Solaris Volume Manager Overview
Solaris Volume Manager (formerly known as Online DiskSuite and Solstice DiskSuite) provides a way to mirror, stripe or RAID-5 local disks. New functionality is constantly being added to the base software. A full discussion is beyond the scope of this article, so we will focus on the most common cases, how to set them up, how to manage them and how to maintain them. Additional information is available in the Solaris Volume Manager Administration Guide.
Solaris Volume Manager uses a state database to store its configuration and state information. (State information refers to the condition of the devices.) Multiple replicas are required for redundancy. At least four should be created on at least two different physical disk devices. It is much better to have at least six replicas on at least three different physical disks, spread across multiple controller channels, if possible.
In the event that the state databases disagree, a majority of configured state databases determines which version of reality is correct. This is why it is important to configure multiple replicas. A minimum of three database replicas must be available in order to boot without human assistance, so it makes sense to create database replicas liberally. They don't take up much space, and there is very little overhead associated with their maintenance. On JBOD (Just a Bunch Of Disks) arrays, I recommend at least two replicas on each disk device.
State database replicas consume between 4 and 16 MB of space, and should ideally be placed on a partition specifically set aside for that purpose. In the event that state database information is lost, it is possible to lose the data stored on the managed disks, so the database replicas should be spread over as much of the disk infrastructure as possible.
State database locations are recorded in /etc/opt/SUNWmd/mddb.cf. Depending on their condition, repair may or may not be possible. Metadevices (the objects which Solaris Volume Manager manipulates) may be placed on a partition with a state database if the state database is there first. The initial state databases can be created by specifying the slices on which they will live as follows:
metadb -a -f -c 2 slice-name1 slice-name2
Because pre-existing partitions are not usable for creating database replicas, it is frequently the case that we will steal space from swap to create a small partition for the replicas. To do so, we need to boot to single-user mode, use swap -d to unmount all swap, and format to re-partition the swap partition, freeing up space for a separate partition for the database replicas. Since the replicas are small, very few cylinders will be required.
The basic types of metadevices are:
- Simple: Stripes or concatenations--consist only of physical slices.
- Mirror: Multiple copies on simple metadevies (submirrors).
- RAID5: Composed of multiple slices; includes distributed parity.
- Trans: Master metadevice plus logging device.
Solaris Volume Manager can build metadevices either by using partitions as the basic building blocks, or by dividing a single large partition into soft partitions. Soft partitions are a way that SVM allows us to carve a single disk into more than 8 slices. We can either build soft partitions directly on a disk slice, or we can mirror (or RAID) slices, then carve up the resulting metadevice into soft partitions to build volumes
Disksets are collections of disks that are managed together, in the same way that a Veritas Volume Manager (VxVM) disk group is managed together. Unlike in VxVM, SVM does not require us to explicitly specify a disk group. If Disksets are configured, we need to specify the set name for monitoring or management commands with a -s setname option. Disksets may be created as shared disksets, where multiple servers may be able to access them. (This is useful in an environment like Sun Cluster, for example.) In that case, we specify some hosts as mediators who determine who owns the diskset. (Note that disks added to shared disksets are re-partitioned in the expectation that we will use soft partitions.)When metadevices need to be addressed by OS commands (like mkfs), we can reference them with device links of the form /dev/md/rdsk/d# or /dev/md/disksetname/rdsk/d#
Volume Manager Commands
Here are the main command line commands within SVM
|metaclear||Deletes active metadevices and hot spare pools.|
|metadb||Manages state database replicas.|
|metadetach||Detaches a metadevice from a mirror or a logging device from a trans-metadevice.|
|metahs||Manages hot spares and hot spare pools.|
|metaoffline||Takes submirrors offline.|
|metaonline||Places submirrors online.|
|metaparam||Modifies metadevice parameters.|
|metarename||Renames and switches metadevice names.|
|metareplace||Replaces slices of submirrors and RAID5 metadevices.|
|metaroot||Sets up system files for mirroring root.|
|metastat||Check metadevice health and state.|
|metattach||Attaches a metadevice to a mirror or a log to a trans-metadevice.|