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
ublksrv  is one of such implementation. It
libublksrv  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  based on
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
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.
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:
delete the device:
ublk del -a ublk del -n $ublk_dev_id
See usage details in README of
ublk driver provides global misc device node (
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
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
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_PARAMSare 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).
/dev/ublkc*. When this command returns, the allocated ublk device number can be reused.
/dev/ublkcis added, the driver creates block layer tagset, so that each queue's affinity info is available. The server sends
UBLK_CMD_GET_QUEUE_AFFINITYto 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_INFO2Same 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_DEVtogether. Only the user owning the requested device can retrieve the device info.
How to deal with userspace/kernel compatibility:
if kernel is capable of handling
If ublk server supports
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_DEVis set given the capability info is stateless, and application should always retrieve it via
If ublk server doesn't support
UBLK_CMD_GET_DEV_INFOis always sent to kernel, and the feature of UBLK_F_UNPRIVILEGED_DEV isn't available for user
if kernel isn't capable of handling
If ublk server supports
UBLK_CMD_GET_DEV_INFO2is tried first, and will be failed, then
UBLK_CMD_GET_DEV_INFOneeds to be retried given
UBLK_F_UNPRIVILEGED_DEVcan't be set
If ublk server doesn't support
UBLK_CMD_GET_DEV_INFOis always sent to kernel, and the feature of
UBLK_F_UNPRIVILEGED_DEVisn't available for user
This command is valid if
UBLK_F_USER_RECOVERYfeature 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_RECOVERYfeature 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_RECOVERYset, 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.
UBLK_F_USER_RECOVERY_REISSUEset, 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_F_USER_RECOVERY_REISSUEis 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
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.
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
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_descto the specified mapped area; then the previous received IO command of this IO tag (either
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_REQback. 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_REQis reused for both fetching request and committing back IO result.
UBLK_F_NEED_GET_DATAenabled, 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_DATAadds 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_REQto the server, ublkdrv needs to copy the server buffer (pages) read to the IO request pages.
Zero copy is a generic requirement for nbd, fuse or similar drivers. A
problem  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.