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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.
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.
ublk requires userspace ublk server to handle real block device logic.
Below is example of using ublksrv to provide ublk-based loop 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 del -a ublk del -n $ublk_dev_id
See usage details in README of ublksrv 4.
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 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 mmaped 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.
ublk driver doesn’t handle any IO logic. Its function is well defined for now and very limited userspace interfaces are needed, which is also well defined too. It is possible to make ublk devices container-aware block devices in future as Stefan Hajnoczi suggested 5, by removing ADMIN privilege.
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.