Saturday, June 15, 2013

Creating a BTRFS filesystem on 2 disks

I know it has nothing to do with Machine Learning, AI or even C++ coding but think about it, it's also part of the job. You just received a massive dataset and you only have small hard disks. In my case I have 2 disks of 400Gb. I know it's small by today's standard and I don't want to bother with a lot of partitions and complex tree structure, especially because I need to store my massive dataset that requires more than... 400 G.

With modern Linux there are at least 2 solutions:
  • LVM the Logical Volume Manager
  • BTRFS a new filesystem that offers incredible features

I have been using LVM for years, so I decided to give BTRFS a try. Here is my simple setup:

  1. I have a desktop computer in which I've just added 2 hard disks of 400 Gb
  2. I want to group them like they would be a single 800 Gb disk
  3. I want RAID0 for speed. RAID0 will split up the data on the 2 hard disks at the same time, making my new hard disk twice as fast as one single hard disk (this is theory, in practice, it is not exactly twice as fast, but still, it is way faster anyway)
  4. I know RAID0 is not safe all the time and I want, at least, the metadata to be RAID1, i.e. to be mirrored on both disks.

BTRFS will do everything for you and it's surprinsingly simple:

  1. let's say my 2 hard disks are /dev/sda and /dev/sdb. It may vary in your case
  2. I create a filesystem with RAID0 and RAID1 for metadata as said before
    • mkfs.btrfs /dev/sda /dev/sdb
  3. I check everything is fine with
    1. btrfs filesystem show /dev/sda
    2. note that if you replace sda by sdb, you will have the same information because the 2 partitions are linked by btrfs
  4. I create a new directory where I want my disk to be mounted
    • mkdir /mnt/newdisc
  5. I mount it
    • mount /dev/sda /mnt/newdisc
    • or mount /dev/sdb/newdisc Again it's the same
  6. I add it to /etc/fstab for it to be mounted at boot time
    •  /dev/sdb /mnt btrfs defaults 0 1
  7. Et voila !

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