disk mirror

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

Syntax

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

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 the SAP ASE 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. serial writes are generally slower than noserial writes.
clear – initializes the mirror device with zeros to guarantee that the underlying filesystem has reserved space for the mirror device. The default value, FALSE, does not clear the mirror, and executing a write to the device might fail through lack of space on the file system. If you specify TRUE, the mirror is cleared, forcing the file system to reserve space for the device.

Examples

Example 1 – tranlog is the logical device name for a raw device. The tranlog device was initialized with disk init and is being used as a transaction log device (as in create database...log on tranlog). The following command mirrors the transaction log device:
```
disk mirror 
  name = "tranlog", 
  mirror = "/dev/rxy1e"
```
Example 2 – Creates a software mirror for the database device user_disk on the file mirror.dat:
```
disk mirror 
    name = "user_disk", 
    mirror = "/server/data/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 the SAP ASE server.
Use dump database to back up the master database 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, the SAP ASE server unmirrors the bad device and prints error messages. The SAP ASE server continues to run, unmirrored. The system administrator must use the disk remirror command to restart mirroring.
The clear option in this command has no effect when used on the NT platform.
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. The SAP ASE 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.
On systems that support asynchronous I/O, the writes option allows you to specify whether writes to the first device must finish before writes to the second device begin (serial) or whether both I/O requests are to be queued immediately, one to each side of the mirror (noserial). In either case, if a write cannot be completed, the I/O error causes the bad device to become unmirrored
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 the SAP ASE 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
```
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 SAP ASE 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

The permission checks for disk mirror differ based on your granular permissions settings. You must be using the master database to use disk mirror.

Setting	Description
Enabled	With granular permissions enabled, you must be a user with `manage disk` privilege.
Disabled	With granular permissions disabled, you must be a user with sa_role. disk mirror permission is not transferable.

Auditing

Values in event and extrainfo columns of sysaudits are:

Information	Values
Event	23
Audit option	disk
Command or access audited	disk mirror
Information in `extrainfo`	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 is in effect