VERITAS Foundation Suite 连载 2 <Virtual Objects>

韦小宝

1.        Physical Disk Structure：\r\n        VTOC stores information about the disk structure and organization. The VTOC is also called the disk label.\r\nthe VTOC is typically less than 200 bytes and resides on the first sector of the disk. A sector is 512 bytes on most systems. \r\nOn the boot disk, the boot block resides within the first 16 sectors (8K). The boot block contains instructions that point to where the second stage of the boot process is located.\r\n        Partitions: After the VTOC, the remainder of a Solaris disk is divided into units called partitions.Information about the size, location, and use of partitions is stored in the VTOC in the partition table.\r\n        Partition table entry 2 contains the entire disk, including the VTOC, and is used to determine the size of the disk.\r\n\r\n2.        Virtual Data Storage：\r\n        A volume is made up of space from one or more physical disks on which the data is physically stored.\r\n        All users and applications access volumes as contiguous address space using special device files in a manner similar to accessing a disk partition.\r\n\r\n3.        Volume Manager can move collections of disks between hosts (disk group import and deport),automatically relocate data in case of disk failure (hot relocation), and automatically detect and use multipathed disk arrays (dynamic multipathing, or DMP).\r\n\r\n4.        The private region stores information, such as disk headers, configuration copies, and kernel logs, that Volume Manager uses to manage virtual objects.\r\nThe default size of private region is 2048 blocks(sectors); the maximum size is 524288 blocks(sectors). With 512-byte blocks,the default size is 1048576 bytes (1 MB), and the maximum size is 268435456 bytes (256 MB).\r\nAn integer number of cylinders is always allocated for the private region.\r\n\r\n5.        If possible, Volume Manager uses partitions 3 and 4 for this purpose.VxVM sets the partition tags, the numeric values that describe the file system mounted on a partition, for the public and private regions:\r\n– Tag 14 is always used for the public region of the disk.\r\n– Tag 15 is always used for the private region of the disk.\r\n\r\n6.        Virtual Objects:\r\n        disk group \r\n        A disk group is a collection of VxVM disks. You group disks into disk groups for management purposes.\r\n        Volume Manager objects cannot span disk groups.For example, a volume’s subdisks, plexes, and disks must be derived from the same disk group as the volume.\r\n        A disk group and its components can be moved as a unit from one host machine to another. Disk drives can be shared by two or more hosts, but can be accessed by only one host at a time.\r\n        Volume Manager (VxVM) disk\r\n        A Volume Manager (VxVM) disk is created from the public region of a physical disk that is under Volume Manager control. Each VxVM disk corresponds to one physical disk.\r\n        Each VxVM disk has a unique virtual disk name called a disk media name.\r\n        A VxVM disk is given a disk media name when it is added to a disk group.\r\n        Once a VxVM disk is assigned a disk media name, the disk is no longer referred to by its physical address of c#t#d#.\r\n        For disks in the rootdg disk group, the default VxVM disk names are disk01, disk02, and so on.\r\n        If you use the command line utilities to administer Volume Manager, then the device nodes are used for the disk names.\r\n        subdisks\r\n        A VxVM disk can be divided into one or more subdisks. subdisks cannot overlap or share the same portions of a VxVM disk.A subdisk is a set of contiguous disk blocks that represent a specific portion of a VxVM disk. A subdisk is a subsection of a disk’s public region. A subdisk is the smallest unit of storage in Volume Manager. Subdisks are the building blocks for Volume Manager objects.\r\n        A subdisk is defined by an offset and a length in sectors on a VxVM disk. The default name for a subdisk takes the form DMname-##.DM: disk media name\r\n        There is no theoretical limit to the number of subdisks that can be attached to a single plex. default to a value of 4096. you can change this default by using the vol_subdisk_num tunable parameter.\r\n        plexes\r\n        Volume Manager uses subdisks to build virtual objects called plexes. A plex is a structured or ordered collection of subdisks that represents one copy of the data in a volume. A plex consists of one or more subdisks located on one or more physical disks.\r\n        The terms plex and mirror can be used interchangeably, even though a plex is only one copy of the data. The terms “mirrored” or “mirroring” imply two or more copies of data.\r\n        The length of a plex is determined by the last block that can be read or written on the last subdisk in the plex. Plex length may not equal volume length to the exact sector, because the plex is aligned to a cylinder boundary.\r\n        The default naming convention for plexes in a volume is volumename-##.\r\n        Plex Types：\r\n        A volume must have at least one complete plex that has a complete copy of the data in the volume with at least one associated subdisk. Other plexes in the volume can be complete, sparse, or log plexes.\r\nA volume can have up to 32 plexes;however, you should never use more than 31 plexes in a single volume. Volume Manager requires one plex for automatic or temporary online operations.\r\n        A complete plex holds a complete copy of a volume and therefore maps the entire address space of the volume.\r\n        A sparse plex is a plex that has a length that is less than the length of the volume or that maps to only part of the address space of a volume.\r\nIn older VxVM versions, sparse plexes are used for performance improvement.\r\n        A log plex is used to speed up data consistency checks and repairs after a system failure. RAID-5 and mirrored volumes typically use a log plex.\r\n        Volume\r\n        A volume is a virtual storage device that is used by applications in a manner similar to a physical disk.\r\n        A volume is composed of one or more plexes.\r\n        A volume must be configured from VxVM disks and subdisks within the same disk group.\r\n        Volume Manager uses the default naming convention vol## for volumes\r\n\r\n7.        A volume’s layout refers to the organization of plexes in a volume. Volume layout is the way plexes are configured to remap the volume address space through which I/O is redirected at run-time.\r\nyou can change the volume layout without disrupting applications or file systems that are using the volume.\r\n\r\n8.        Supported volume layouts include:\r\n        Concatenated\r\n        Striped\r\n        Mirrored\r\n        RAID-5\r\n        Layered\r\n\r\n9.        Stripes are equally-sized fragments that are allocated alternately and evenly to the subdisks of a single plex.\r\n\r\n10.        In comparison to the performance of striped volumes, write throughput of RAID-5 volumes decreases,because parity information needs to be updated each time data is accessed.\r\n\r\n11.        A layered volume is a virtual Volume Manager object that nests other virtual objects inside of itself. Layered volumes provide better redundancy by mirroring data at a more granular level.