Chapter 1: Commands

disk mirror

Description

Creates a software mirror that immediately takes over when the primary device fails.

Syntax

disk mirror 
	name = "device_name" , 
	mirror = "physicalname" 
	[, writes = { serial | noserial }]

Parameters

name: is the name of the database device to mirror. This is recorded in the name column of the sysdevices table. The name must be enclosed in single or double quotes.

mirror: is the full path name of the database mirror device that is to be your secondary device. It must be enclosed in single or double quotes. If the secondary device is a file, physicalname should be a path specification that clearly identifies the file, which Adaptive Server creates. The value of physicalname cannot be an existing file.

writes: allows you to choose whether to enforce serial writes to the devices. In the default case (serial), the write to the primary database device is guaranteed to finish before the write to the secondary device begins. If the primary and secondary devices are on different physical devices, serial writes can ensure that at least one of the disks is unaffected in the event of a power failure.

Examples

Example 1

disk mirror 
	name = "user_disk", 
	mirror = "/server/data/mirror.dat"

Creates a software mirror for the database device user_disk on the file mirror.dat.

Usage

Disk mirroring creates a software mirror of a user database device, the master database device, or a database device used for user database transaction logs. If a database device fails, its mirror immediately takes over.

Disk mirroring does not interfere with ongoing activities in the database. You can mirror or unmirror database devices without shutting down Adaptive Server.
Back up the master database with the dump database command after each use of disk mirror. This makes recovery easier and safer in case master is damaged.
When a read or write to a mirrored device is unsuccessful, Adaptive Server unmirrors the bad device and prints error messages. Adaptive Server continues to run, unmirrored. The System Administrator must use the disk remirror command to restart mirroring.
You can mirror the master device, devices that store data, and devices that store transaction logs. However, you cannot mirror dump devices.
Devices are mirrored; databases are not.
A device and its mirror constitute one logical device. Adaptive Server stores the physical name of the mirror device in the mirrorname column of the sysdevices table. It does not require a separate entry in sysdevices and should not be initialized with disk init.
To retain use of asynchronous I/O, always mirror devices that are capable of asynchronous I/O to other devices capable of asynchronous I/O. In most cases, this means mirroring raw devices to raw devices and operating system files to operating system files.

If the operating system cannot perform asynchronous I/O on files, mirroring a raw device to a regular file produces an error message. Mirroring a regular file to a raw device works, but does not use asynchronous I/O.
Mirror all default database devices so that you are still protected if a create database or alter database command affects a database device in the default list.
For greater protection, mirror the database device used for transaction logs.
Always put user database transaction logs on a separate database device. To put a database’s transaction log (that is, the system table syslogs) on a device other than the one on which the rest of the database is stored, name the database device and the log device when you create the database. Alternatively, use alter database to extend the database onto a second device, then run sp_logdevice.
If you mirror the database device for the master database, you can use the -r option and the name of the mirror for UNIX, when you restart Adaptive Server with the dataserver utility program. Add this to the RUN_servername file for that server so that the startserver utility program knows about it. For example, to start a master device named master.dat and its mirror, mirror.dat enter:
```
dataserver -dmaster.dat -rmirror.dat
```
For more information, see dataserver and startserver in the Utility Guide.
If you mirror a database device that has unallocated space (room for additional create database and alter database statements to allocate part of the device), disk mirror begins mirroring these allocations when they are made, not when the disk mirror command is issued.
For a report on all Adaptive Server devices on your system (user database devices and their mirrors, as well as dump devices), execute sp_helpdevice.

Standards

ANSI SQL – Compliance level: Transact-SQL extension.

Permissions

disk mirror permission defaults to the System Administrator and is not transferable. You must be using the master database to use disk mirror.

Auditing

Values in event and extrainfo columns of sysaudits are:

Event	Audit option	Command or access audited	Information in extrainfo
23	disk	disk mirror	Roles – Current active roles Keywords or options – disk mirror Previous value – NULL Current value – NULL Other information – Name of the disk Proxy information – Original login name, if set proxy in effect