Description of commonly used RAID levels

Article:TECH54996  |  Created: 2008-01-25  |  Updated: 2013-10-24  |  Article URL http://www.symantec.com/docs/TECH54996
Article Type
Technical Solution


Environment

Issue



Description of commonly used RAID levels


Solution



The following are descriptions of commonly used RAID levels

RAID Level 0: Stripe sets
In a stripe set, data is stored a block at a time, evenly and sequentially among all the disks in the set. The boot or system partition cannot be on a stripe set. A stripe set appears as a single drive letter to the NT user.

To create a stripe set:

1. Start Disk Administrator.
2. Click a block of free space on one of your hard disks.
3. Press and hold the Ctrl key, and then click another block of free space on a different hard disk.
 

Note: Stripe sets can be created on as few as 2 hard disks, or as many as 32. You can select as many blocks of free space as you like. However, you may not select more than 1 block of free space per hard disk.

 
4. On the Partition menu, click Create Stripe Set.
5. In the Create Stripe Set dialog box, type the desired size for the stripe set.
6. On the Partition menu, click Commit Changes Now.
 
7. Format the stripe set.

 
Stripe sets cannot be recovered gracefully; data must be restored from a backup.

RAID Level 1: Disk mirroring
Disk mirroring enables operations to continue when one hard disk fails. Any partition can be mirrored, including Windows NT system and boot partitions. The partition must be mirrored to another hard drive; it cannot be mirrored to the same hard drive. Disk mirrors cannot maintain their fault tolerance if both hard disks fail.

 
To create a mirror set:
 
1. At a command line prompt on the disk you are creating a mirror of, type the following command:

chkdsk /f /v

This will help to speed up data synchronization during the mirroring process.

2. Start Disk Administrator, and click the partition that you want to mirror.
3. Press and hold the Ctrl key, and then click a block of free space on a different hard disk.
4. This block of free space must be at least as large as the partition that you want to mirror.
 
5. On the Fault Tolerance menu, click Establish Mirror.

 
Depending on the size of the partition that you are mirroring, the mirroring process may take quite a while. Additionally, you may have to restart the computer twice before the mirror is complete.

 
When the mirror set is established, you must create a fault-tolerant boot diskette so that you can recover your system if your hard disk ever crashes.

 
To create a fault-tolerant boot diskette from a Windows NT 4.0 mirrored drive
1. Format a diskette in Windows NT.
2. Copy NTLDR, Ntdetect.com, and Boot.ini to the diskette.
3. If the drives are SCSI and their controller BIOS is disabled, you will also need to copy Ntbootdd.sys to the diskette.
4. Edit Boot.ini to point to the mirrored partition that you just created.

In the event that one of the mirrored partitions fails, it can be recovered.

To recover a mirrored partition
1. Boot the computer (using the fault-tolerant boot diskette if necessary).
2. Start Disk Administrator.
3. In the mirror set, click the partition that is still operational.
4. On the Fault Tolerance menu, click Break Mirror.
5. Click Yes when you are prompted.
6. On the Partition menu, click Commit Changes Now.
7. Power down the system, replace the damaged hard disk, and re-establish the mirror set.


RAID Level 5: Stripe sets with parity

Striping with parity is the most common method of fault tolerance. In a stripe set with parity, data is not only distributed one block at a time, evenly and sequentially among all the disks in the set, but parity information is also written across all of the disks in the set. Neither the boot nor system partitions can be on a stripe set with parity.

Stripe sets with parity are created in the same fashion as a typical stripe set (RAID level 0).

It is possible to recover a stripe set with parity, provided that only one partition of the set has been damaged.

To recover a stripe set with parity
1. Either replace the failed hard disk, or delete the corrupted partition.
2. Start Disk Administrator.
3. Click any portion of the existing stripe set with parity that is still operational.
4. Press and hold the Ctrl key and click a block of free space on the new hard disk.
5. If you deleted the corrupted partition on the failed hard disk, click the block of free space that contained the corrupted partition.
6. On the Fault Tolerance menu, click Regenerate.
7. Click an operational partition within the stripe set with parity.

It may take a significant amount of time to regenerate the data. During the regeneration process, various messages are displayed at the bottom of the Disk Administrator window. These include "RECOVERABLE," "REGENERATING," and "HEALTHY." The process is complete when "HEALTHY" is displayed. In some cases, it will be necessary to reboot the system if the status remains at "RECOVERABLE" for an unreasonable length of time.

Volume sets
Volume sets are generally used when a drive becomes full and you want to extend its capacity. Volume sets are similar to stripe sets, except that files in volume sets are generally confined to one hard disk, making disk operations slower than with stripe sets. Also, as with stripe sets, volume sets cannot include boot and system partitions, and they cannot be recovered gracefully--data must be restored from backup.

You can either create new volume sets or extend existing volume sets; extending existing volume sets is by far the more common practice.


To extend a volume set
1. Start Disk Administrator.
2. Click an NTFS partition on one of your hard disks.
3. Only NTFS partitions can be extended.
4. Press and hold the Ctrl key, and then click one or more blocks of free space on other hard disks.
5. On the Partition menu, click Extend Volume Set.
6. Specify the desired size for the volume set.
7. Reboot the system.



 



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