Uuid And Other Persistent Identifiers; Removing A Storage Device - Red Hat ENTERPRISE LINUX 5.5 - ONLINE STORAGE GUIDE Manual

Online Storage Guide
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Device-mapper-multipath automatically maintains the proper mapping of each WWID-based device
name to its corresponding /dev/sd name on the system. These names are persistent across path
changes, and they are consistent when accessing the device from different systems.
When the user_friendly_names feature (of device-mapper-multipath) is used, the WWID is
mapped to a name of the form /dev/mapper/mpathn. By default, this mapping is maintained in the
file /var/lib/multipath/bindings. These mpathn names are persistent as long as that file is
maintained.
Warning
The multipath bindings file (by default, /var/lib/multipath/bindings) must be
available at boot time. If /var is a separate filesystem from /, then you must change the
default location of the file. For more information, refer to
DOC-17650.
Important
If you use user_friendly_names, then additional steps are required to obtain
consistent names in a cluster. Refer to the
Using Device-Mapper Multipath
the
In addition to these persistent names provided by the system, you can also use udev rules to
implement persistent names of your own, mapped to the WWID of the storage. For more information
about this, refer to http://kbase.redhat.com/faq/docs/DOC-7319.

4.2. UUID and Other Persistent Identifiers

If a storage device contains a filesystem, then that filesystem may provide one or both of the following:
• Universally Unique Identifier (UUID)
• Filesystem label
These identifiers are persistent, and based on metadata written on the device by certain applications.
They may also be used to access the device using the symlinks maintained by the operating system
in the /dev/disk/by-label/ (e.g. boot -> ../../sda1 ) and /dev/disk/by-uuid/ (e.g.
f8bf09e3-4c16-4d91-bd5e-6f62da165c08 -> ../../sda1) directories.
md and LVM write metadata on the storage device, and read that data when they scan devices. In
each case, the metadata contains a UUID, so that the device can be identified regardless of the
path (or system) used to access it. As a result, the device names presented by these facilities are
persistent, as long as the metadata remains unchanged.

5. Removing a Storage Device

Before removing access to the storage device itself, it is advisable to back up data from the device
first. Afterwards, flush I/O and remove all operating system references to the device (as described
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http://kbase.redhat.com/faq/docs/
Consistent Multipath Device Names
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