Userspace block device driver (ublk driver)


ublk is a generic framework for implementing block device logic from userspace. The motivation behind it is that moving virtual block drivers into userspace, such as loop, nbd and similar can be very helpful. It can help to implement new virtual block device such as ublk-qcow2 (there are several attempts of implementing qcow2 driver in kernel).

Userspace block devices are attractive because:

  • They can be written many programming languages.

  • They can use libraries that are not available in the kernel.

  • They can be debugged with tools familiar to application developers.

  • Crashes do not kernel panic the machine.

  • Bugs are likely to have a lower security impact than bugs in kernel code.

  • They can be installed and updated independently of the kernel.

  • They can be used to simulate block device easily with user specified parameters/setting for test/debug purpose

ublk block device (/dev/ublkb*) is added by ublk driver. Any IO request on the device will be forwarded to ublk userspace program. For convenience, in this document, ublk server refers to generic ublk userspace program. ublksrv 1 is one of such implementation. It provides libublksrv 2 library for developing specific user block device conveniently, while also generic type block device is included, such as loop and null. Richard W.M. Jones wrote userspace nbd device nbdublk 3 based on libublksrv 2.

After the IO is handled by userspace, the result is committed back to the driver, thus completing the request cycle. This way, any specific IO handling logic is totally done by userspace, such as loop’s IO handling, NBD’s IO communication, or qcow2’s IO mapping.

/dev/ublkb* is driven by blk-mq request-based driver. Each request is assigned by one queue wide unique tag. ublk server assigns unique tag to each IO too, which is 1:1 mapped with IO of /dev/ublkb*.

Both the IO request forward and IO handling result committing are done via io_uring passthrough command; that is why ublk is also one io_uring based block driver. It has been observed that using io_uring passthrough command can give better IOPS than block IO; which is why ublk is one of high performance implementation of userspace block device: not only IO request communication is done by io_uring, but also the preferred IO handling in ublk server is io_uring based approach too.

ublk provides control interface to set/get ublk block device parameters. The interface is extendable and kabi compatible: basically any ublk request queue’s parameter or ublk generic feature parameters can be set/get via the interface. Thus, ublk is generic userspace block device framework. For example, it is easy to setup a ublk device with specified block parameters from userspace.

Using ublk

ublk requires userspace ublk server to handle real block device logic.

Below is example of using ublksrv to provide ublk-based loop device.

  • add a device:

    ublk add -t loop -f ublk-loop.img
  • format with xfs, then use it:

    mkfs.xfs /dev/ublkb0
    mount /dev/ublkb0 /mnt
    # do anything. all IOs are handled by io_uring
    umount /mnt
  • list the devices with their info:

    ublk list
  • delete the device:

    ublk del -a
    ublk del -n $ublk_dev_id

See usage details in README of ublksrv 4.


Control plane

ublk driver provides global misc device node (/dev/ublk-control) for managing and controlling ublk devices with help of several control commands:


    Add a ublk char device (/dev/ublkc*) which is talked with ublk server WRT IO command communication. Basic device info is sent together with this command. It sets UAPI structure of ublksrv_ctrl_dev_info, such as nr_hw_queues, queue_depth, and max IO request buffer size, for which the info is negotiated with the driver and sent back to the server. When this command is completed, the basic device info is immutable.


    Set or get parameters of the device, which can be either generic feature related, or request queue limit related, but can’t be IO logic specific, because the driver does not handle any IO logic. This command has to be sent before sending UBLK_CMD_START_DEV.


    After the server prepares userspace resources (such as creating per-queue pthread & io_uring for handling ublk IO), this command is sent to the driver for allocating & exposing /dev/ublkb*. Parameters set via UBLK_CMD_SET_PARAMS are applied for creating the device.


    Halt IO on /dev/ublkb* and remove the device. When this command returns, ublk server will release resources (such as destroying per-queue pthread & io_uring).


    Remove /dev/ublkc*. When this command returns, the allocated ublk device number can be reused.


    When /dev/ublkc is added, the driver creates block layer tagset, so that each queue’s affinity info is available. The server sends UBLK_CMD_GET_QUEUE_AFFINITY to retrieve queue affinity info. It can set up the per-queue context efficiently, such as bind affine CPUs with IO pthread and try to allocate buffers in IO thread context.


    For retrieving device info via ublksrv_ctrl_dev_info. It is the server’s responsibility to save IO target specific info in userspace.

  • UBLK_CMD_GET_DEV_INFO2 Same purpose with UBLK_CMD_GET_DEV_INFO, but ublk server has to provide path of the char device of /dev/ublkc* for kernel to run permission check, and this command is added for supporting unprivileged ublk device, and introduced with UBLK_F_UNPRIVILEGED_DEV together. Only the user owning the requested device can retrieve the device info.

    How to deal with userspace/kernel compatibility:

    1. if kernel is capable of handling UBLK_F_UNPRIVILEGED_DEV

    If ublk server supports UBLK_F_UNPRIVILEGED_DEV:

    ublk server should send UBLK_CMD_GET_DEV_INFO2, given anytime unprivileged application needs to query devices the current user owns, when the application has no idea if UBLK_F_UNPRIVILEGED_DEV is set given the capability info is stateless, and application should always retrieve it via UBLK_CMD_GET_DEV_INFO2

    If ublk server doesn’t support UBLK_F_UNPRIVILEGED_DEV:

    UBLK_CMD_GET_DEV_INFO is always sent to kernel, and the feature of UBLK_F_UNPRIVILEGED_DEV isn’t available for user

    1. if kernel isn’t capable of handling UBLK_F_UNPRIVILEGED_DEV

    If ublk server supports UBLK_F_UNPRIVILEGED_DEV:

    UBLK_CMD_GET_DEV_INFO2 is tried first, and will be failed, then UBLK_CMD_GET_DEV_INFO needs to be retried given UBLK_F_UNPRIVILEGED_DEV can’t be set

    If ublk server doesn’t support UBLK_F_UNPRIVILEGED_DEV:

    UBLK_CMD_GET_DEV_INFO is always sent to kernel, and the feature of UBLK_F_UNPRIVILEGED_DEV isn’t available for user


    This command is valid if UBLK_F_USER_RECOVERY feature is enabled. This command is accepted after the old process has exited, ublk device is quiesced and /dev/ublkc* is released. User should send this command before he starts a new process which re-opens /dev/ublkc*. When this command returns, the ublk device is ready for the new process.


