tcm_loop

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LinuxIO
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tcm_loop SCSI Emulation Device
Original author(s) Nicholas Bellinger
Developer(s) Datera, Inc.
Initial release May 18, 2011 (2011-05-18)
Stable release 4.1.0 / June 20, 2012;
8 years ago (2012-06-20)
Preview release 4.2.0-rc5 / June 28, 2012;
8 years ago (2012-06-28)
Development status Production
Written in C
Operating system Linux
Type Fabric module
License GNU General Public License, version 2 (GPLv2)
Website datera.io
See LIO for a complete overview over all fabric modules.
FILEIO with a tcm_loop SAS port on KVM x86_64 host running v2.6.34 with the new LSI Megasas 8708EM2 PPC HBA emulation into a Windows 7 x64 KVM Guest.

tcm_loop is a fabric module that can emulate any supported fabric type locally for LinuxIO.

Contents

Overview

tcm_loop provides full local SCSI device emulation (SPC-3 and SPC-4) for raw block storage objects. It thus makes any raw block device apear to the LinuxIO as a full SPC-3/SPC-4 SCSI compliant device with the corresponding CDB-level SAS emulation, for access from any SCSI LLD driver.

tcm_loop minimizes overhead - it delivers >12 GB/s (native) and >2 GB/s (from a single KVM guest, without hardware assist) on Intel Nehalem platforms. This provides e.g., unmodified guest operating systems, such as VMware ESX or VMware vSphere, to use the full SPC-3/SPC-4 SCSI functionality on any type of local raw block storage object they require.

The tcm_loop fabric module (loopback.ko, Linux kernel driver database) for LIO was released with Linux kernel 2.6.39 on May 18, 2011 (2011-05-18).[1]

targetcli

targetcli from Datera, Inc. is used to configure tcm_loop targets. targetcli aggregates LIO service modules via a core library, and exports them through an API, to provide a unified single-node SAN configuration shell, independently of the underlying fabric(s).

LIO Admin Manual
The LIO Admin Manual provides comprehensive background and many examples on using targetcli and on programming the Datera library.

Startup

targetcli is invoked by running targetcli as root from the command prompt of the underlying LIO shell. An initial setup as described in targetcli is assumed. 

# targetcli
Welcome to targetcli:

 Copyright (c) 2014 by Datera, Inc.
 All rights reserved.

Visit us at http://www.datera.io.

Using ib_srpt fabric module.
Using qla2xxx fabric module.
Using iscsi fabric module.
Using loopback fabric module.

/> loopback/ info
Fabric module name: loopback
ConfigFS path: /sys/kernel/config/target/loopback
Allowed WWN list type: naa
Fabric module specfile: /var/target/fabric/loopback.spec
Fabric module features: nexus
Corresponding kernel module: tcm_loop
/>

Cheat sheet

Command Comment
/backstores/iblock create my_disk /dev/sdb Create the LUN my_disk on the device /dev/sdb
In /backstores/iblock/my_disk>:
/loopback create 		Create a local loopback target
/saveconfig Commit the configuration

Display the object tree

Use ls to list the object hierarchy, which is initially empty: 

/> ls
o- / ..................................................................... [...]
  o- backstores .......................................................... [...]
  | o- fileio ............................................... [0 Storage Object]
  | o- iblock ............................................... [0 Storage Object]
  | o- pscsi ................................................ [0 Storage Object]
  | o- rd_dr ................................................ [0 Storage Object]
  | o- rd_mcp ............................................... [0 Storage Object]
  o- ib_srpt ........................................................ [0 Target]
  o- iscsi .......................................................... [0 Target]
  o- loopback ....................................................... [0 Target]
  o- qla2xxx ........................................................ [0 Target]
/>
Global parameter auto_cd_after_create
After the creation of a new object, automatically enter its object context.

Per default, auto_cd_after_create is set to true, which automatically enters an object context (or working directory) after its creation. The examples here are modeled after this behavior.

Optionally, set auto_cd_after_create=false to prevent targetcli from automatically entering new object contexts after their creation: 

/> set global auto_cd_after_create=false
Parameter auto_cd_after_create is now 'false'.
/>

Create a backstore

Create a backstore using the IBLOCK or FILEIO type devices.

For instance, enter the top-level backstore context and create an IBLOCK backstore from a /dev/sdb block device: 

/> cd backstores/
/backstores> iblock/ create name=my_disk dev=/dev/sdb
Generating a wwn serial.
Created iblock storage object my_disk using /dev/sdb.
Entering new node /backstores/iblock/my_disk.
/backstores/iblock/my_disk>

targetcli automatically creates a WWN serial ID for the backstore device and then changes the working context to it.

The resulting object hierarchy looks as follows (displayed from the root object): 

/> ls
o- / ..................................................................... [...]
  o- backstores .......................................................... [...]
  | o- fileio ............................................... [0 Storage Object]
  | o- iblock ............................................... [1 Storage Object]
  | | o- my_disk .......................................... [/dev/sdb activated]
  | o- pscsi ................................................ [0 Storage Object]
  | o- rd_dr ................................................ [0 Storage Object]
  | o- rd_mcp ............................................... [0 Storage Object]
  o- ib_srpt ........................................................ [0 Target]
  o- iscsi .......................................................... [0 Target]
  o- loopback ....................................................... [0 Target]
  o- qla2xxx ........................................................ [0 Target]
/>

Alternatively, any LVM logical volume can be used as a backstore, please refer to the LIO Admin Manual on how to create them properly.

