21.9. Scanning Storage Interconnects

There are several commands available that allow you to reset and/or scan one or more interconnects, potentially adding and removing multiple devices in one operation. This type of scan can be disruptive, as it can cause delays while I/O operations timeout, and remove devices unexpectedly. As such, Red Hat recommends that this type of scan be used only when necessary. In addition, the following restrictions must be observed when scanning storage interconnects:

All I/O on the effected interconnects must be paused and flushed before executing the procedure, and the results of the scan checked before I/O is resumed.
As with removing a device, interconnect scanning is not recommended when the system is under memory pressure. To determine the level of memory pressure, run the command vmstat 1 100; interconnect scanning is not recommended if free memory is less than 5% of the total memory in more than 10 samples per 100. It is also not recommended if swapping is active (non-zero si and so columns in the vmstat output). The command free can also display the total memory.

The following commands can be used to scan storage interconnects.

echo "1" > /sys/class/fc_host/host/issue_lip: This operation performs a Loop Initialization Protocol (LIP) and then scans the interconnect and causes the SCSI layer to be updated to reflect the devices currently on the bus. A LIP is, essentially, a bus reset, and will cause device addition and removal. This procedure is necessary to configure a new SCSI target on a Fibre Channel interconnect.
Bear in mind that issue_lip is an asynchronous operation. The command may complete before the entire scan has completed. You must monitor /var/log/messages to determine when it is done.
The lpfc and qla2xxx drivers support issue_lip. For more information about the API capabilities supported by each driver in Red Hat Enterprise Linux, refer to Table 21.1, “Fibre-Channel API Capabilities”.
/usr/bin/rescan-scsi-bus.sh: This script was included as of Red Hat Enterprise Linux 5.4. By default, this script scans all the SCSI buses on the system, updating the SCSI layer to reflect new devices on the bus. The script provides additional options to allow device removal and the issuing of LIPs. For more information about this script (including known issues), refer to Section 21.15, “Adding/Removing a Logical Unit Through rescan-scsi-bus.sh”.
echo "- - -" > /sys/class/scsi_host/hosth/scan: This is the same command described in Section 21.6, “Adding a Storage Device or Path” to add a storage device or path. In this case, however, the channel number, SCSI target ID, and LUN values are replaced by wildcards. Any combination of identifiers and wildcards is allowed, allowing you to make the command as specific or broad as needed. This procedure will add LUNs, but not remove them.
rmmod driver-name or modprobe driver-name: These commands completely re-initialize the state of all interconnects controlled by the driver. Although this is extreme, it may be appropriate in some situations. This may be used, for example, to re-start the driver with a different module parameter value.