tcm_loop

The Linux SCSI Target Wiki

LIO Target

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
Website	datera.io

See Target 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.

FILEIO with a tcm_loop SCSI port on KVM x86_64 host running v2.6.36-rc3+ with QEMU lsi53c895a HBA emulation into an OS/2 Warp v4 guest. A HPFS partition has been formatted and mounted as DRIVE D:.

tcm_loop fabric module running on LIO v3.2 on a Linux Host providing shared virtual SAS Target emulation into a VM hypervisor to multiple VM guests of RHEL 5.4 x86_64 and NetBSD 5.x 32-bit.

tcm_loop fabric module running on LIO v3.2 on Linux Host v2.6.32-rc5 providing persistent reservations to RHEL 5.4 x86_64 KVM Guest using scsi generic passthrough with Vmware Workstation 6.5.

tcm_loop is a pluggable virtual fabric module (tcm_loop.ko) for the LIO multiprotocol Target engine.

It 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.

tcm_loop was initially released on 10/19/2009,^[1] and it went upstream into the Linux 2.6.39 kernel on 3/18/2011.^[2]

Specification compliance

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

Target 4 tcm_loop also 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, SAS or FC device identifiers for the SCSI LUN protocol identifiers in the EVPD 0x83 device identifier page.

The entire corresponding logic resides in target_core_fabric_lib.c, and is shared by the different target fabric modules. The code simply contains magic numbers defined for different protocols in SPC-4, and is commented using pieces directly from SPC-4.

Setup

targetcli from Datera, Inc. is a comprehensive, powerful, yet easy storage management tool that can efficiently handle complex storage installations. The tcm_loop fabric module can be configured as described in tcm_loop/targetcli.

Results

See tcm_loop/configFS for the configFS and kernel ringbuffer output of a working tcm_loop driver.

Other healthy results should look similar to the following examples, which are provided as a reference.

SCSI devices

The list of resulting SCSI devices with lsscsi -v output is:

[9:0:1:0]    disk    LIO-ORG  IBLOCK           3.1   /dev/sdg 
  dir: /sys/bus/scsi/devices/9:0:1:0  [/sys/devices/tcm_loop_0/tcm_loop_adapter_0/host9/target9:0:1/9:0:1:0]
[10:0:1:0]   disk    LIO-ORG  IBLOCK           3.1   /dev/sdh 
  dir: /sys/bus/scsi/devices/10:0:1:0  [/sys/devices/tcm_loop_0/tcm_loop_adapter_1/host10/target10:0:1/10:0:1:0]

Block devices

The list of resulting block devices with sg_inq -i /dev/sdg is:

VPD INQUIRY: Device Identification page
Designation descriptor number 1, descriptor length: 20
  id_type: NAA,  code_set: Binary
  associated with the addressed logical unit
    NAA 6, IEEE Company_id: 0x1405
    Vendor Specific Identifier: 0xa97e4ce21
    Vendor Specific Identifier Extension: 0xc0711de829b000c2
    [0x6001405a97e4ce21c0711de829b000c2]
Designation descriptor number 2, descriptor length: 52
  id_type: T10 vendor identification,  code_set: ASCII
  associated with the addressed logical unit
    vendor id: LIO-ORG
    vendor specific: IBLOCK:a97e4ce21c0711de829b000c2943d57b
Designation descriptor number 3, descriptor length: 8
  transport: Serial Attached SCSI (SAS)
  id_type: Relative target port,  code_set: Binary
  associated with the target port
    Relative target port: 0x8
Designation descriptor number 4, descriptor length: 8
  transport: Serial Attached SCSI (SAS)
  id_type: Target port group,  code_set: Binary
  associated with the target port
    Target port group: 0x0
Designation descriptor number 5, descriptor length: 8
  id_type: Logical unit group,  code_set: Binary
  associated with the addressed logical unit
    Logical unit group: 0x0
Designation descriptor number 6, descriptor length: 36
    transport: Serial Attached SCSI (SAS)
    id_type: SCSI name string,  code_set: UTF-8
    associated with the target port
    SCSI name string:
    naa.600140554cf3a18e,t,0x0001

Modules

The resulting loadable kernel modules (LKMs) with lsmod output are:

Module                  Size  Used by
tcm_loop               11606  9 
iscsi_target_mod      205640  44 
target_core_mod       226770  75 tcm_loop,iscsi_target_mod
scsi_tgt                8806  1 target_core_mod
configfs               21168  4 tcm_loop,iscsi_target_mod,target_core_mod
dm_mirror              10356  0 
dm_region_hash          7562  1 dm_mirror
dm_log                  6769  2 dm_mirror,dm_region_hash
dm_snapshot            18562  0 
pcnet32                25510  0 
mii                     3430  1 pcnet32

Timeline

See also

• targetcli
• ConfigFS: tcm_loop/configFS (low level kernel API)
• Other fabric modules: iSCSI, Fibre Channel, FCoE, InfiniBand, IBM vSCSI
• Target

Notes

External links

• RisingTide Systems Official website
• RTSadmin Reference Guide
• RTSlib Referemce Guide