RAID (Redundant Array of Independent Disks) Primer
RAID 0 - Data Striping
RAID 0 allows a number of disk drives to be combined and presented as one large disk. RAID 0 does not provide any data redundancy - if one drive fails, all data is lost.
Access Time Very Good
Transfer Rate Good
Redundancy None
Cost Per Megabyte None
Penalty
Applications Large disk requirements, high performance databases
RAID 1 - Disk Mirroring/Disk Duplexing
RAID 1 mirrors (shadows) one disk drive to another. All data is stored twice on two or more identical disk drives.
When one disk drive fails, all data is immediately available on the other without any impact on the data integrity -performance in degraded mode is also degraded. Performance is gained by splitting of functions. If multiple read requests are pending, the RAID controller will allows reads from different disk drives. If one disk is busy writing the other disk drive can supply read data, at a later time. The RAID controller will update the read drive with data from the already written disk drive.
If each disk drive is connected with a separate SCSI channel, this is called "Disk Duplexing" (additional security and performance). RAID 1 represents a simple
and highly efficient solution for data security and system availability. Use RAID 1 when large volumes of data are not required.
Access Time Very Good
Transfer Rate Good
Redundancy Yes
Cost Per Megabyte 100% or more
Penalty
Applications Small disk capacities that require redundancy
RAID 0 + 1 - Combination of RAID 1 and RAID 0
The idea behind RAID 0+1 is simply based on the combination of RAID 0 (Performance) and RAID 1 (Data Security). RAID 0+1 disk sets offer good performance and data security. Similar as in RAID 0, optimum performance is achieved in highly sequential load situations.
The major draw back is a 100% "Cost Per Megabyte Penalty".
Access Time Very Good
Transfer Rate Good
Redundancy Yes
Cost Per Megabyte 100%
Penalty
Applications Multiuser environments, database servers, file serving, web site hosting.
RAID 3 Data Bit Striping With a Dedicated Parity Drive
The data is striped at a byte/bit level across the disk drives.
Additionally, the controller calculates parity information which is stored on a separate disk drive (aP, bP, ...). Even when one disk drive fails, all data is fully available. The missing data can be recalculated from the data still available and the parity information.
This data calculation can also be used to restore data to a replaced defective disk. Because the data must be presented at the same time, the disk drive spindles must be synchronized for RAID 3 to be effective. This represents a practical implementation problem for RAID 3.
Many RAID controller manufacturers are moving to a RAID 4 solution or using the term RAID 3 merely as a recognized marketing term for high data transfer capability.
Access Time Good
Transfer Rate Very Good
Redundancy Yes
Cost Per Megabyte Varies. 5 drive set = 20%, 6 drive set = 17%, 10 drive set = 10%
Penalty
Applications Imaging, geological, seismological, video
RAID 4 - Data Striping With a Dedicated Parity Drive
RAID 4 works just like RAID 0. The data is striped across disk drives. Additionally, the controller calculates parity information which is stored on a separate disk drive (P1, P2, ...). Even when one disk drive fails, all data is fully available. The missing data can be recalculated from the data still available and the parity information. This data calculation can also be used to restore data to replaced defective disk. RAID 4 offers excellent transfer rates when used with large contiguous blocks of data. When used with with many small data blocks, the parity disk drive becomes a throughput bottle-neck because of it's fixed position.A RAID 4 disk set can only lose one disk from it's RAID set. Loosing another disk drive, before a replacement is restored, will loose all data in the RAID set.
Access Time Good
Transfer Rate Very Good
Redundancy Yes
Cost Per Megabyte Varies. 5 drive set = 20%, 6 drive set = 17%, 10 drive set = 10%
Penalty
Applications Imaging, geological, seismological, video
RAID 5 - Data Striping with Striped Parity
The data is striped across disk drives. Unlike RAID 4, the parity data in a RAID 5 set is striped across all disk drives. RAID 5 is designed to handle small data blocks. This makes RAID 5 the level of choice for multitasking, multiuser and database environments.
RAID 5 offers the same level of security as RAID 4: when one disk drive fails, all data is fully available, the missing data is recalculated from the data still available and the parity information. This data calculation can also be used to restore data to replaced defective disks. RAID 5 is particularly suited for systems with medium to large capacity requirements, with their "Cost Per Megabyte Penalty" is relatively low. A RAID 5 disk set can only lose one disk from it's RAID set. Loosing another disk drive, before a replacement is restored, will loose all data in the RAID set.
Access Time Very Good
Transfer Rate Good
Redundancy Yes
Cost Per Megabyte Varies. 5 drive set = 20%, 6 drive set = 17%, 10 drive set = 10%
Penalty
Applications Multiuser environments, database servers, file serving, web site hosting
JBOD - Just a Bunch Of DiskS
An allowance was made by virtually all RAID control manufacturers for adding a single disk inder
the RAID controller that would not be a part of any RAID Set. A "JBOD" disk drive appears to the host as an add-on disk drive. Using JBODs is a convenient way of adding quick storage. If a JBOD disk drive breaks all data is lost.
Access Time Good
Transfer Rate Good
Redundancy No
Cost Per Megabyte None
Penalty
Applications Quick increase in capacity
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by Awgeewhiz  edited by vkr
last modified: 2002-05-23 23:30:10 |
