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Enhanced Availability With RAID
CC5493/7493
RAID
• Redundant Array of Independent Disks • RAID is implemented to improve: – IO throughput (speed) and – Availability of a file system.
RAID Implementation
• Software – often criticized as not being a true RAID implementation.
• Hardware – A special RAID controller is required.
RAID: Stripe
• The stripe takes on two meanings within the context of a RAID system: – Stripe
width
(number of independent drives) – Stripe
size
(storage block size) Both stripe width and stripe size are adjusted to enhance IO throughput.
RAID Stripe Width
• Stripe
width
refers to the number of disks used in parallel for IO transfers to and from the array.
Raid Stripe Size
• Stripe
size
refers to the size of the storage units organized on the disk surface. • The stripe size is adjusted to optimize the speed of the IO transfers.
Common RAID Types
• RAID-0 • RAID-1 • RAID-1+0, RAID-0+1 • RAID-5 • RAID-6
RAID-0
• AKA disk striping • Does not provide redundancy • Degrades data availability, reduces MTF • Improves IO throughput (average IO transfer rate improves)
RAID-0
• Ideal for temporary storage requiring fast data access.
-Engineering/Scientific calculations on large data volumes. However, the data is a redundant temporary copy.
RAID-1
• AKA mirroring • Requires two independent disk devices – The first disk stores the data – The second disk is an image of the first – Can double the overall read throughput
• width = 1
RAID-1
RAID-1 Advantages
• Improves data availability.
• Dual-channel controller allows for two simultaneous read operations.
• Allows for error detection on read.
• Administrative advantages for service on one drive while the other remains available.
• Fault tolerance is one drive.
RAID-1 Disadvantages
• Writes have a slight performance penalty compared to no RAID.
• Doubles the cost of storage.
• Storage efficiency = 50%
RAID-1
• Ideal for data that is read more often than written: – Some database information that is not updated often.
– Web Server information (lots of reads, few writes)
RAID-1+0
• Enhances IO throughput and data availability.
• Requires 2(n+1) separate disk devices, where n = 1, 2, 3, 4, … – Minimum of 4 disks required (n=1)
Width=2
RAID-1+0
• Width = 4
RAID-1+0
RAID-1+0
• RAID-1+0 has a higher fault tolerance compared to RAID-0,1, & 5.
• Storage efficiency is 50%
RAID-0+1
• Requires the same hardware as RAID 1+0, but less fault tolerant.
• However, there is better read throuthput from RAID-0+1 compared to RAID-1+0.
RAID-0+1
• Duplicate RAID-0 arrays. Allows simultaneous reads
RAID-5
• RAID-5 enhances – IO data throughput – Data availability • Parity information enhances availability • Requires a minimum of 3 independent disk devices.
Parity Information
• Based on the logical exclusive-or operation.
RAID-5 Configuration
• Stripe Width = 4
RAID-5
• The most common implementation of RAID.
• Ideal for a disk-server providing general storage.
• A good balance between reliability and speed.
• Often implemented using high quality disk drives (SCSI, 15k-rpm, high MTF)
RAID-5 Limitations
• Overhead occurs during writes due to the parity calculation and parity write.
• Storage efficiency is not 100% due to the parity storage requirements. storage efficiency = (n-1)/n, where n = number of drives.
RAID-5 (S)ATA Limitations
• Large capacity (S)ATA drives are more likely to contain bad blocks.
• After a disk failure, the bad blocks make it impossible to rebuild the array from the remaining drives.
RAID-6
• Contains two sets of parity.
• Tolerates two simultaneous disk failures.
• A better solution for (S)ATA arrays where each disk has a large capacity (multiple TB).
• Stripe Width = 6
RAID-6
• Higher availability at the cost of greater IO overhead due to complex parity calculations and storage.
• Storage efficiency = (n-2)/n • Becoming more popular for large storage capacity (S)ATA arrays
RAID-6 Disadvantages
• More expensive to implement due to extra parity information • Slower write operations compared to other RAID-5
RAID Disk Swapping
• Hot Swap • Warm Swap • Cold Swap
Hot Swap
• The ability to swap out a failed disk from a RAID array without an interruption of service from the array.
• Performance will be slower due to the operations required to rebuild the new replacement disk.
Warm Swap
• The array is not accessible while a drive is being serviced, but the system does not need to be shut down.
Cold Swap
• System must be shutdown to service the array.
Spare Disk: Hot Spare
• Some RAID controllers can be configured to immediately recover from a disk failure if a hot-spare disk is connected to the controller at all times.
RAID Disk Failure and Performance
• When a failed disk is replaced in an array, there is a performance hit as the new disk must be re-populated with the required data for the complete array.
RAID Summary
• RAID-0 : for temporary storage only • RAID-1 : ideal for disk services that provide mostly read operations like data base services and web services.
• RAID-5 : general purpose disk-server • RAID-6 : for very large data requirement environments (multiple T-Bytes).
RAID Summary
• RAID 1+0 : general purpose disk server where RAID-5 & 6 are not adequate.
– Better fault tolerance – More IO throughput
Other?
• RAID 1+1, mirror a mirrored RAID-1 – Triples the cost of storage – Excellent fault tolerance.
– Excellent read throughput.
– Writes will suffer