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Documentation: ublk: document UBLK_F_PER_IO_DAEMON

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Explain the restrictions imposed on ublk servers in two cases:
1. When UBLK_F_PER_IO_DAEMON is set (current ublk_drv)
2. When UBLK_F_PER_IO_DAEMON is not set (legacy)

Remove most references to per-queue daemons, as the new
UBLK_F_PER_IO_DAEMON feature renders that concept obsolete.

Signed-off-by: Uday Shankar <ushankar@purestorage.com>
Reviewed-by: Caleb Sander Mateos <csander@purestorage.com>
Reviewed-by: Ming Lei <ming.lei@redhat.com>
Link: https://lore.kernel.org/r/20250529-ublk_task_per_io-v8-9-e9d3b119336a@purestorage.com
Signed-off-by: Jens Axboe <axboe@kernel.dk>
This commit is contained in:
Uday Shankar 2025-05-29 17:47:18 -06:00 committed by Jens Axboe
parent 17574aa2a0
commit 08652bd86e
1 changed files with 24 additions and 11 deletions
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35
Documentation/block/ublk.rst
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@ -115,15 +115,15 @@ managing and controlling ublk devices with help of several control commands:
- ``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
After the server prepares userspace resources (such as creating I/O handler
threads & 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.
- ``UBLK_CMD_STOP_DEV``
Halt IO on ``/dev/ublkb*`` and remove the device. When this command returns,
ublk server will release resources (such as destroying per-queue pthread &
ublk server will release resources (such as destroying I/O handler threads &
io_uring).
- ``UBLK_CMD_DEL_DEV``
@ -208,15 +208,15 @@ managing and controlling ublk devices with help of several control commands:
modify how I/O is handled while the ublk server is dying/dead (this is called
the ``nosrv`` case in the driver code).
With just ``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
With just ``UBLK_F_USER_RECOVERY`` set, after the ublk server exits,
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`` additionally set, after one ubq_daemon
(ublk server's io handler) is dying, contrary to ``UBLK_F_USER_RECOVERY``,
With ``UBLK_F_USER_RECOVERY_REISSUE`` additionally set, after the ublk server
exits, 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
@ -241,10 +241,11 @@ 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 ublk server should create dedicated threads for handling I/O. Each
thread should have its own io_uring through which it is notified of new
I/O, and through which it can complete I/O. These dedicated threads
should focus 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*``.
@ -265,6 +266,18 @@ with specified IO tag in the command data:
destined to ``/dev/ublkb*``. This command is sent only once from the server
IO pthread for ublk driver to setup IO forward environment.
Once a thread issues this command against a given (qid,tag) pair, the thread
registers itself as that I/O's daemon. In the future, only that I/O's daemon
is allowed to issue commands against the I/O. If any other thread attempts
to issue a command against a (qid,tag) pair for which the thread is not the
daemon, the command will fail. Daemons can be reset only be going through
recovery.
The ability for every (qid,tag) pair to have its own independent daemon task
is indicated by the ``UBLK_F_PER_IO_DAEMON`` feature. If this feature is not
supported by the driver, daemons must be per-queue instead - i.e. all I/Os
associated to a single qid must be handled by the same task.
- ``UBLK_IO_COMMIT_AND_FETCH_REQ``
When an IO request is destined to ``/dev/ublkb*``, the driver stores