    This command is valid if UBLK_F_USER_RECOVERY feature is enabled. This command is accepted after ublk device is quiesced and a new process has opened /dev/ublkc* and get all ublk queues be ready. When this command returns, ublk device is unquiesced and new I/O requests are passed to the new process.

  • user recovery feature description

    Two new features are added for user recovery: UBLK_F_USER_RECOVERY and UBLK_F_USER_RECOVERY_REISSUE.

    With UBLK_F_USER_RECOVERY set, after one ubq_daemon(ublk server’s io handler) is dying, ublk does not delete /dev/ublkb* during the whole recovery stage and ublk device ID is kept. It is ublk server’s responsibility to recover the device context by its own knowledge. Requests which have not been issued to userspace are requeued. Requests which have been issued to userspace are aborted.

    With UBLK_F_USER_RECOVERY_REISSUE set, after one ubq_daemon(ublk server’s io handler) is dying, contrary to UBLK_F_USER_RECOVERY, requests which have been issued to userspace are requeued and will be re-issued to the new process after handling UBLK_CMD_END_USER_RECOVERY. UBLK_F_USER_RECOVERY_REISSUE is designed for backends who tolerate double-write since the driver may issue the same I/O request twice. It might be useful to a read-only FS or a VM backend.

Unprivileged ublk device is supported by passing UBLK_F_UNPRIVILEGED_DEV. Once the flag is set, all control commands can be sent by unprivileged user. Except for command of UBLK_CMD_ADD_DEV, permission check on the specified char device(/dev/ublkc*) is done for all other control commands by ublk driver, for doing that, path of the char device has to be provided in these commands’ payload from ublk server. With this way, ublk device becomes container-ware, and device created in one container can be controlled/accessed just inside this container.

Data plane

ublk server needs to create per-queue IO pthread & io_uring for handling IO commands via io_uring passthrough. The per-queue IO pthread focuses on IO handling and shouldn’t handle any control & management tasks.

The’s IO is assigned by a unique tag, which is 1:1 mapping with IO request of /dev/ublkb*.

UAPI structure of ublksrv_io_desc is defined for describing each IO from the driver. A fixed mmapped area (array) on /dev/ublkc* is provided for exporting IO info to the server; such as IO offset, length, OP/flags and buffer address. Each ublksrv_io_desc instance can be indexed via queue id and IO tag directly.

The following IO commands are communicated via io_uring passthrough command, and each command is only for forwarding the IO and committing the result with specified IO tag in the command data:


    Sent from the server IO pthread for fetching future incoming IO requests destined to /dev/ublkb*. This command is sent only once from the server IO pthread for ublk driver to setup IO forward environment.


    When an IO request is destined to /dev/ublkb*, the driver stores the IO’s ublksrv_io_desc to the specified mapped area; then the previous received IO command of this IO tag (either UBLK_IO_FETCH_REQ or UBLK_IO_COMMIT_AND_FETCH_REQ) is completed, so the server gets the IO notification via io_uring.

    After the server handles the IO, its result is committed back to the driver by sending UBLK_IO_COMMIT_AND_FETCH_REQ back. Once ublkdrv received this command, it parses the result and complete the request to /dev/ublkb*. In the meantime setup environment for fetching future requests with the same IO tag. That is, UBLK_IO_COMMIT_AND_FETCH_REQ is reused for both fetching request and committing back IO result.


    With UBLK_F_NEED_GET_DATA enabled, the WRITE request will be firstly issued to ublk server without data copy. Then, IO backend of ublk server receives the request and it can allocate data buffer and embed its addr inside this new io command. After the kernel driver gets the command, data copy is done from request pages to this backend’s buffer. Finally, backend receives the request again with data to be written and it can truly handle the request.

    UBLK_IO_NEED_GET_DATA adds one additional round-trip and one io_uring_enter() syscall. Any user thinks that it may lower performance should not enable UBLK_F_NEED_GET_DATA. ublk server pre-allocates IO buffer for each IO by default. Any new project should try to use this buffer to communicate with ublk driver. However, existing project may break or not able to consume the new buffer interface; that’s why this command is added for backwards compatibility so that existing projects can still consume existing buffers.

  • data copy between ublk server IO buffer and ublk block IO request

    The driver needs to copy the block IO request pages into the server buffer (pages) first for WRITE before notifying the server of the coming IO, so that the server can handle WRITE request.

    When the server handles READ request and sends UBLK_IO_COMMIT_AND_FETCH_REQ to the server, ublkdrv needs to copy the server buffer (pages) read to the IO request pages.

Future development

Zero copy

Zero copy is a generic requirement for nbd, fuse or similar drivers. A problem 6 Xiaoguang mentioned is that pages mapped to userspace can’t be remapped any more in kernel with existing mm interfaces. This can occurs when destining direct IO to /dev/ublkb*. Also, he reported that big requests (IO size >= 256 KB) may benefit a lot from zero copy.