Installation guide

Chapter 21. Online Storage Management
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• the Logical Unit Number (LUN)
This path-based address is not persistent. It may change any time the system is reconfigured (either
by on-line reconfiguration, as described in this manual, or when the system is shutdown, reconfigured,
and rebooted). It is even possible for the path identifiers to change when no physical reconfiguration
has been done, as a result of timing variations during the discovery process when the system boots, or
when a bus is re-scanned.
The operating system provides several non-persistent names to represent these access paths
to storage devices. One is the /dev/sd name; another is the major:minor number. A third is
a symlink maintained in the /dev/disk/by-path/ directory. This symlink maps from the path
identifier to the current /dev/sd name. For example, for a Fibre Channel device, the PCI info and
Host:BusTarget:LUN info may appear as follows:
pci-0000:02:0e.0-scsi-0:0:0:0 -> ../../sda
For iSCSI devices, by-path/ names map from the target name and portal information to the sd
name.
It is generally not appropriate for applications to use these path-based names. This is because the
storage device these paths reference may change, potentially causing incorrect data to be written
to the device. Path-based names are also not appropriate for multipath devices, because the path-
based names may be mistaken for separate storage devices, leading to uncoordinated access and
unintended modifications of the data.
In addition, path-based names are system-specific. This can cause unintended data changes when
the device is accessed by multiple systems, such as in a cluster.
For these reasons, several persistent, system-independent, methods for identifying devices have been
developed. The following sections discuss these in detail.
21.3.1. WWID
The World Wide Identifier (WWID) can be used in reliably identifying devices. It is a persistent,
system-independent ID that the SCSI Standard requires from all SCSI devices. The WWID identifier is
guaranteed to be unique for every storage device, and independent of the path that is used to access
the device.
This identifier can be obtained by issuing a SCSI Inquiry to retrieve the Device Identification Vital
Product Data (page 0x83) or Unit Serial Number (page 0x80). The mappings from these WWIDs
to the current /dev/sd names can be seen in the symlinks maintained in the /dev/disk/by-id/
directory.
For example, a device with a page 0x83 identifier would have:
scsi-3600508b400105e210000900000490000 -> ../../sda
Or, a device with a page 0x80 identifier would have:
scsi-SSEAGATE_ST373453LW_3HW1RHM6 -> ../../sda
Red Hat Enterprise Linux automatically maintains the proper mapping from the WWID-based device
name to a current /dev/sd name on that system. Applications can use the /dev/disk/by-id/
name to reference the data on the disk, even if the path to the device changes, and even when
accessing the device from different systems.