Zoned mode

Since version 5.12 btrfs supports so called zoned mode. This is a special on-disk format and allocation/write strategy that’s friendly to zoned devices. In short, a device is partitioned into fixed-size zones and each zone can be updated by append-only manner, or reset. As btrfs has no fixed data structures, except the super blocks, the zoned mode only requires block placement that follows the device constraints. You can learn about the whole architecture at .

The devices are also called SMR/ZBC/ZNS, in host-managed mode. Note that there are devices that appear as non-zoned but actually are, this is drive-managed and using zoned mode won’t help.

The zone size depends on the device, typical sizes are 256MiB or 1GiB. In general it must be a power of two. Emulated zoned devices like null_blk allow to set various zone sizes.

Requirements, limitations

  • all devices must have the same zone size

  • maximum zone size is 8GiB

  • minimum zone size is 4MiB

  • mixing zoned and non-zoned devices is possible, the zone writes are emulated, but this is namely for testing

  • the super block is handled in a special way and is at different locations than on a non-zoned filesystem:

    • primary: 0B (and the next two zones)

    • secondary: 512GiB (and the next two zones)

    • tertiary: 4TiB (4096GiB, and the next two zones)

Incompatible features

The main constraint of the zoned devices is lack of in-place update of the data. This is inherently incompatible with some features:

  • NODATACOW - overwrite in-place, cannot create such files

  • fallocate - preallocating space for in-place first write

  • mixed-bg - unordered writes to data and metadata, fixing that means using separate data and metadata block groups

  • booting - the zone at offset 0 contains superblock, resetting the zone would destroy the bootloader data

Initial support lacks some features but they’re planned:

  • only single (data, metadata) and DUP (metadata) profile is supported

  • fstrim - due to dependency on free space cache v1

Super block

As said above, super block is handled in a special way. In order to be crash safe, at least one zone in a known location must contain a valid superblock. This is implemented as a ring buffer in two consecutive zones, starting from known offsets 0B, 512GiB and 4TiB.

The values are different than on non-zoned devices. Each new super block is appended to the end of the zone, once it’s filled, the zone is reset and writes continue to the next one. Looking up the latest super block needs to read offsets of both zones and determine the last written version.

The amount of space reserved for super block depends on the zone size. The secondary and tertiary copies are at distant offsets as the capacity of the devices is expected to be large, tens of terabytes. Maximum zone size supported is 8GiB, which would mean that e.g. offset 0-16GiB would be reserved just for the super block on a hypothetical device of that zone size. This is wasteful but required to guarantee crash safety.


Real hardware

The WD Ultrastar series 600 advertises HM-SMR, i.e. the host-managed zoned mode. There are two more: DA (device managed, no zoned information exported to the system), HA (host aware, can be used as regular disk but zoned writes improve performance). There are not many devices available at the moment, the information about exact zoned mode is hard to find, check data sheets or community sources gathering information from real devices.

Note: zoned mode won’t work with DM-SMR disks.

Emulated: null_blk

The driver null_blk provides memory backed device and is suitable for testing. There are some quirks setting up the devices. The module must be loaded with nr_devices=0 or the numbering of device nodes will be offset. The configfs must be mounted at /sys/kernel/config and the administration of the null_blk devices is done in /sys/kernel/config/nullb. The device nodes are named like /dev/nullb0 and are numbered sequentially. NOTE: the device name may be different than the named directory in sysfs!


modprobe configfs
modprobe null_blk nr_devices=0

Create a device mydev, assuming no other previously created devices, size is 2048MiB, zone size 256MiB. There are more tunable parameters, this is a minimal example taking defaults:

cd /sys/kernel/config/nullb/
mkdir mydev
cd mydev
echo 2048 > size
echo 1 > zoned
echo 1 > memory_backed
echo 256 > zone_size
echo 1 > power

This will create a device /dev/nullb0 and the value of file index will match the ending number of the device node.

Remove the device:

rmdir /sys/kernel/config/nullb/mydev

Then continue with mkfs.btrfs /dev/nullb0, the zoned mode is auto-detected.

For convenience, there’s a script wrapping the basic null_blk management operations, the above commands become:

nullb setup
nullb create -s 2g -z 256
mkfs.btrfs /dev/nullb0
nullb rm nullb0

Emulated: TCMU runner

TCMU is a framework to emulate SCSI devices in userspace, providing various backends for the storage, with zoned support as well. A file-backed zoned device can provide more options for larger storage and zone size. Please follow the instructions at .

Compatibility, incompatibility

  • the feature sets an incompat bit and requires new kernel to access the filesystem (for both read and write)

  • superblock needs to be handled in a special way, there are still 3 copies but at different offsets (0, 512GiB, 4TiB) and the 2 consecutive zones are a ring buffer of the superblocks, finding the latest one needs reading it from the write pointer or do a full scan of the zones

  • mixing zoned and non zoned devices is possible (zones are emulated) but is recommended only for testing

  • mixing zoned devices with different zone sizes is not possible

  • zone sizes must be power of two, zone sizes of real devices are e.g. 256MiB or 1GiB, larger size is expected, maximum zone size supported by btrfs is 8GiB

Status, stability, reporting bugs

The zoned mode has been released in 5.12 and there are still some rough edges and corner cases one can hit during testing. Please report bugs to .