Types of Volume Layouts

VxVM allows you to create volumes with the following layout types:

• Concatenated---A volume whose subdisks are arranged both sequentially and contiguously within a plex. Concatenation allows a volume to be created from multiple regions of one or more disks if there is not enough space for an entire volume on a single region of a disk. For more information, see Concatenation and Spanning.
• Striped---A volume with data spread evenly across multiple disks. Stripes are equal-sized fragments that are allocated alternately and evenly to the subdisks of a single plex. There must be at least two subdisks in a striped plex, each of which must exist on a different disk. Throughput increases with the number of disks across which a plex is striped. Striping helps to balance I/O load in cases where high traffic areas exist on certain subdisks. For more information, see Striping (RAID-0).
• Mirrored---A volume with multiple data plexes that duplicate the information contained in a volume. Although a volume can have a single data plex, at least two are required for true mirroring to provide redundancy of data. For the redundancy to be useful, each of these data plexes should contain disk space from different disks. For more information, see Mirroring (RAID-1).
• RAID-5---A volume that uses striping to spread data and parity evenly across multiple disks in an array. Each stripe contains a parity stripe unit and data stripe units. Parity can be used to reconstruct data if one of the disks fails. In comparison to the performance of striped volumes, write throughput of RAID-5 volumes decreases since parity information needs to be updated each time data is accessed. However, in comparison to mirroring, the use of parity to implement data redundancy reduces the amount of space required. For more information, see RAID-5 (Striping with Parity).
• Mirrored-stripe---A volume that is configured as a striped plex and another plex that mirrors the striped one. This requires at least two disks for striping and one or more other disks for mirroring (depending on whether the plex is simple or striped). The advantages of this layout are increased performance by spreading data across multiple disks and redundancy of data. Striping Plus Mirroring (Mirrored-Stripe or RAID-0+1).
• Layered Volume---A volume constructed from other volumes. Non-layered volumes are constructed by mapping their subdisks to VM disks. Layered volumes are constructed by mapping their subdisks to underlying volumes (known as storage volumes), and allow the creation of more complex forms of logical layout. Examples of layered volumes are striped-mirror and concatenated-mirror volumes. For more information, see Layered Volumes.

A striped-mirror volume is created by configuring several mirrored volumes as the columns of a striped volume. This layout offers the same benefits as a non-layered mirrored-stripe volume. In addition it provides faster recovery as the failure of single disk does not force an entire striped plex offline. For more information, see Mirroring Plus Striping (Striped-Mirror, RAID-1+0 or RAID-10).

A concatenated-mirror volume is created by concatenating several mirrored volumes. This provides faster recovery as the failure of a single disk does not force the entire mirror offline.

Supported Volume Logs and Maps

VERITAS Volume Manager supports the use of several types of logs and maps with volumes:

• FastResync Maps are used to perform quick and efficient resynchronization of mirrors (see FastResync for details). These maps are supported either in memory (Non-Persistent FastResync), or on disk as part of a DCO volume (Persistent FastResync). Two types of DCO volume are supported:
• Version 0 DCO volumes only support Persistent FastResync for the traditional third-mirror break-off type of volume snapshot. See Version 0 DCO Volume Layout, and Creating a Volume with a Version 0 DCO Volume for more information.
• Version 20 DCO volumes, introduced in VxVM 4.0, support DRL logging (see below) and Persistent FastResync for full-sized and space-optimized instant volume snapshots. See Version 20 DCO Volume Layout, and Creating a Volume with a Version 20 DCO Volume for more information.

See Enabling FastResync on a Volume for information on how to enable Persistent or Non-Persistent FastResync on a volume.

• Dirty region logs allow the fast recovery of mirrored volumes after a system crash (see Dirty Region Logging (DRL) for details). These logs are supported either as DRL log plexes, or as part of a version 20 DCO volume. Refer to the following sections for information on creating a volume on which DRL is enabled:
• Creating a Volume with Dirty Region Logging Enabled for creating a volume with DRL log plexes.
• Creating a Volume with a Version 20 DCO Volume for creating a volume with DRL configured within a version 20 DCO volume.
• RAID-5 logs are used to prevent corruption of data during recovery of RAID-5 volumes (see RAID-5 Logging for details). These logs are configured as plexes on disks other than those that are used for the columns of the RAID-5 volume.

See Creating a RAID-5 Volume for information on creating a RAID-5 volume together with RAID-5 logs.