Storage Foundation 7.4.1 Administrator's Guide - Linux

Creating a RAID-5 volume

A RAID-5 volume requires space to be available on at least as many disks in the disk group as the number of columns in the volume. Additional disks may be required for any RAID-5 logs that are created.

You can create RAID-5 volumes by using either the vxassist command (recommended) or the vxmake command. This section describes using the preferred method, the vxassist command.

For information about using the vxmake command, see the vxmake(1M) manual page.

A RAID-5 volume contains a RAID-5 data plex that consists of three or more subdisks located on three or more physical disks. Only one RAID-5 data plex can exist per volume. A RAID-5 volume can also contain one or more RAID-5 log plexes, which are used to log information about data and parity being written to the volume.

To create a RAID-5 volume, use the following command:

# vxassist [-b] [-g diskgroup] make volume length layout=raid5 \ 
  [ncol=number_of_columns] [stripewidth=size] [nlog=number] \
  [loglen=log_length]

Specify the -b option if you want to make the volume immediately available for use.

For example, to create the RAID-5 volume volraid together with 2 RAID-5 logs in the disk group, mydg, use the following command:

# vxassist -b -g mydg make volraid 10g layout=raid5 nlog=2

This creates a RAID-5 volume with the default stripe unit size on the default number of disks. It also creates two RAID-5 logs rather than the default of one log.

If you require RAID-5 logs, you must use the logdisk attribute to specify the disks to be used for the log plexes.

RAID-5 logs can be concatenated or striped plexes, and each RAID-5 log associated with a RAID-5 volume has a complete copy of the logging information for the volume. To support concurrent access to the RAID-5 array, the log should be several times the stripe size of the RAID-5 plex.

It is suggested that you configure a minimum of two RAID-5 log plexes for each RAID-5 volume. These log plexes should be located on different disks. Having two RAID-5 log plexes for each RAID-5 volume protects against the loss of logging information due to the failure of a single disk.

If you use ordered allocation when creating a RAID-5 volume on specified storage, you must use the logdisk attribute to specify on which disks the RAID-5 log plexes should be created. Use the following form of the vxassist command to specify the disks from which space for the logs is to be allocated:

# vxassist [-b] [-g diskgroup] -o ordered make volume length \ 
  layout=raid5 [ncol=number_columns] [nlog=number] \ 
  [loglen=log_length] logdisk=disk[,disk,...] \
  storage_attributes

For example, the following command creates a 3-column RAID-5 volume with the default stripe unit size on disks mydg04, mydg05 and mydg06. It also creates two RAID-5 logs on disks mydg07 and mydg08.

# vxassist -b -g mydg -o ordered make volraid 10g layout=raid5 \
  ncol=3 nlog=2 logdisk=mydg07,mydg08 mydg04 mydg05 mydg06

The number of logs must equal the number of disks that is specified to logdisk.

See the vxassist(1M) manual page.

You can add more logs to a RAID-5 volume at a later time.

To add a RAID-5 log to an existing volume, use the following command:

# vxassist [-b] [-g diskgroup] addlog volume [loglen=length]

If you specify the -b option, adding the new log is a background task.

When you add the first log to a volume, you can specify the log length. Any logs that you add subsequently are configured with the same length as the existing log.

For example, to create a log for the RAID-5 volume volraid, in the disk group mydg, use the following command:

# vxassist -g mydg addlog volraid