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This allows userspace to request the fabric drivers do direct submissions
if they support it. With the new device file, submit_type, users can
write 0 - 2 to control how commands are submitted to the backend:
0 - TARGET_FABRIC_DEFAULT_SUBMIT - LIO will use the fabric's default
submission type. This is the default for compat.
1 - TARGET_DIRECT_SUBMIT - LIO will submit the cmd to the backend from the
calling context if the fabric the cmd was received on supports it,
else it will use the fabric's default type.
2 - TARGET_QUEUE_SUBMIT - LIO will queue the cmd to the LIO submission
workqueue which will pass it to the backend.
When using an NVMe drive and vhost-scsi with direct submission we see
around a 20% improvement in 4K I/Os:
fio jobs 1 2 4 8 10
--------------------------------------------------
defer 94K 190K 394K 770K 890K
direct 128K 252K 488K 950K -
And when using the queueing mode, we now no longer see issues like where
the iSCSI tx thread is blocked in the block layer waiting on a tag so it
can't respond to a nop or perform I/Os for other LUs.
Signed-off-by: Mike Christie <michael.christie@oracle.com>
Link: https://lore.kernel.org/r/20230928020907.5730-6-michael.christie@oracle.com
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>
Linux kernel
============
There are several guides for kernel developers and users. These guides can
be rendered in a number of formats, like HTML and PDF. Please read
Documentation/admin-guide/README.rst first.
In order to build the documentation, use ``make htmldocs`` or
``make pdfdocs``. The formatted documentation can also be read online at:
https://www.kernel.org/doc/html/latest/
There are various text files in the Documentation/ subdirectory,
several of them using the Restructured Text markup notation.
Please read the Documentation/process/changes.rst file, as it contains the
requirements for building and running the kernel, and information about
the problems which may result by upgrading your kernel.