Teradata RAID

RAID 1 [Disk Mirroring Technique]	RAID 5 [Party checking Technique]
In this process for every primary disk the is a three is mirror disk available	Here, for every 3 blocks of data there is a parity maintained in the 4th disk.
Whenever the main disk went down the array controller redirects all instructions from main disk to mirror disk.	Whenever the main disk went down it reconstructs its blocks from the parity and remaining two blocks.
Adv: Recovers Fast	Only 25% additional memory it occupies
Drawbacks: Double memory occupies, and double the operations (both primary and mirror disk)	Recovery is slow

RAID PROTECTION

RAID 1 (Mirroring)

·         Each physical disk in the array has an exact copy in the same array.

·         The array controller can read from either disk and write to both.

·         When one disk of the pair fails. There is no change in performance.

·         Mirroring reduces available disk space by 50%.

·         Array controller reconstructs failed disks quickly.

RAID 5 (Parity)

·         Data and parity are striped across a rank if 4 disks.

·         If a disk fails, any missing block may be reconstructed using the other three disks.

·         Parity reduces available disk space by 25 in a 4-disk rank.

·         Array controller reconstruction of failed disks takes longer than RAID 1.

Summary

RAID-1- Good Performance with disk failures Higher cost in terms of disk space RAID-5- Reduce performance with disk failures Lower cost in terms of disk space

                                                                   FALLBACK

1.      A Fallback table is fully available in the event of an unavailable AMP.

2.      A Fallback row is a copy of a primary row stored on a different AMP in the same cluster.

Benefits of Fallback:

·         Permits access to table data during AMP off-line period

·         Adds a level of data protection beyond disk array RAID 1 & 5

·         Highest level of data protection is RAID 1 and Fallback

·         Automatically restores data changed during AMP off-line

·         Critical for high availability applications

Costs of Fallback:

·         Twice the disk space for table storage is needed

·         Twice the I/O for INSERTs, UPDATEs and DELETEs is needed

FALLBACK CLUSTER

·         A defined number of AMPs treated as a fault-tolerant unit.

·         Fallback rows for AMPs in a cluster reside in the cluster.

·         Loss of an AMP in the cluster permits continued table access.

·         Loss of two AMPs in the cluster causes the RDBMS to halt.

TRANSIENT JOURNAL

Transient Journal:

·         Consists of a journal of transaction before images.

·         Provides rollback in the event of TXN failure.

·         Is automatic and transparent.

·         Before images are reapplied to table if TXN fails.

·         Before images are discarded upon TXN completion.

PERMIANENT JOURNALS

An optional, user-specified, system-maintained journal used for database recovery to a specified point in time.

·         Used for recovery from unexpected hardware of software disasters.

·         May be specified for:

o   One or more tables

o   One or more database

·         Permits capture of before images for database rollback.

·         Permits capture of after images for database roll forward.

·         Permits archiving change image during table maintenance.

·         Reduces need for full-table backups.

·         Provides a means of recovering NO FALLBACK table.

·         Requires additional disk space for change images.

·         Requires user intervention for archive and recovery activity.