HP 3000 Enterprise Virtual Array - Manual (Page 83)

Variations in process scheduling and network delay can result in iSCSI targets being mapped to
different SCSI device nodes every time the driver is started. Because of this variability, configuring
applications or operating system utilities to use the standard SCSI device nodes to access iSCSI devices
can result in sending SCSI commands to the wrong target or logical unit.
To provide consistent naming, the iSCSI driver scans the system to determine the mapping from SCSI
device nodes to iSCSI targets. The iSCSI driver creates a tree of directories and symbolic links under
/dev/iscsi
to make it easier to use a particular iSCSI target's logical unit.
The directory tree under /dev/iscsi contains subdirectories for each iSCSI bus number, each target
id number on the bus, and each logical unit number for each target. For example, the whole disk
device for bus 0, target ID 0, and LUN 0 would be /dev/iscsi/bus0/target0/LUN0/
disk
.
In each logical unit directory there is a symbolic link for each SCSI device node that can be connected
to that particular logical unit. These symbolic links are modeled after the Linux devfs naming
convention:
• The symbolic link disk maps to the whole-disk SCSI device node such as /dev/sda or /dev/
sdb
.
• The symbolic links part1 through part15 maps to each partition of that SCSI disk. For example,
a symbolic link can map to partitions /dev/sda1, dev/sda15, or to as many partitions as ne-
cessary.
NOTE:
These symbolic links exist regardless of the number of disk partitions. Opening the partition devices
results in an error if the partition does not actually exist on the disk.
• The symbolic link mt maps to the auto-rewind SCSI tape device node for the LUN /dev/st0, for
example. Additional links for mtl, mtm, and mta map to the other auto-rewind devices /dev/
st0l, /dev/st0m, /dev/st0a
, regardless of whether these device nodes actually exist or
could be opened.
• The symbolic link mtn maps to the no-rewind SCSI tape device node, if any. For example, this
LUN maps to /dev/nst0. Additional links formtln, mtmn, and mtan map to the other no-rewind
devices such as /dev/nst0l, /dev/nst0m, /dev/nst0a, regardless of whether those device
nodes actually exist or could be opened.
• The symbolic link cd maps to the SCSI CD-ROM device node, if any, for the LUN /dev/scd0 for
example.
• The symbolic link generic maps to the SCSI generic device node, if any, for the LUN /dev/
sg0
.
Because the symlink creation process must open all of the SCSI device nodes in /dev in order to
determine which nodes map to iSCSI devices, you may see many modprobe messages logged to
syslog
indicating that modprobe could not find a driver for a particular combination of major and
minor numbers. This message can be ignored. The messages occur when Linux is unable to find a
driver to associate with a SCSI device node that the iSCSI daemon is opening as part of its symlink
creation process. To prevent these messages from occurring, remove the SCSI device nodes that do
not contain an associated high-level SCSI driver.
EVA iSCSI Connectivity User Guide 83