Understanding SCSI addressing on Red Hat Enterprise Linux
In common with the SCSI protocol definitions Linux represents scsi addresses as a sequence of 4 ID numbers that uniquely identify the location of a SCSI device within the system. These are referred to as Host, Bus, Target, Lun (HBTL) or Host, Channel, Id, Lun (HCIL ) SCSI addresses. HBTL is the most commonly used acronym in the SCSI literature however some commands and log files label this HCIL instead (the numeric address values are identical in either case). HBTL is also sometimes known as a SCSI address quad.
For example:
The format used is the following :
[Host:Bus:Target:Lun] or [H:B:T:L]
In the examples below, taken from the sg_map command (included in the sg3_utils package available from Red Hat Network), we can see that the SCSI Generic devices sg0 through sg3 are mapped in the system and using different modules of the upper SCSI layer. For instance, we can see:
# sg_map -x
/dev/sg0 0 0 0 0 8
/dev/sg1 0 0 1 0 1 /dev/nst0
/dev/sg2 1 0 0 0 0 /dev/sda
/dev/sg3 1 0 1 0 0 /dev/sdb
sgN H:B:T:L ^ ^^^
| +---- scsi upper layer device name
|
+------------- scsi device type numberThe output of sg_map shows the SCSI generic device number (sgN device name), the SCSI H:B:T:L value, the SCSI device type value, and then if any, the device name for the device using the SCSI upper layer driver (ie, sda for sd_mod (scsi disk), sr0 for sr_mod (scsi cdrom/dvd) , st0 for st_mod (scsi tape), etc.) on the /dev filesystem. To find out the values and corresponding device types displayed on fifth numeric column of the sg_map command please refer to the file /usr/include/scsi/scsi.h and look for the DEVICE TYPES section. For example:
/dev/sg0has H:B:T:L [0:0:0:0], device type 0x8 which is a TYPE_MEDIUM_CHANGER and no extra/deventry because it is only accessed as a SCSI generic device./dev/sg1has H:B:T:L [0:0:1:0], device type 0x1 which is a TYPE_TAPE and/dev/nst0because it is accessed as a SCSI tape device viast_mod.kolinked in the kernel./dev/sg2has H:B:T:L [1:0:0:0], device type 0x0 which is a TYPE_DISK and/dev/sdabecause it is accessed as a SCSI disk device viasd_mod.kolinked in the kernel./dev/sg3has H:B:T:L [1:0:1:0], device type 0x0 which is a TYPE_DISK and/dev/sdbbecause it is accessed as a SCSI disk device viasd_mod.kolinked in the kernel.
where :
H: SCSI Host ID is the ID of the SCSI Host Bus Adapter (HBA) card, also referred to as the scsi controller. This identifier is assigned by the kernel in order of HBA card discovery order (1st scsi HBA assigned scsi host0, 2nd scsi host1, and so forth).- `B': Bus used within the HBA. It is also known as Channel. This identifier is assigned by the kernel and reflects part of the hardware/firmware architecture of the HBA design. Most scsi controllers with have only 1 bus/channel assigned index 0. Backplane raid controllers such as HP's Smart Array or LSI MegaRaid controllers are one example of an HBA that can have multiple channels.
T: Target is the destination device within this bus also known as Id of the device. This identifier, similar to hostN, is assigned by the kernel in order of target discovery... within a given HBA. Therefore these values are a relative scsi target index associated with the scsi host. So the first scsi storage target discovered attached to host0 is assigned scsi target (relative index) 0, then 1, and 2 and so forth. Similarly the first scsi target discovered attached to host1 is also assigned scsi target (relative index) 0, then 1, and 2 and so forth. This means that scsi target id is scsi hostN relative. Within FC and FCoE, the scsi target is a storage port within the san storage box.L: Logical Unit Number (LUN) is the logical device within the SCSI target, as seen by the operating system. Typically a logical disk, or a tape device. The LUN number is assigned by storage -- the only part of the scsi address that is not assigned by the kernel.
The following illustrates the nature of a scsi address providing a "route" from host to scsi device.
HBA (Host Bus Adapter) SCSI
e.g. PCI storage controller address
+---------------------------+ +----------------------------+ h:b:t:l
| SCSI | Bus/Channel(0)+------------->+ SCSI Target (0) | LUN 0 | << 0:0:0:0
| Host (0) +---------------+ | +----------+
| | Bus/Channel(1)+------+ | | LUN 1 | << 0:0:0:1
+---------------------------+ | | +----------+
| | : |
| | +----------+
| | | LUN n | << 0:0:0:n
| +----------------------------+
|
|
| +----------------------------+
+------>+ SCSI Target (0) | LUN 0 | << 0:1:0:0
| +----------+
| | LUN 1 | << 0:1:0:1
| +----------+
| | LUN 2 | << 0:1:0:2
| +----------+
| : |
| +----------+
| | LUN n | << 0:1:0:n
+----------------------------+
A couple things to note within the above: each subsequent piece of the scsi address is unique and relative to the peice before it. So while there is only one scsi host 0 within a server, there could be multiple bus/channel 0s -- one associated with each unique scsi host. The same holds true for scsi targets: in the above diagram there are two SCSI target 0s - but one is 0:0:0 (host:bus:target) and the other is 0:1:0 (host:bus:target) and thus represent unique individual routes.
