store extra information that can be used to rebuild information lost in a disk failure
Basic strategy: mirroring, parity
Mean time to data loss depends on mean time to failure, and mean time to repair
10010010 1 Data Parity
RAID Levels Twice the Read transaction rate of single disks, same Write transaction rate as single disks. Due to its cost and complexity, level 2 never really "caught on". Therefore, much of the information below is based upon theoretical analysis, not empirical evidence.
RAID Levels (cont) Very high read data transfer rate and very high write data transfer rate, but Controller design is fairly complex. Very high read data transaction rate, but quite complex controller design. Difficult and inefficient data rebuild in the event of disk failure.
RAID Levels (cont) Highest read data transaction rate and medium write data transaction rate. Most complex controller design. Difficult to rebuild in the event of a disk failure (compared to RAID level 1) RAID 6 is essentially an extension of RAID level 5 which allows for additional fault tolerance by using a second independent distributed parity scheme (dual parity)
Suitable for applications that require sequential processing of the entire file. The records in the file are ordered by a search-key. Need to reorganize the file from time to time to restore sequential order.
Deletion – use pointer chains
Insertion – must locate the position in the file where the record is to be inserted
– If there is free space insert there
– If no free space, insert the record in an overflow block
– In either case, pointer chain must be updated
Clustering File Organization
Simple file structure stores each relation in a separate file
Can instead store several relations in one file using a clustering file organization