The metadb command creates and deletes replicas of the metadevice state database. State database replicas can be created on dedicated slices, or on slices that will later become part of a simple metadevice (concatenation or stripe), RAID5 metadevice, or trans metadevice.

The metadevice state database contains the configuration of all metadevices and hot spare pools in the system. Additionally, the metadevice state database keeps track of the current state of metadevices and hot spare pools, and their components. Solaris Volume Manager automatically updates the metadevice state database when a configuration or state change occurs. A submirror failure is an example of a state change. Creating a new metadevice is an example of a configuration change.

The metadevice state database is actually a collection of multiple, replicated database copies. Each copy, referred to as a replica, is subject to strict consistency checking to ensure correctness.

Replicated databases have an inherent problem in determining which database has valid and correct data. To solve this problem, Volume Manager uses a majority consensus algorithm. This algorithm requires that a majority of the database replicas be available before any of them are declared valid. This algorithm strongly encourages the presence of at least three initial replicas, which you create. A consensus can then be reached as long as at least two of the three replicas are available. If there is only one replica and the system crashes, it is possible that all metadevice configuration data can be lost.

The majority consensus algorithm is conservative in the sense that it will fail if a majority consensus cannot be reached, even if one replica actually does contain the most up-to-date data. This approach guarantees that stale data will not be accidentally used, regardless of the failure scenario. The majority consensus algorithm accounts for the following: the system will stay running with exactly half or more replicas; the system will panic when less than half the replicas are available; the system will not reboot without one more than half the total replicas.

When used with no options, the metadb command gives a short form of the status of the metadevice state database. Use metadb -i for an explanation of the flags field in the output.

The initial state database is created using the metadb command with both the -a and -f options, followed by the slice where the replica is to reside. The -a option specifies that a replica (in this case, the initial) state database should be created. The -f option forces the creation to occur, even though a state database does not exist. (The -a and -f options should be used together only when no state databases exist.)

Additional replicas beyond those initially created can be added to the system. They contain the same information as the existing replicas, and help to prevent the loss of the configuration information. Loss of the configuration makes operation of the metadevices impossible. To create additional replicas, use the metadb -a command, followed by the name of the new slice(s) where the replicas will reside. All replicas that are located on the same slice must be created at the same time.

To delete all replicas that are located on the same slice, the metadb -d command is used, followed by the slice name.

When used with the -i option, metadb displays the status of the metadevice state databases. The status can change if a hardware failure occurs or when state databases have been added or deleted.

To fix a replica in an error state, delete the replica and add it back again.

Root privileges are required for all of the following options except -h and -i.

The following options can be used with the metadb command. Not all the options are compatible on the same command line. Refer to the above synopsis line to see the supported use of the options.

-a

Attach a new database device. The /kernel/drv/md.conf file is automatically updated with the new information and the /etc/lvm/mddb.cf file is updated as well. An alternate way to create replicas is by defining them in the /etc/lvm/md.tab file and specifying the assigned name at the command line in the form, mddbnn, where nn is a two-digit number given to the replica definitions. Refer to the md.tab(4) man page for instructions on setting up replicas in that file.

-c number

Specifies the number of replicas to be placed on each device. The default number of replicas is 1.

-d

Deletes all replicas that are located on the specified slice. The /kernel/drv/md.conf file is automatically updated with the new information and the /etc/lvm/mddb.cf file is updated as well.

-f

The -f option is used to create the initial state database. It is also used to force the deletion of replicas below the minimum of one. (The -a and -f options should be used together only when no state databases exist.)

-h

Displays a usage message.

-i

Inquire about the status of the replicas. The output of the -i option includes characters in front of the device name that represent the status of the state database. Explanations of the characters are displayed following the replica status and are as follows:

d

replica does not have an associated device ID.

o

replica active prior to last mddb configuration change

u

replica is up to date

l

locator for this replica was read successfully

c

replica's location was in /etc/lvm/mddb.cf

p

replica's location was patched in kernel

m

replica is master, this is replica selected as input

W

replica has device write errors

a

replica is active, commits are occurring to this

M

replica had problem with master blocks

D

replica had problem with data blocks

F

replica had format problems

S

replica is too small to hold current database

R

replica had device read errors

-k system-file

Specifies the name of the kernel file where the replica information should be written. The default system-file is /kernel/drv/md.conf. This option is for use with the local diskset only.

-l length

Specifies the size of each replica. The default length is 8192 blocks, which should be appropriate for most configurations.

-p

Specifies updating the system file (/kernel/drv/md.conf) with entries from the /etc/lvm/mddb.cf file. This option is normally used to update a newly built system before it is booted for the first time. If the system has been built on a system other than the one where it will run, the location of the mddb.cf on the local machine can be passed as an argument. The system file to be updated can be changed using the -k option. This option is for use with the local diskset only.

-s setname

Specifies the name of the diskset on which the metadb command will work. Using the -s option will cause the command to perform its administrative function within the specified diskset. Without this option, the command will perform its function on local database replicas.

slice

Specifies the logical name of the physical slice (partition), such as /dev/dsk/c0t0d0s2.

/etc/lvm/mddb.cf

Contains the location of each copy of the metadevice state database.

/etc/lvm/md.tab

Workspace file for metadevice database configuration.

/kernel/drv/md.conf

Contains database replica information for all metadevices on a system. Also contains Solaris Volume Manager configuration information.

The following exit values are returned:

0

successful completion

>0

an error occurred

See attributes(5) for descriptions of the following attributes:

metaclear(1M), metadetach(1M), metahs(1M), metainit(1M), metaoffline(1M), metaonline(1M), metaparam(1M), metareplace(1M), metaroot(1M), metaset(1M), metastat(1M), metasync(1M), metattach(1M), md.tab(4), md.cf(4), mddb.cf(4)

Solaris Volume Manager Administration Guide