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
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RAID-0
RAID-1
RAID-1+0, RAID-0+1
RAID-5
RAID-6
RAID-0
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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
RAID-1
• width = 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)
RAID-1+0
Width=2
RAID-1+0
• Width = 4
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 RAID1+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