For instance, create an IBLOCK backstore on a logical volume (under /dev/<volume_group_name>/<logical_volume_name>): 

/backstores> iblock/ create name=my_disk dev=/dev/vg0/lv1
Generating a wwn serial.
Created iblock storage object my_disk using /dev/vg0/lv1.
Entering new node /backstores/iblock/my_disk.
/backstores/iblock/my_disk>

Again, targetcli automatically creates a WWN serial ID for the backstore device and then changes the working context to it.

More backstore examples
More examples on creating backstores can be found in targetcli.

Instantiate a target

Instantiate a loopback target on the existing IBLOCK backstore device my_disk (as set up in targetcli): 

/backstores/iblock/my_disk> /loopback create
Successfully created LUN 0.
Entering new node /loopback/naa.60014057f822d991/luns/lun0.
/loopback/naa...991/luns/lun0>

targetcli automatically assigns the default ID '0' to the LUN, and then changes the working context to the new LUN. Now the target is created, and exports /dev/sdb as LUN 0.

Return to the root context, as no attributes need to be set or modified for a standard loopback target: 

/loopback/naa...991/luns/lun0> cd /
/>

Display the object tree

The resulting object hierarchy looks as follows (displayed from the root object): 

/> ls
o- / ..................................................................... [...]
  o- backstores .......................................................... [...]
  | o- fileio ............................................... [0 Storage Object]
  | o- iblock ............................................... [1 Storage Object]
  | | o- my_disk .......................................... [/dev/sdb activated]
  | o- pscsi ................................................ [0 Storage Object]
  | o- rd_dr ................................................ [0 Storage Object]
  | o- rd_mcp ............................................... [0 Storage Object]
  o- ib_srpt ........................................................ [0 Target]
  o- iscsi .......................................................... [0 Target]
  o- loopback ....................................................... [1 Target]
  | o- naa.60014057f822d991 ............................. [naa.60014053485b9490]
  | | o- luns .......................................................... [1 LUN]
  | |   o- lun0 .................................... [iblock/my_disk (/dev/sbd)]
  o- qla2xxx ........................................................ [0 Target]
/>

Persist the configuration

Don't forget to use saveconfig!
Without saveconfig, the LIO configuration will be lost upon rebooting or unloading the target service, as it will revert back to the last saved one.

Use saveconfig from the root context to persist the LIO configuration across OS reboots: 

/> saveconfig
WARNING: Saving rtsnode1 current configuration to disk will overwrite your boot settings.
The current target configuration will become the default boot config.
Are you sure? Type 'yes': yes
Making backup of srpt/ConfigFS with timestamp: 2012-02-27_23:19:37.660264
Successfully updated default config /etc/target/srpt_start.sh
Making backup of qla2xxx/ConfigFS with timestamp: 2012-02-27_23:19:37.660264
Successfully updated default config /etc/target/qla2xxx_start.sh
Making backup of loopback/ConfigFS with timestamp: 2012-02-27_23:19:37.660264
Successfully updated default config /etc/target/loopback_start.sh
Making backup of LIO-Target/ConfigFS with timestamp: 2012-02-27_23:19:37.660264
Successfully updated default config /etc/target/lio_backup-2012-02-27_23:19:37.660264.sh
Making backup of Target_Core_Mod/ConfigFS with timestamp: 2012-02-27_23:19:37.660264
Successfully updated default config /etc/target/tcm_backup-2012-02-27_23:19:37.660264.sh
Generated Target_Core_Mod config: /etc/target/backup/tcm_backup-2012-02-27_23:19:37.660264.sh
Successfully updated default config /etc/target/lio_start.sh
Successfully updated default config /etc/target/tcm_start.sh
/>

Spec file

Datera spec files define the fabric-dependent feature set, capabilities and available target ports of the specific underlying fabric. In particular, the tcm_loop spec file /var/target/fabric/loopback.spec is included via RTSlib. 

# WARNING: This is a draft specfile supplied for demo purposes only.

# The loopback fabric module uses the default feature set
features = nexus

# Use naa WWNs
wwn_type = naa

# Non-standard module naming scheme
kernel_module = tcm_loop

# The configfs group
configfs_group = loopback

Specifications

The type of SCSI fabric features that are available with tcm_loop (target mode emulated SAS ports) includes the complete set of SPC-3/SPC-4 logic in the LIO engine, including advanced features such as PRs, ALUA, VAAI, etc.

tcm_loop supports high level fabric emulation. That is, depending on which WWN is used to create the ConfigFS group in /sys/kernel/config/target/loopback/$WWN, it configures itself to return iSCSI, FC etc. device identifiers for the SCSI LUN protocol identifiers device identifier page.

See also

Notes

  1. ↑ Linus Torvalds (2011-05-18). "Linux 2.6.39". lkml.org. 

External links

Timeline of the LinuxIO
Release Details 2011 2012 2013 2014 2015
123456789101112 123456789101112 123456789101112 123456789101112 123456789101112
4.x Version 4.0 4.1
Feature LIO Core Loop back FCoE iSCSI Perf SRP
 		CM WQ FC
USB
1394 		vHost Perf Misc 16 GFC iSER Misc VAAI Misc DIF Core
NPIV 		DIF iSER DIF FC vhost TCMU Xen Misc Misc virtio 1.0 Misc NVMe OF
Linux 2.6.38 2.6.39 3.0 3.1 3.2 3.3 3.4 3.5 3.6 3.7 3.8 3.9 3.10 3.11 3.12 3.13 3.14 3.15 3.16 3.17 3.18 3.19 3.20 3.21 3.22
Retrieved from "http://wwvw.linux-iscsi.org/wiki/Tcm_loop"
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