⚠️ IMPORTANT - Please read this section carefully if you are currently using or plan to use gluster-block or want to contribute to the project. ⚠️
As of gluster-block release v5.1 the project is considered to be in maintenance only status. This means that only major bugs, performance regressions, or security defects are being considered for inclusion by the project team. Please also expect slow replies to issues.
You can run gluster-blockd as systemd service, note '/etc/sysconfig/gluster-blockd' is the configuration file which gets dynamicaly reloaded on changing any options.
* Read More on how to [create a gluster volume](https://gluster.readthedocs.io/en/latest/Administrator%20Guide/Setting%20Up%20Volumes/#creating-replicated-volumes).
[Gluster](http://gluster.readthedocs.io/en/latest/) is a well known scale-out distributed storage system, flexible in its design and easy to use. One of its key goals is to provide high availability of data. Gluster is very easy to setup and use. Addition and removal of storage servers from a Gluster cluster is intuitive. These capabilities along with other data services that Gluster provides makes it a reliable software defined storage platform.
We can access glusterfs via [FUSE](https://en.wikipedia.org/wiki/Filesystem_in_Userspace) module. However to perform a single filesystem operation various context switches are required which can often exhibit performance issues. [Libgfapi](http://blog.gluster.org/2014/04/play-with-libgfapi-and-its-python-bindings/) is a userspace library for accessing data in Glusterfs. It can perform I/O on gluster volumes without the FUSE module, kernel VFS layer and hence requires no context switches. It exposes a filesystem like API for accessing gluster volumes. Samba, NFS-Ganesha, QEMU and now the tcmu-runner all use libgfapi to integrate with Gluster.
> A unique distributed storage solution build on traditional filesystems
1. Create a file in the gluster volume (Block Hosting Volume)
2. We expose the file in the gluster volume as tcmu backstore using tcmu-runner, exporting the target file as iSCSI LUN and
3. From the initiator we login to the exported LUN and play with the block device
#### Background
The [SCSI](http://searchstorage.techtarget.com/definition/SCSI) subsystem uses a form of client-server model. The Client/Initiator request I/O happen through target which is a storage device. The SCSI target subsystem enables a computer node to behave as a SCSI storage device, responding to storage requests by other SCSI initiator nodes.
In simple terms SCSI is a set of standards for physically connecting and transferring data between computers and peripheral devices.
The most common implementation of the SCSI target subsystem is an iSCSIserver, [iSCSI](http://searchstorage.techtarget.com/definition/iSCSI) transports block level data between the iSCSI initiator and the target which resides on the actual storage device. iSCSi protocol wraps up the SCSI commands and sends it over TCP/IP layer. Up on receiving the packets at the other end it disassembles them to form the same SCSI commands, hence on the OS’es it seen as local SCSI device.
> In other words iSCSI is SCSI over TCP/IP.
The [LIO](http://linux-iscsi.org/wiki/LIO) project began with the iSCSI design as its core objective, and created a generic SCSI target subsystem to support iSCSI. LIO is the SCSI target in the Linux kernel. It is entirely kernel code, and allows exported SCSI logical units (LUNs) to be backed by regular files or block devices.
> LIO is Linux IO target, is an implementation of iSCSI target.
[TCM](https://www.kernel.org/doc/Documentation/target/tcmu-design.txt) is another name for LIO, an in-kernel iSCSI target (server). As we know existing TCM targets run in the kernel. TCMU (TCM in Userspace) allows userspace programs to be written which act as iSCSI targets. These enables wider variety of backstores without kernel code. Hence the TCMU userspace-passthrough backstore allows a userspace process to handle requests to a LUN. TCMU utilizes the traditional UIO subsystem, which is designed to allow device driver development in userspace.
> One such backstore with best clustered network storage capabilities is GlusterFS
Any TCMU userspace-passthrough can utilize the TCMU framework handling the messy details of the TCMU interface.
One such passthrough is [Tcmu-runner](https://github.com/open-iscsi/tcmu-runner) (Thanks to Andy Grover). Tcmu-runner has a glusterfs handler that can interact with the backed file in gluster volume over gluster libgfapi interface and can show it as a target (over network).
Some responsibilities of userspace-passthrough include,
Discovering and configuring TCMU UIO devices
waiting for the events on the device and
managing the command ring buffers
[TargetCli](https://github.com/Datera/targetcli) is the general management platform for the LIO/TCM/TCMU. TargetCli with its shell interface is used to configure LIO.
> Think it like a shell which makes life easy in configuring LIO core
gluster-block is licensed to you under your choice of the GNU Lesser General Public License, version 3 or any later version ([LGPLv3](https://opensource.org/licenses/lgpl-3.0.html) or later), or the GNU General Public License, version 2 ([GPLv2](https://opensource.org/licenses/GPL-2.0)), in all cases as published by the Free Software Foundation.
