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Private and public disk regions

Most VM disks consist of the following regions:

private region  

A small area where configuration information is stored. A disk header label, configuration records for VxVM objects (such as volumes, plexes and subdisks), and an intent log for the configuration database are stored here. The default private region size is 32 megabytes, which is large enough to record the details of several thousand VxVM objects in a disk group. 

Under most circumstances, the default private region size should be sufficient. For administrative purposes, it is usually much simpler to create more disk groups that contain fewer volumes, or to split large disk groups into several smaller ones. 

See "Splitting disk groups" on page 214. 

If required, the value for the private region size may be overridden when you add or replace a disk using the vxdiskadm command. 

Each disk that has a private region holds an entire copy of the configuration database for the disk group. The size of the configuration database for a disk group is limited by the size of the smallest copy of the configuration database on any of its member disks. 

public region  

An area that covers the remainder of the disk, and which is used for the allocation of storage space to subdisks. 

A disk's type identifies how VxVM accesses a disk, and how it manages the disk's private and public regions.

The following disk access types are used by VxVM:


When the vxconfigd daemon is started, VxVM obtains a list of known disk device addresses from the operating system and configures disk access records for them automatically. 


There is no private region (only a public region for allocating subdisks). This is the simplest disk type consisting only of space for allocating subdisks. Such disks are most useful for defining special devices (such as RAM disks, if supported) on which private region data would not persist between reboots. They can also be used to encapsulate disks where there is insufficient room for a private region. The disks cannot store configuration and log copies, and they do not support the use of the vxdisk addregion command to define reserved regions. VxVM cannot track the movement of nopriv disks on a SCSI chain or between controllers. 


The public and private regions are on the same disk area (with the public area following the private area). 


The public and private regions are on different disk partitions. 

Auto-configured disks (with disk access type auto) support the following disk formats:


The disk is formatted as a Cross-platform Data Sharing (CDS) disk that is suitable for moving between different operating systems. This is the default format for disks that are not used to boot the system.Typically, most disks on a system are configured as this disk type. However, it is not a suitable format for boot, root or swap disks, for mirrors or hot-relocation spares of such disks, or for Extensible Firmware Interface (EFI) disks. 


The disk is formatted as a simple disk that can be converted to a CDS disk. 


The disk is formatted as a sliced disk. This format can be applied to disks that are used to boot the system. The disk can be converted to a CDS disk if it was not initialized for use as a boot disk. 

The vxcdsconvert utility can be used to convert disks to the cdsdisk format.

See the vxcdsconvert(1M) manual page.

Warning: The CDS disk format is incompatible with EFI disks. If a disk is initialized by VxVM as a CDS disk, the CDS header occupies the portion of the disk where the VTOC would usually be located. If you subsequently use a command such as fdisk or format to create a partition table on a CDS disk, this erases the CDS information and could cause data corruption.

By default, auto-configured non-EFI disks are formatted as cdsdisk disks when they are initialized for use with VxVM. You can change the default format by using the vxdiskadm command to update the /etc/default/vxdisk defaults file

See "Displaying or changing default disk layout attributes" on page 101.

See the vxdisk(1M) manual page.

Auto-configured EFI disks are formatted as sliced disks by default.

VxVM initializes each new disk with the smallest possible number of partitions. For non-EFI disks of type sliced, VxVM usually configures partition s3 as the private region, s4 as the public region, and s2 as the entire physical disk. An exception is an encapsulated root disk, on which s3 is usually configured as the public region and s4 as the private region.