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More tagging documentation, with some cluster examples.
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doc/tagging.txt
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doc/tagging.txt
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Tagging aims
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============
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1) Ability to attach an unordered list of tags to LVM metadata objects.
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2) Ability to add or remove tags easily.
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3) Ability to select LVM objects for processing according to presence/absence
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of specific tags.
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4) Ability to control through the config file which VGs/LVs are activated
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on different machines using names or tags.
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5) Ability to overlay settings from different config files e.g. override
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some settings in a global config file locally.
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Clarifications
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==============
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1) Tag character set: A-Za-z0-9_+.-
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Can't start with hyphen & max length is 128 (NAME_LEN).
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2) LVM object types that can be tagged:
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VG, LV, LV segment
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PV - tags are stored in VG metadata so disappear when PV becomes orphaned
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Snapshots can't be tagged, but their origin may be.
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3) A tag can be used in place of any command line LVM object reference that
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accepts (a) a list of objects; or (b) a single object as long as the
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tag expands to a single object. This is not supported everywhere yet.
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Duplicate arguments in a list after argument expansion may get removed
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retaining the first copy of each argument.
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4) Wherever there may be ambiguity of argument type, a tag must be prefixed
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by '@'; elsewhere an '@' prefix is optional.
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5) LVM1 objects cannot be tagged, as the disk format doesn't support it.
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6) Tags can be added or removed with --addtag or --deltag.
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Config file Extensions
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======================
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To define host tags in config file:
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tags {
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# Set a tag with the hostname
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hosttags = 1
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tag1 { }
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tag2 {
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# If no exact match, tag is not set.
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host_list = [ "hostname", "dbase" ]
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}
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}
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Activation config file example
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==============================
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activation {
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volume_list = [ "vg1/lvol0", "@database" ]
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}
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Matches against vgname, vgname/lvname or @tag set in *metadata*.
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@* matches exactly against *any* tag set on the host.
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The VG or LV only gets activated if a metadata tag matches.
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The default if there is no match is not to activate.
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If volume_list is not present and any tags are defined on the host
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then it only activates if a host tag matches a metadata tag.
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If volume_list is not present and no tags are defined on the host
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then it does activate.
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Multiple config files
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=====================
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(a) lvm.conf
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(b) lvm_<host_tag>.conf
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At startup, load lvm.conf.
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Process tag settings.
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If any host tags were defined, load lvm_tag.conf for each tag, if present.
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When searching for a specific config file entry, search order is (b)
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then (a), stopping at the first match.
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Within (b) use reverse order tags got set, so file for last tag set is
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searched first.
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New tags set in (b) *do* trigger additional config file loads.
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Usage Examples
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==============
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1) Simple activation control via metadata with static config files
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lvm.conf: (Identical on every machine - global settings)
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tags {
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hostname_tags = 1
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}
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From any machine in the cluster, add db1 to the list of machines that
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activate vg1/lvol2:
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lvchange --tag @db1 vg1/lvol2
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(followed by lvchange -ay to actually activate it)
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2) Multiple hosts.
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Activate vg1 only on the database hosts, db1 and db2.
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Activate vg2 only on the fileserver host fs1.
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Activate nothing initially on the fileserver backup host fsb1, but be
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prepared for it to take over from fs1.
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Option (i) - centralised admin, static configuration replicated between hosts
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# Add @database tag to vg1's metadata
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vgchange --tag @database vg1
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# Add @fileserver tag to vg2's metadata
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vgchange --tag @fileserver vg2
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lvm.conf: (Identical on every machine)
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tags {
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database {
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host_list = [ "db1", "db2" ]
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}
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fileserver {
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host_list = [ "fs1" ]
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}
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fileserverbackup {
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host_list = [ "fsb1" ]
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}
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}
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activation {
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# Only activate if host has a tag that matches a metadata tag
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volume_list = [ "@*" ]
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}
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In the event of the fileserver host going down, vg2 can be brought up
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on fsb1 by running *on any node* 'vgchange --tag @fileserverbackup vg2'
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followed by 'vgchange -ay vg2'
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Option (ii) - localised admin & configuation
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(i.e. each host holds *locally* which classes of volumes to activate)
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# Add @database tag to vg1's metadata
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vgchange --tag @database vg1
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# Add @fileserver tag to vg2's metadata
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vgchange --tag @fileserver vg2
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lvm.conf: (Identical on every machine - global settings)
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tags {
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hosttags = 1
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}
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lvm_db1.conf: (only needs to be on db1 - could be symlink to lvm_db.conf)
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activation {
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volume_list = [ "@database" ]
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}
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lvm_db2.conf: (only needs to be on db2 - could be symlink to lvm_db.conf)
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activation {
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volume_list = [ "@database" ]
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}
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lvm_fs1.conf: (only needs to be on fs1 - could be symlink to lvm_fs.conf)
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activation {
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volume_list = [ "@fileserver" ]
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}
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If fileserver goes down, to bring a spare machine fsb1 in as fileserver,
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create lvm_fsb1.conf on fsb1 (or symlink to lvm_fs.conf):
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activation {
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volume_list = [ "@fileserver" ]
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}
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and run 'vgchange -ay vg2' or 'vgchange -ay @fileserver'
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