Tag: raid

Btrfs raid1 vs. mdadm raid1

RAID is about up-time. Or about the chance to avoid having to restore from backup if you are lucky. RAID is not a backup, though. There are also no backups, just successful or failed restores. (Those are the most important proverbs that come to my mind right now.)

Given that I’m obsessed with backups, but also “lazy” in the sense that I want to avoid having to actually restore from my backups, I’ve been using RAID1 in my data store for at least 15 years now. For the first 12 years of them, I’ve been using ext3/4 on top of mdadm managed RAID1. About 3-4 years ago, I switched most of my storage to Btrfs, using the filesystem’s built-in RAID1 mode.

In this article I want to give a short reasoning for this. I initially wanted this article to kick-off a mini-series on blog-posts on Btrfs features that you might find usable, but due to some discussion on Mastodon, I already previously posted my article about speeding up Btrfs RAID 1 up using LVM cache. You should check that one out as well.

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Speeding up Btrfs RAID1 with LVM Cache

Logical Volume Manager 2 (lvm2) is a very powerful toolset to manage physical storage devices and logical volumes. I’ve been using that instead of disk partitions for over a decade now. LVM gives you full control where logical volumes are placed, and a ton of other features I have not even tried out yet. It can provide software RAID, it can provide error correction, you can move around logical volumes while they are being actively used. In short, LVM is an awesome tool that should be in every Linux-admin’s toolbox.

Today I want to show how I used LVM’s cache volume feature to drastically speed up a Btrfs RAID1 situated on two slow desktop HDDs, using two cheap SSDs also attached to the same computer, while still maintaining reasonable error resilience against single failing devices.

Creating the cached LVs and Btrfs RAID1

The setup is as follows:

  • 2x 4TB HDD (slow), /dev/sda1, /dev/sdb1
  • 2x 128GB SSD (consumer-grade, SATA), /dev/sdc1, /dev/sdd1
  • All of these devices are part of the Volume Group vg0
  • Goal is to use Btrfs RAID1 mode instead of a MD RAID or lvmraid, because Btrfs has built-in checksums and can detect and correct problems a little bit better because it can determine which leg of the mirror is the correct one.
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