Veritas Volume Manager 5.1 SP1 Administrator"s Guide (5900-1506, April 2011)

■ At least two sites must be configured in the disk group before site consistency
is turned on.
See “Making an existing disk group site consistent” on page 471.
■ All the disks in a disk group must be registered to one of the sites before you
can set the siteconsistent attribute on the disk group.
About site tags
In a Remote Mirror configuration, each storage device in the disk group must be
tagged with site information. The site tag indicates the site to which the device
is associated. VxVM provides a facility to tag VxVM-initialized disks with an
arbitrary name-value pair. The tag name site is reserved by VxVM and is used
to identify the site information of tagged disks. The command vxdisk settag
can be used to tag multiple disks or all disks from an enclosure or disks from
multiple enclosures. The tagging can be restricted to disks in a disk group by
specifying the disk group with the command.
You can use automatic site tagging to assign site tags to disks when adding them
to a disk group. When automatic site tagging is on, newly added disks or LUNs
inherit the site tag from the site-enclosure mapping stored in the disk group. To
use automatic site tagging, turn on automatic site tagging for a disk group, and
then assign the site names to the enclosures in the disk group. Any disks or LUNs
in that disk group inherit the tag from the enclosure to which they belong.
See “Configuring automatic site tagging for a disk group” on page 476.
About the site read policy
To enhance read performance, VxVM will service reads from the plexes at the
local site where an application is running, if the siteread read policy is set on a
volume. Writes are written to plexes at all sites. By tagging hosts with site
information, VxVM identifies which hosts belong to which site. Reads initiated
by a host from one site are then satisfied by disks which are tagged with the same
site. Tagging hosts and disks with correct site information gives you maximum
read performance when siteread read policy is used.
If a license enabling the Site Awareness feature is available on all the hosts in the
Remote Mirror configuration, the disk group is configured for site consistency
with several sites enabled, and the allsites=on attribute is specified for a volume,
the default read policy is siteread.
If the siteread policy is not set, use the following command to set the read policy
to siteread policy on a volume:
# vxvol [-g diskgroup] rdpol siteread volume
Administering sites and remote mirrors
About sites and remote mirrors
470