[PATCH 1/1] Documentation: document ublk user recovery feature

ZiyangZhang
2022-10-18T04:55:10Z

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>
---
Documentation/block/ublk.rst | 36 ++++++++++++++++++++++++++++++++++++
1 file changed, 36 insertions(+)

diff --git a/Documentation/block/ublk.rst b/Documentation/block/ublk.rst
index 2122d1a4a541..ba45c46cc0da 100644
--- a/Documentation/block/ublk.rst
+++ b/Documentation/block/ublk.rst
@@ -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
----------

--
2.27.0