System information

When the user_fri end l y_names feature (of device- map p er- mu lt ip at h ) is used, the WWID is
mapped to a name of the form /d ev/mapper/mpathn. By default, this mapping is maintained in the
file /etc/mul ti path/bi nd ing s. These mpathn names are persistent as long as that file is
maintained.
Important
If you use user_fri end l y_names, then additional steps are required to obtain consistent
names in a cluster. Refer to the Consistent Multipath Device Names in a Cluster section in the
Using DM Multipath Configuration and Administration book.
In addition to these persistent names provided by the system, you can also use ud ev 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.
25.3.2. UUID and Ot her Persist ent Ident ifiers
If a storage device contains a file system, then that file system may provide one or both of the
following:
Universally Unique Identifier (UUID)
File system 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 /d ev/d isk/by-l abel / (e.g. bo o t -> . . /. . /sd a1) and /d ev/d i sk/by-uui d / (e.g.
f8bf0 9e3-4 c16 -4 d 9 1-bd 5e-6 f6 2d a16 5c0 8 -> . . /. . /sd a1) 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.
25.4. Removing a St orage 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
below). If the device uses multipathing, then do this for the multipath "pseudo device"
(Section 25.3.1, “WWID” ) and each of the identifiers that represent a path to the device. If you are
only removing a path to a multipath device, and other paths will remain, then the procedure is
simpler, as described in Section 25.6, “ Adding a Storage Device or Path” .
Removal of a storage device is not recommended when the system is under memory pressure, since
the I/O flush will add to the load. To determine the level of memory pressure, run the command
vmstat 1 10 0 ; device removal is not recommended if:
Free memory is less than 5% of the total memory in more than 10 samples per 100 (the command
free can also be used to display the total memory).
Swapping is active (non-zero si and so columns in the vmstat output).
The general procedure for removing all access to a device is as follows:
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