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XXXXVVVVMMMM((((7777MMMM)))) XXXXVVVVMMMM((((7777MMMM)))) NNNNAAAAMMMMEEEE XVM - logical volume manager SSSSYYYYNNNNOOOOPPPPSSSSIIIISSSS _////_dddd_eeee_vvvv_////_xxxx_vvvv_mmmm_////_**** _////_dddd_eeee_vvvv_////_rrrr_xxxx_vvvv_mmmm_////_**** DDDDEEEESSSSCCCCRRRRIIIIPPPPTTTTIIIIOOOONNNN 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. TTTTeeeerrrrmmmmiiiinnnnoooollllooooggggyyyy objects XVM objects are divided into three categories: physical volumes, 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. PPPPaaaaggggeeee 1111 XXXXVVVVMMMM((((7777MMMM)))) XXXXVVVVMMMM((((7777MMMM)))) 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. Subvolumes 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 _dddd_aaaa_tttt_aaaa, _llll_oooo_gggg, or _rrrr_tttt 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 cannot 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 maintaining 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 children. pieces The terms _p_i_e_c_e and _c_h_i_l_d 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 children. Pieces are 0- based, meaning piece 0 is the leftmost child of a ve. OOOObbbbjjjjeeeecccctttt NNNNaaaammmmiiiinnnngggg 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 ambiguities in the XVM object name space. When an ambiguous name is supplied to an XVM command, the command will generate an error. To remove ambiguity, object names may be prefixed with a type followed by a '/'. For example: _cccc_oooo_nnnn_cccc_aaaa_tttt_////_cccc_oooo_nnnn_cccc_aaaa_tttt_1111. PPPPaaaaggggeeee 2222 XXXXVVVVMMMM((((7777MMMM)))) XXXXVVVVMMMM((((7777MMMM)))) The following type prefixes are recognized: pppprrrreeeeffffiiiixxxx oooobbbbjjjjeeeecccctttt ttttyyyyppppeeee __________________________ 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: _ssss_uuuu_bbbb_vvvv_oooo_llll_////_v_o_l__n_a_m_e_////_s_u_b_v_o_l__n_a_m_e. 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: _vvvv_oooo_llll_////_ffff_rrrr_eeee_dddd_////_dddd_aaaa_tttt_aaaa_////_cccc_oooo_nnnn_cccc_aaaa_tttt_0000_////_pppp_hhhh_yyyy_ssss_ssss_0000 refers to the ve _pppp_hhhh_yyyy_ssss_ssss_0000, whose parent is _cccc_oooo_nnnn_cccc_aaaa_tttt_0000, which is the data subvolume of volume _ffff_rrrr_eeee_dddd. Additionally, _cccc_oooo_nnnn_cccc_aaaa_tttt_0000_////_0000 refers to the zero-th (leftmost) piece of the ve _cccc_oooo_nnnn_cccc_aaaa_tttt_0000. 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, _cccc_oooo_nnnn_cccc_aaaa_tttt_0000_////_...._.... would refer to the parent of concat0. The parent 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. CCCCrrrreeeeaaaattttiiiinnnngggg TTTTooooppppoooollllooooggggiiiieeeessss 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. AAAAuuuuttttoooommmmaaaattttiiiicccc VVVVoooolllluuuummmmeeee &&&& SSSSuuuubbbbvvvvoooolllluuuummmmeeee GGGGeeeennnneeeerrrraaaattttiiiioooonnnn When new volume elements are created, they are automaticly associated with a volume and data subvolume. This eliminates the need to explicitly 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 _dddd_aaaa_tttt_aaaa. The _xxxx_vvvv_mmmm(1M) 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. SSSSuuuubbbbvvvvoooolllluuuummmmeeee bbbblllloooocccckkkk////cccchhhhaaaarrrr PPPPaaaatttthhhh nnnnaaaammmmeeeessss The subvolume block and character special files exist under the following paths: _////_dddd_eeee_vvvv_////_xxxx_vvvv_mmmm_////_v_o_l_n_a_m_e______s_u_b_v_o_l_n_a_m_e (blk-special) _////_dddd_eeee_vvvv_////_rrrr_xxxx_vvvv_mmmm_////_v_o_l_n_a_m_e______u_b_v_o_l_n_a_m_e (chr-special) PPPPaaaaggggeeee 3333 XXXXVVVVMMMM((((7777MMMM)))) XXXXVVVVMMMM((((7777MMMM)))) The shorter name _v_o_l_n_a_m_e can optionally be used instead of vvvvoooollllnnnnaaaammmmeeee_____ssssuuuubbbbvvvvoooollllnnnnaaaammmmeeee when referring to the data subvolume. For example, _////_dddd_eeee_vvvv_////_xxxx_vvvv_mmmm_////_ffff_rrrr_eeee_dddd______dddd_aaaa_tttt_aaaa and _////_dddd_eeee_vvvv_////_xxxx_vvvv_mmmm_////_ffff_rrrr_eeee_dddd both refer to the block-special file of the data subvolume under volume fred. AAAAddddmmmmiiiinnnniiiisssstttteeeerrrriiiinnnngggg XXXXVVVVMMMM ddddeeeevvvviiiicccceeeessss Physvols and ve's are created and manipulated with the _xxxx_vvvv_mmmm(1M) command. FFFFIIIILLLLEEEESSSS _////_dddd_eeee_vvvv_////_xxxx_vvvv_mmmm_////_**** _////_dddd_eeee_vvvv_////_rrrr_xxxx_vvvv_mmmm_////_**** SSSSEEEEEEEE AAAALLLLSSSSOOOO xvm(1M). PPPPaaaaggggeeee 4444 
