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Top Forums UNIX for Dummies Questions & Answers why need to mirror disk if we had backups ? Post 302186431 by Dave Miller on Thursday 17th of April 2008 08:12:10 AM
Old 04-17-2008
Disasters come in two forms: Hardware failures and operator errors.

Mirroring is the ideal backup for hardware failures, while tapes are the ideal backup for operator errors.


If a hard drive goes down, the mirrored drive takes over. Often, depending on the hardware, the bad drive can be replaced without taking the system down.

This happened to me at my last job. I got a red light on one of the drives. Called IBM and they sent a new drive overnight, with instuctions to simply remove the old drive, 'wait a minute', and insert the new drive.

The only part that surprised me was that I expected to have to issue some type of console command or something. But, no. A few seconds after the new drive was installed, the lights started blinking like crazy as it copied all the data to make it a good mirror. About an hour later, normal blinking resumed.

The important thing was, users never even knew there was a problem.


Tapes provide historical data.

If someone deletes a file, or does some other equally stupid thing, a mirror is no help since that copy will also be bad.
 

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volcheck(1)                                                        User Commands                                                       volcheck(1)

NAME
volcheck - checks for media in a drive and by default checks all floppy media SYNOPSIS
volcheck [-v] [-i secs] [-t secs] pathname DESCRIPTION
The volcheck utility tells Volume Management to look at each dev/pathname in sequence and determine if new media has been inserted in the drive. The default action is to volcheck all checkable media managed by volume management. OPTIONS
The following options are supported: -i secs Set the frequency of device checking to secs seconds. The default is 2 seconds. The minimum frequency is 1 second. -t secs Check the named device(s) for the next secs seconds. The maximum number of seconds allowed is 28800, which is 8 hours. The fre- quency of checking is specified by -i. There is no default total time. -v Verbose. OPERANDS
The following operands are supported: pathname The path name of a media device. EXAMPLES
Example 1: A sample of the volcheck command. The following example example% volcheck -v /dev/diskette /dev/diskette has media asks Volume Management to examine the floppy drive for new media. The following example example% volcheck -i 2 -t 600 /dev/diskette1 & asks Volume Management if there is a floppy in the floppy drive every 2 seconds for 600 seconds (10 minutes). FILES
/dev/volctl Volume Management control port ATTRIBUTES
See attributes(5) for descriptions of the following attributes: +-----------------------------+-----------------------------+ | ATTRIBUTE TYPE | ATTRIBUTE VALUE | +-----------------------------+-----------------------------+ |Availability |SUNWvolu | +-----------------------------+-----------------------------+ SEE ALSO
eject(1), volcancel(1), volmissing(1) rmmount(1M), vold(1M), rmmount.conf(4), vold.conf(4), attributes(5), volfs(7FS) WARNINGS
Due to a hardware limitation in many floppy drives, the act of checking for media causes mechanical action in the floppy drive. Continu- ous polling of the floppy drive will cause the drive to wear out. It is recommended that polling the drive only be performed during periods of high use. SunOS 5.10 21 Feb 1997 volcheck(1)
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