tcm_loop
The Linux SCSI Target Wiki
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![]() tcm_loop SCSI Emulation Device | |
Original author(s) | Nicholas Bellinger |
Developer(s) | Datera, Inc. |
Initial release | May 18, 2011 |
Stable release | 4.1.0 / June 20, 2012 |
Preview release | 4.2.0-rc5 / June 28, 2012 |
Development status | Production |
Written in | C |
Operating system | Linux |
Type | Fabric module |
License | GNU General Public License |
Website | datera.io |
- See Target for a complete overview over all fabric modules.
tcm_loop is a fabric module that can emulate any supported fabric type locally for the Linux Unified Target.
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 Target 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 the Unified Target was released with Linux kernel 2.6.39 on May 18, 2011 .[1]
targetcli
targetcli from Datera, Inc. is used to configure tcm_loop targets. targetcli aggregates service modules via a core library, and exports them through an API to the Unified Target, to provide a unified single-node SAN configuration shell, independently of the underlying fabric(s).
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RTS OS Admin Manual The RTS OS Admin Manual provides comprehensive background and many examples on using targetcli and on programming the RTS library. |
Startup
targetcli is invoked by running targetcli as root from the command prompt of the underlying OS shell. An initial setup as described in targetcli is assumed.
# targetcli Welcome to targetcli: Copyright (c) 2012 by RisingTide Systems, Inc. All rights reserved. Visit us at http://www.risingtidesystems.com. 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 />
Cookbook
The following is a cheat sheet for reference purposes.
Command | Comment |
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/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] />
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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 RTS OS 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.
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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
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Don't forget to use saveconfig! Without saveconfig, the target configuration will be lost upon rebooting or unloading the target service, as the target configuration will revert back to the last saved one. |
Use saveconfig from the root context to persist the target 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
RTS 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 Target engine including advanced features such as PRs, ALUA, 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
- RTS OS, targetcli
- Target
- Fibre Channel, FCoE, iSCSI, iSER, SRP, vHost
- ConfigFS (low level kernel API)
Notes
- ↑ Linus Torvalds (2011-05-18). "Linux 2.6.39". lkml.org.
External links
- RisingTide Systems Official website
- RTS OS Admin Manual
- RTSlib Reference Guide [HTML][PDF]
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Release | Details | 2011 | 2012 | 2013 | 2014 | 2015 | |||||||||||||||||||||||||||||||||||||||||||||||||||||||
1 | 2 | 3 | 4 | 5 | 6 | 7 | 8 | 9 | 10 | 11 | 12 | 1 | 2 | 3 | 4 | 5 | 6 | 7 | 8 | 9 | 10 | 11 | 12 | 1 | 2 | 3 | 4 | 5 | 6 | 7 | 8 | 9 | 10 | 11 | 12 | 1 | 2 | 3 | 4 | 5 | 6 | 7 | 8 | 9 | 10 | 11 | 12 | 1 | 2 | 3 | 4 | 5 | 6 | 7 | 8 | 9 | 10 | 11 | 12 | ||
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 |