Using Btrfs with Multiple Devices

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Revision as of 17:22, 4 April 2011

Contents

Multiple Devices

A Btrfs filesystem can be created on top of many devices, and more devices can be added after the FS has been created.

By default, metadata will be mirrored across two devices and data will be striped across all of the devices present.

If only one device is present, metadata will be duplicated on that one device.

Current Status

Btrfs can add and remove devices online. Adding devices at mkfs time gives the most control over the raid levels used.

Btrfs can do raid0, raid1, raid10 and it can duplicate metadata on a single spindle. When blocks are read in, checksums are verified and if there are any errors, Btrfs tries to read from an alternate copy.

See the Gotchas page for some current issues when using btrfs with multiple volumes of differing sizes in a RAID1 style setup.

Creating a Multi-device FS

mkfs.btrfs will accept more than one device on the command line. It has options to control the raid configuration for data and metadata. Valid choices are raid0, raid1, raid10 and single. Single means that no duplication of metadata is done, which may be desired when using hardware raid.

Raid10 requires at least 4 devices.

# Create a filesystem across four drives (metadata mirrored, data striped)
mkfs.btrfs /dev/sdb /dev/sdc /dev/sdd /dev/sde
# Stripe the metadata without mirroring
mkfs.btrfs -m raid0 /dev/sdb /dev/sdc
# Use raid10 for both data and metadata
mkfs.btrfs -m raid10 -d raid10 /dev/sdb /dev/sdc /dev/sdd /dev/sde
# Don't duplicate metadata on a single drive
mkfs.btrfs -m single /dev/sdb

Once you create a multi-device filesystem, you can use any device in the FS for the mount command:

mkfs.btrfs /dev/sdb /dev/sdc /dev/sde
mount /dev/sde /mnt

If you want to mount a multi-device filesystem using a loopback device, it's not sufficient to use mount -o loop. Instead, you'll have to set up the loopbacks manually:

# Create and mount a filesystem made of several disk images
mkfs.btrfs img0 img1 img2
losetup /dev/loop0 img0
losetup /dev/loop1 img1
losetup /dev/loop2 img2
mount /dev/loop0 /mnt/btrfs

After a reboot or reloading the btrfs module, you'll need to use btrfs device scan to discover all multi-device filesystems on the machine (see below)

Finding and Listing Multi-device Filesystems

Note: the new command btrfs is replacing the old ones btrfsctl and btrfs-show. In the examples the btrfs commands is used. Below the old commands compared to the new one:
OLD                         NEW
---------------------       --------------------------
btrfsctl -a                 btrfs device scan
btrfsctl -A /dev/sdb        btrfs device scan /dev/sdb
btrfs-show                  btrfs filesystem show 

btrfs device scan is used to scan all of the block devices under /dev and probe for Btrfs volumes. This is required after loading the btrfs module if you're running with more than one device in a filesystem.

# Scan all devices
btrfs device scan
# Scan a single device
btrfs device scan /dev/sdb

btrfs filesystem show will print information about all of the Btrfs filesystems on the machine.

Adding New Devices

Note: the new command btrfs is replacing the old ones btrfs-vol and btrfs-show. In the examples the btrfs commands is used. Below the old commands compared to the new one:
OLD                              NEW
--------------------------       ----------------------------------------
btrfs-show                       btrfs filesystem show 
btrfs-vol -a <dev> <path>        btrfs device add <dev [<dev>...] <path>
btrfs-vol -b <path>              btrfs filesystem balance <path>


btrfs filesystem show gives you a list of all the btrfs filesystems on the systems and which devices they include.

btrfs device add is used to add new devices to a mounted filesystem.

btrfs filesystem balance can balance (restripe) the allocated extents across all of the existing devices. For example:

mkfs.btrfs /dev/sdb
mount /dev/sdb /mnt
# Create some files
btrfs- device add /dev/sdc /mnt

At this point we have a filesystem with two devices, but all of the metadata and data are still stored on /dev/sdb. The filesystem must be balanced to spread the files across all of the devices.

btrfs filesystem balance /mnt

The balance operation will take some time. It reads in all of the FS data and metadata and rewrites it across the new device.

Removing Devices

Note: the new command btrfs is replacing the old one btrfs-vol. In the examples the btrfs commands is used. Below the old command compared to the new one:
OLD                              NEW
--------------------------       ----------------------------------------
btrfs-vol -r <dev> <path>        btrfs device delete <dev [<dev>...] <path>

btrfs device remove is used to remove devices online. It redistributes the any extents in use on the device being removed to the other devices in the filesystem. Example:

mkfs.btrfs /dev/sdb /dev/sdc /dev/sdd /dev/sde
mount /dev/sdb /mnt
# Put some data on the filesystem here
btrfs device delete /dev/sdc /mnt

Replacing Failed Devices

Note: the new command btrfs is replacing the old one btrfs-vol. In the examples below the btrfs commands is used. Below the old command compared to the new one:
OLD                              NEW
--------------------------       ----------------------------------------
btrfs-vol -r missing <path>        btrfs device delete missing <path>


The example above can be used to remove a failed device if the super block can still be read. But, if a device is missing or the super block has been corrupted, the filesystem will need to be mounted in degraded mode:

mkfs.btrfs -m raid1 /dev/sdb /dev/sdc /dev/sdd /dev/sde
#
# sdd is destroyed or removed, use -o degraded to force the mount
# to ignore missing devices
#
mount -o degraded /dev/sdb /mnt
#
# 'missing' is a special device name
#
btrfs device delete missing /mnt

btrfs device delete missing tells btrfs to remove the first device that is described by the filesystem metadata but not present when the FS was mounted.

In case of raidXX layout, you cannot go below the minimum number of the device required. So before removing a device (even the missing one) you may need to add a new one. For example if you have a raid1 layout with two device, and a device fails, you must:

  • mount in degraded mode
  • add a new device
  • remove the missing device

Mounting a multi-device filesystem from /etc/fstab

If you don't have an initrd, or your initrd doesn't perform a btrfs device scan, you can still mount a multi-volume btrfs filesystem by passing all the devices in the filesystem explicitly to the mount command. A suitable /etc/fstab entry would be:

/dev/sdb     /mnt    btrfs    device=/dev/sdb,device=/dev/sdc,device=/dev/sdd,device=/dev/sde    0 0
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