IT 4100 : File Systems and Storage Technologies

BTRFS

BTRFS

• butterFS
• RAID 0,1,10
• COW
• Incremental backup
• Online defrag
• gzip and LZO compression
• Space-efficient packing of small files
• Dynamic inode allocation
• Checksums on data and metadata
• Shrink and grow storage volumes
• Snapshots
• 16 EiB maximum file size

What is COW

A COW – copy on write – filesystem is extra-careful with writing your data. When you make a change to a file, the old data are not overwritten. Instead, the filesystem allocates new blocks for the new data, and only the changed data are given a new allocation.

BTRFS (subvolumes) (from manpage)

A subvolume is a part of filesystem with its own and independent file/directory hierarchy. Subvolumes can share file extents. A snapshot is also subvolume, but with a given initial content of the original subvolume.

Note: A subvolume in btrfs is not like an LVM logical volume, which is block-level snapshot while btrfs subvolumes are file extent-based.

A subvolume in btrfs can be accessed in two ways:

• like any other directory that is accessible to the user
• like a separately mounted filesystem (options subvol or subvolid)

BTRFS commands (viewing information)

• btrfs filesystem df /
• btrfs device usage /
• btrfs filesystem show /
• btrfs subvolume list /

BTRFS (formatting)

• mkfs.btrfs -L butters /dev/sdb12
  • formats device sdb12 as a btrfs system, with a name label of butters
• mkfs.btrfs -d raid0 -L mydata /dev/sdc /dev/sdd

BTRFS subvolumes

• btrfs subvolume create /mnt/svol
• Put stuff in it
• Unmount parent
• Mount subvolume
  • mount -o subvol=test /dev/sda21 /mnt/test #subvol, not subvolume
• Is the stuff still there?
• Can mount parent and subvolume at the same time

BTRFS snapshots

• btrfs subvolume snapshot /mnt/test /mnt/snap

Then if you break something, you can unmount the volume and copy over your snapshot to the old one.

• cp -r mntbtrfs/snapshots/test mntbtrfs/test and remount

BTRFS (snapper)

Utility to help manage snapshots.

• show configs available with:
  • snapper list-configs, or
  • ls -l /etc/snapper/configs/
• Create a new config with:
  • snapper -c configname create-config /path/to/subvolume
  • Example: snapper -c test create-config /mnt/test
  • Example: snapper -c root create-config /
• delete a config with:
  • snapper -c config delete-config

BTRFS (snapper)

• With a config you can take snapshots like:
  • snapper -c config create --description foo
• can automate snapshots, or pre/post snapshots
• delete a snapshot
  • snapper -c config delete N # will delete snapshot number N

AWS Demo commands (after adding some volumes to ec2 instance)

    mkfs.btrfs -L testfs /dev/xvdf -f
    mount /dev/xvdf /mnt/
    df
    btrfs filesystem show /mnt
    cd /mnt
    fallocate -l 726M garbaged
    md5sum garbaged 
    df -h 
    btrfs filesystem df /mnt
    btrfs device add -f /dev/xvdg /mnt 
    btrfs device add -f /dev/xvdh /mnt 
    btrfs device add -f /dev/xvdi /mnt 
    btrfs filesystem df /mnt
    btrfs balance start /mnt
    btrfs filesystem show /mnt
    btrfs balance start -dconvert=raid1 -mconvert=raid1 /mnt
    btrfs filesystem show /mnt
    md5sum garbaged