The following illustrates the nature of scsi address providing a multiple paths or "routes" from multiple HBA to the same set of scsi devices within a Fibre Channel environment.
HBA (Host Bus Adapter)
e.g. PCI storage controller SCSI
+---------------------------+ +-----+ +----------------------------+ Addresses
| SCSI | Bus/Channel(0)+--->+ F +-->+ SCSI Target (n) | LUN 0 | << 0:0:0:0 AND 1:0:2:0
| Host (0) | | | C | | +----------+
| | | | | | | LUN 1 | << 0:0:0:1 AND 1:0:2:1
+---------------------------+ | S | | +----------+
| w | | : |
+---------------------------+ | i | | +----------+
| SCSI | Bus/Channel(0)+--->+ t | | | LUN n | << 0:0:0:n AND 1:0:2:n
| Host (1) | | | c | +----------------------------+
| | | | h |
+---------------------------+ +-----+
While SCSI host 0 may discover the SCSI target "n" as its first SCSI target and thus is assigned target id 0, SCSI host 1 in the diagram above discovered this particular SCSI target as the 3rd target and thus was assigned an id of 2 (after target ids 0 and 1 were assigned to previously disovered targets accessible via host1 -- target ids 1:0:0 and 1:0:1 not shown).
You will be able to find the scsi LUNs in /proc/scsi/scsi. For example, still on the same host as above, we can see:
# cat /proc/scsi/scsi
Attached devices:
Host: scsi0 Channel: 00 Id: 00 Lun: 00
Vendor: ADIC Model: FastStor DLT Rev: D116
Type: Medium Changer ANSI SCSI revision: 02
Host: scsi0 Channel: 00 Id: 01 Lun: 00
Vendor: Quantum Model: DLT4000 Rev: D782
Type: Sequential-Access ANSI SCSI revision: 02
Host: scsi1 Channel: 00 Id: 00 Lun: 00
Vendor: FUJITSU Model: MAJ3091MC Rev: 5508
Type: Direct-Access ANSI SCSI revision: 04
Host1: scsi1 Channel: 00 Id: 01 Lun: 00
Vendor: FUJITSU Model: MAJ3091MC Rev: 5508
Type: Direct-Access ANSI SCSI revision: 04
Host: scsi1 Channel: 00 Id: 02 Lun: 00
Vendor: IBM Model: DDYS-T36950M Rev: S96H
Type: Direct-Access ANSI SCSI revision: 03
Host: scsi1 Channel: 00 Id: 06 Lun: 00
Vendor: DELL Model: 1x5 U2W SCSI BP Rev: 1.16
Type: Processor ANSI SCSI revision: 02as expected on:
Host: scsi0 Channel: 00 Id: 00 Lun: 00 -> HBTL [0:0:0:0] - Medium Changer
Host: scsi0 Channel: 00 Id: 01 Lun: 00 -> HBTL [0:0:1:0] - Sequential-Access (TAPE)
Host: scsi1 Channel: 00 Id: 00 Lun: 00 -> HBTL [1:0:0:0] - Direct-Access (Disk) Another command to output the same data is lsscsi from the package of the same name:
# lsscsi -g
[0:0:0:0] changer ADIC FastStor DLT D116 /dev/sg0
[0:0:1:0] tape Quantum DLT4000 D782 /dev/st0 /dev/sg1
[1:0:0:0] disk FUJITSU MAJ3091MC 5508 /dev/sda /dev/sg2
[1:0:1:0] disk IBM DDYS-T36950M S96H /dev/sdb /dev/sg3
:
As another illustrative example, below shows two S-ATA disks, on two scsi controllers:
# cat /proc/scsi/scsi
Attached devices:
Host: scsi0 Channel: 00 Id: 00 Lun: 00
Vendor: ATA Model: WDC WD2500JS-75N Rev: 10.0
Type: Direct-Access ANSI SCSI revision: 05
Host: scsi1 Channel: 00 Id: 00 Lun: 00
Vendor: ATA Model: WDC WD7500AACS-0 Rev: 01.0
Type: Direct-Access ANSI SCSI revision: 05
In the first entry, we can see an ATA disk of model WDC WD2500JS-75N, which is located on:
Host: scsi0 Channel: 00 Id: 00 Lun: 00 or simply [0:0:0:0]
On Red Hat Enterprise Linux 5 most of the information for SCSI devices can be found within the /sys filesystem. For further information, refer to the article below for the systool command, which is designed to traverse the /sys filesystem gracefully.