This chapter provides information about Solaris Volume Manager soft partitions. For information about related tasks, see Chapter 12, Soft Partitions (Tasks).
This chapter contains the following information:
As disks become larger, and disk arrays present ever larger logical devices to Solaris systems, users need to be able to subdivide disks or logical volumes into more than eight partitions, often to create manageable file systems or partition sizes. Solaris Volume Manager's soft partition feature addresses this need.
Solaris Volume Manager can support up to 8192 logical volumes per disk set (including the local, or unspecified, disk set), but is configured for 128 (d0–d127) by default. To increase the number of logical volumes, see Changing Solaris Volume Manager Defaults.
Do not increase the number of possible logical volumes far beyond the number that you will actually use. Solaris Volume Manager creates a device node (/dev/dsk/md/*) and associated data structures for every logical volume that is permitted by the maximum value. These additional possible volumes can result in a substantial performance impact.
You use soft partitions to divide a disk slice or logical volume into as many partitions as needed. You must provide a name for each division or soft partition, just like you do for other storage volumes, such as stripes or mirrors. A soft partition, once named, can be accessed by applications, including file systems, as long as the soft partition is not included in another volume. Once included in a volume, the soft partition should no longer be directly accessed.
Soft partitions can be placed directly above a disk slice, or on top of a mirror, stripe or RAID 5 volume. A soft partition may not be both above and below other volumes. For example, a soft partition built on a stripe with a mirror built on the soft partition is not allowed.
A soft partition appears to file systems and other applications to be a single contiguous logical volume. However, the soft partition actually comprises a series of extents that could be located at arbitrary locations on the underlying media. In addition to the soft partitions, extent headers (also called system recovery data areas) on disk record information about the soft partitions to facilitate recovery in the event of a catastrophic system failure.
Slices that are used for soft partitions cannot be used for other purposes.
When you partition a disk and build file systems on the resulting slices, you cannot later extend a slice without modifying or destroying the disk format. With soft partitions, you can extend the soft partitions up to the amount of space on the underlying device without moving or destroying data on other soft partitions.
While it is technically possible to manually place extents of soft partitions at arbitrary locations on disk (as you can see in the output of metastat -p, described in Viewing the Solaris Volume Manager Configuration), you should allow the system to place them automatically.
Although you can build soft partitions on any slice, creating a single slice that occupies the entire disk and then creating soft partitions on that slice is the most efficient way to use soft partitions at the disk level.
Because the maximum size of a soft partition is limited to the size of the slice or logical volume on which it is built, you should build a volume on top of your disk slices, then build soft partitions on top of the volume. This strategy allows you to add components to the volume later, then expand the soft partitions as needed.
For maximum flexibility and high availability, build RAID 1 (mirror) or RAID 5 volumes on disk slices, then create soft partitions on the mirror or RAID 5 volume.
Soft partitions provide tools with which to subdivide larger storage spaces into more managable spaces. For example, in other scenarios (Scenario—RAID 1 Volumes (Mirrors) or Scenario—RAID 5 Volumes), large storage aggregations provided redundant storage of many Gigabytes. However, many possible scenarios would not require so much space—at least at first. Soft partitions allow you to subdivide that storage space into more manageable sections. Each of those sections can have a complete file system. For example, you could create 1000 soft partitions on top of a RAID 1 or RAID 5 volume so that each of your users can have a home directory on a separate file system. If a user needs more space, simply expand the soft partition.