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sheepdog: fine grained locking in write path

Current sheepdog driver has a big write lock section. It harms write
performance significantly. This patch removes a needless big write
lock section with checking submitted requests can cause COW or
not. The write performance can be improved significantly.

In a case of 4k rand write by fio:
 - before: 1144 IOPS
 - after:  19074 IOPS

Signed-off-by: Hitoshi Mitake <mitake.hitoshi@lab.ntt.co.jp>
Signed-off-by: FUJITA Tomonori <fujita.tomonori@lab.ntt.co.jp>
latest commit 0c531dfe18
@mitake mitake authored committed

README

Introduction
-------------
Linux target framework (tgt) aims to simplify various SCSI target
driver (iSCSI, Fibre Channel, SRP, etc) creation and maintenance.

Currently, tgt supports the following target drivers:

- iSCSI software target driver for Ethernet NICs

- iSER software target driver for Infiniband and RDMA NICs

- IBM System p VIO server

- FCoE software target driver for Ethernet NICs (in progress)

- Qlogic qla2xxx FC target driver (in progress)

Tgt consists of kernel modules, user-space daemon, and user-space
tools. iSCSI, iSER, and FCoE target drivers use only user-space daemon
and tools (i.e. they are just user-space applications. They don't need
any kernel support).

tgt can emulate the following device types:

- SBC: a virtual disk drive that can use a file to store the content.

- SMC: a virtual media jukebox that can be controlled by the "mtx"
tool.

- MMC: a virtual DVD drive that can read DVD-ROM iso files and create
burnable DVD+R. It can be combined with SMC to provide a fully
operational DVD jukebox.

- SSC: a virtual tape device (aka VTL) that can use a file to store
the content.

- OSD: a virtual object-based storage device that can use a file to
store the content (in progress).

The code is under the GNU General Public License version 2.


Preparation
-------------
Linux kernel 2.6.22 or newer are recommended because tgt can get
better performance with signalfd.

Target drivers have their own ways to build, configure, etc. Please
find an appropriate documentation in the doc directory. You might find
other useful information on tgt's site:

http://stgt.sourceforge.net/


Developer Notes
-------------
The central resource for tgt development is the mailing list
(stgt@vger.kernel.org).

First, please read the following documents (in short, follow Linux
kernel development rules):

https://www.kernel.org/doc/Documentation/CodingStyle
https://www.kernel.org/doc/Documentation/SubmittingPatches

Then, check your patches with the patch style checker prior to
submission (scripts/checkpatch.pl) like the following example.

fujita@arbre:~/git/tgt$ ./scripts/checkpatch.pl ~/0001-add-bidi-support.patch
Your patch has no obvious style problems and is ready for submission.
Something went wrong with that request. Please try again.