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Documentation: document ublk user recovery feature

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Add documentation for user recovery feature of ublk subsystem.

Signed-off-by: ZiyangZhang <ZiyangZhang@linux.alibaba.com>
Reviewed-by: Ming Lei <ming.lei@redhat.com>
Link: https://lore.kernel.org/r/20221018045346.99706-2-ZiyangZhang@linux.alibaba.com
Signed-off-by: Jens Axboe <axboe@kernel.dk>
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ZiyangZhang 2022-10-18 12:53:46 +08:00 committed by Jens Axboe
parent 76dd298094
commit e0539ae012
1 changed files with 36 additions and 0 deletions
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Documentation/block/ublk.rst
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@ -144,6 +144,42 @@ managing and controlling ublk devices with help of several control commands:
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_START_USER_RECOVERY``
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.
- ``UBLK_CMD_END_USER_RECOVERY``
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.
Data plane
----------