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Documentation: ublk: document UBLK_F_USER_RECOVERY_FAIL_IO

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Signed-off-by: Uday Shankar <ushankar@purestorage.com>
Reviewed-by: Ming Lei <ming.lei@redhat.com>
Link: https://lore.kernel.org/r/20241007182419.3263186-6-ushankar@purestorage.com
Signed-off-by: Jens Axboe <axboe@kernel.dk>
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Uday Shankar 2024-10-07 12:24:18 -06:00 committed by Jens Axboe
parent 59eaa01ce7
commit 69f407ee8d
1 changed files with 17 additions and 5 deletions
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Documentation/block/ublk.rst
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@ -199,24 +199,36 @@ managing and controlling ublk devices with help of several control commands:
- user recovery feature description
Two new features are added for user recovery: ``UBLK_F_USER_RECOVERY`` and
``UBLK_F_USER_RECOVERY_REISSUE``.
Three new features are added for user recovery: ``UBLK_F_USER_RECOVERY``,
``UBLK_F_USER_RECOVERY_REISSUE``, and ``UBLK_F_USER_RECOVERY_FAIL_IO``. To
enable recovery of ublk devices after the ublk server exits, the ublk server
should specify the ``UBLK_F_USER_RECOVERY`` flag when creating the device. The
ublk server may additionally specify at most one of
``UBLK_F_USER_RECOVERY_REISSUE`` and ``UBLK_F_USER_RECOVERY_FAIL_IO`` to
modify how I/O is handled while the ublk server is dying/dead (this is called
the ``nosrv`` case in the driver code).
With ``UBLK_F_USER_RECOVERY`` set, after one ubq_daemon(ublk server's io
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
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``,
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``,
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.
With ``UBLK_F_USER_RECOVERY_FAIL_IO`` additionally set, after the ublk server
exits, requests which have issued to userspace are failed, as are any
subsequently issued requests. Applications continuously issuing I/O against
devices with this flag set will see a stream of I/O errors until a new ublk
server recovers the device.
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