tact. Failure can and will happen. To keep your data safe, you must plan for failure to happen:
Expect a power surge to fry all your disks at the same time.
Expect your house to burn down or flood.
Expect to run the wrong command and istantly hose your entire array.
Expect your backup server to get ransomware'd.
...
Only if you effectively mitigate these dangers will your data stay safe.
Thanks for the info!
Ops. Missed that part.
Are there some advantages of btrfs over raid? I understand how raid works but btrfs for redundancy is foreign to me.
BTRFS has RAID built into the file system - instead of using MD you use BTRFS profiles which tell the system how to handle data.
For instance
With this set up you could lose one device (of n, the total doesn’t matter), and not lose any data, and still be able to boot to recover with too much hassle.
BTRFS does block checksums, can scan for bit rot and recover from it, and generally tries to make your data safe. It technically supports raid5/6 for user data, the issue is around unclean shutdowns and a potential write hole where you could lose data, but if your system has a UPS backup and is on a relatively recent kernel it’s not any more dangerous than MD raid5/6 as I understand it.
I use BTRFS for snapshots, and auto compression. Maybe it can be done with raids with LVM? AFAIK BTRFS redundancy is basically the same as traditional RAID, similar to using mdadm. Still, you would want a backup strat instead relying on the disk redundancy. I learn that the hardway.