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NAME
       XVM - logical volume manager

SYNOPSIS
       /dev/xvm/*
       /dev/rxvm/*

NOTE
       This  man page has not yet been updated for Linux.  Some details may be
       Irix-specific.

DESCRIPTION
       XVM devices provide a logical organization to disk storage that is  not
       based  on  disk partitions, enabling the administrator to divide a disk
       into an arbitrary number of slices.

       XVM is cluster-aware, providing a replicated image of the  XVM  devices
       across  all  cells in a cluster, and allowing for administration of XVM
       devices from any cell in the cluster.

       There is a kernel driver, referred to  as  XVM,  an  api  library,  and
       administrative  commands to interface with the kernel.  The driver is a
       pseudo device, not directly associated with any physical hardware;  its
       function  is to map requests on logical volume devices into requests to
       underlying disk devices.

       XVM configuration information is stored on-disk in the form of  labels,
       in a reserved area of each XVM disk.

   Terminology
       objects
              XVM  objects  are  divided into three categories:  physical vol-
              umes, volume elements, and unlabeled disks.

       unlabeled disk
              An unlabeled disk is a disk that either has not been labeled for
              use by xvm, or a disk that has been labeled, but has not had its
              labels read by the XVM driver.

       physvol
              Physical volumes (physvol's) are disks that  have  been  labeled
              for use by XVM.

       ve     Volume  elements (ve's) are the building blocks of an XVM device
              topology.  Ve's are further divided into specific ve types.

       topology
              A collection of related ve's arranged in a treelike fashion with
              volume  ve's  at  the  root  of  the tree, and slice ve's at the
              leaves.

       volume A volume is the topmost ve in an XVM topology.  Volumes are used
              to group multiple subvolumes under a single name.

       subvolume
              Subvolumes  are  the main io and control entry points for an xvm
              topology.  Block and character special files  are  tied  to  the
              subvolumes  to  provide the XVM io and ioctl interface.  Subvol-
              umes have only a single child ve.  Associated with  a  subvolume
              is  a  subvolume  type.   The  subvolume type is a number in the
              range 0-255.  Types 0-16 are system-defined,  and  are  used  to
              mark a subvolume as being a XFS data, log, or rt subvolume.  The
              remaining system subvolume types are reserved  for  future  use.
              Types 17-255 are available for administrators to tag a subvolume
              with a generic type.

       System-defined subvolumes must be unique within a volume ie.  you  can-
       not have two data subvolumes under the same volume.

       slice  Slices  reside at the bottom of an XVM topology, and represent a
              range of contiguous data blocks on a physvol.

       logical ve's
              Between subvolumes and slices are logical volume  elements  that
              are  used  to  impose  certain  characteristics to the subvolume
              address space.  Concat ve's are used to  concatenate  underlying
              ve's  into  a  larger address space.  Stripe ve's can be used to
              improve io transfer rates by dividing io requests up  among  its
              underlying  ve's.   Mirror ve's are for data redundancy by main-
              taining identical images of data on each underlying ve.

       In general, the logical ve's can be stacked arbitrarily, although
              there is a limit of 10 levels to a ve topology from  the  volume
              ve  through  a  slice.  Mirror ve's also have a limit of 8 chil-
              dren.

       pieces The terms piece and child refer to a child of a  ve.   All  ve's
              except  slices  can  have  children.  Volumes are limited to 255
              children, subvolumes are limited to 1  child,  and  mirrors  are
              limited  to  8  children.  Other ve's are limited to 65536 chil-
              dren.  Pieces are 0-based, meaning piece 0 is the leftmost child
              of a ve.

   Object Naming
       All  XVM  objects  except  slices can have user-supplied names given to
       them (if user names are not supplied, default names will be generated).
       Generally, objects are specified using the object name, with subvolumes
       being an exception.  Subvolume objects must be specified  by  prefixing
       the  subvolume name with its volume name followed by '/'.  For example:
       fred/data.

       Because user-defined names are allowed, it is possible to have ambigui-
       ties  in the XVM object name space.  When an ambiguous name is supplied
       to an XVM command, the command will generate an error.  To remove ambi-
       guity, object names may be prefixed with a type followed by a '/'.  For
       example:  concat/concat1.

       The following type prefixes are recognized:

       prefix      object type
       ---------------------------
       vol         volume ve
       subvol      subvolume ve
       concat      concat ve
       stripe      stripe ve
       mirror      mirror ve
       slice       slice ve
       phys        physvol
       unlabeled   unlabeled disk

       Note that unambiguous subvolumes are  a  3-tuple:  subvol/vol_name/sub-
       vol_name.

       XVM  volume  elements  can  also be specified using a path-like syntax,
       where the components of the path are ve names or  piece  numbers  under
       the  parent.   For example:  vol/fred/data/concat0/physs0 refers to the
       ve physs0, whose parent is concat0, which is the data subvolume of vol-
       ume  fred.   Additionally,  concat0/0  refers to the zero-th (leftmost)
       piece of the ve concat0.  The piece syntax is helpful when you want  to
       target  a ve without having to know its name.  A component of .. refers
       to the parent of the object referenced by the path up  to  that  point.
       For example, concat0/.. would refer to the parent of concat0.  The par-
       ent of a volume is considered to be the volume  itself.   There  is  no
       concept  of  current object, so .. can only be used when the preceeding
       portion of the path can be translated into an object.

   Creating Topologies
       XVM topologies can be built top-down, or bottom-up.  It should be noted
       that  any  tree  or  subtree that does not end in a slice will not have
       labels written to disk, and therefore will  not  be  persistent  across
       reboots.

   Automatic Volume & Subvolume Generation
       When  new  volume elements are created, they are automaticly associated
       with a volume and data subvolume.  This eliminates the need to  explic-
       itly create volume and subvolumes for every ve.  By default, the volume
       name will be automaticly named, and the subvolume will be of type  data
       with  the  name data.  The xvm(8) ve creation commands take options for
       naming the volume explicitly.  Note that if  a  volume  is  automaticly
       named,  a  new,  possibly  different  name is generated when the system
       reboots.

   Subvolume block/char Path names
       The subvolume block and character special files exist under the follow-
       ing paths:

       /dev/xvm/volname_subvolname   (blk-special)
       /dev/rxvm/volname_ubvolname   (chr-special)

       The shorter name volname can optionally be used instead of volname_sub-
       volname  when  referring  to  the   data   subvolume.    For   example,
       /dev/xvm/fred_data  and  /dev/xvm/fred  both refer to the block-special
       file of the data subvolume under volume fred.

   Administering XVM devices
       Physvols and ve's are created and manipulated with the xvm(8)  command.

FILES
       /dev/xvm/*
       /dev/rxvm/*

SEE ALSO
       xvm(8).

                                                                        XVM(5)

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