[RESEND PATCH V5 7/7] Documentation: document ublk user recovery feature

ZiyangZhang
2022-09-23T06:17:54Z

Add documentation for user recovery feature of ublk subsystem.

Signed-off-by: ZiyangZhang <ZiyangZhang@linux.alibaba.com>
---
Documentation/block/ublk.rst | 32 ++++++++++++++++++++++++++++++++
1 file changed, 32 insertions(+)

diff --git a/Documentation/block/ublk.rst b/Documentation/block/ublk.rst
index 2122d1a4a541..c3dde087e601 100644
--- a/Documentation/block/ublk.rst
+++ b/Documentation/block/ublk.rst
@@ -144,6 +144,38 @@ 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 closed. User should send this command before he starts
+ a new process which 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 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(ublksrv io handler) is
+ dying, ublk does not release ``/dev/ublkc*`` or ``/dev/ublkb*`` but requeues all
+ inflight requests which have not been issued to userspace. Requests which have
+ been issued to userspace are aborted.
+
+ With ``UBLK_F_USER_RECOVERY_REISSUE`` set, after one ubq_daemon(ublksrv 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