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Intelligent Disk Subsystem 1
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After completing this chapter, you will be able to: Internal Disk subsystem structure
Hard disk and internal I/O channels
JBOD: Just a Bunch Of Disk Storage virtualization using RAID
Different RAID levels
Caching: Acceralaration of Hard Disk Access
Intelligence and Availability of Disk Sub system
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Small storage device is replaced by largestorage subsystem.
Performance limitation of small storagedevice.
An individual drive has a certain lifeexpectancy Measured in MTBF
Example If the MTBF of a drive is 750,000 hours, and there are
1000 drives in the array, then the MTBF of the arraybecomes 750,000 /1000, or 750 hours
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Storage consolidation.
High Availability.
High performance Instant copies
Remote mirroring at reasonable price
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Disk subsystem visualized as hard disk server. Servers are connected to a disk subsystem using
standard I/O techniques. SCSI: Small computer system interface
iSCSI: Internet SCSI
Fibre Channel (SAN)
Storage Pooling: Free disk space can be flexiblyassigned to each server connected.
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Intelligent Disk Subsystem - 6
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The controller
increase the data availability
data access performance with
RAID.
facilitates copying services
instant copy and remote
mirroring.
uses a cache in an attempt toaccelerate read and write accesses
to the server.
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All servers share
the storage capacity
of a disk subsystem.
Each server can
be assigned freestorage more flexibly
as required.
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Size of the hard disk used limits the maximumcapacity of the overall disk subsystem.
Maximum Performance Vs Maximum capacity
Maximum disk (C,I)
More disk more load in terms of R/W operationsand I/O channels
Factors impacting disk sub system or individualdisk: Size Speed Cache I/O communication medium.
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For increased fault tolerance: I/O Channels techniques Active
Active/Passive
Active/Active (No Load sharing)
Active/ Active (Load Sharing).
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Disk Subsystem complexity No controller
RAID controller
Intelligent controller
JBOD: No internal controller.
Only full enclosure of disks
Normally 8 to 16 hard disk space. Every hard disk has a separate address space
Does not support RAID or any other virtualization
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Individual hard disk are slow and has less lifeexpectancy.
Stripping: Increases performance by strippingand
Mirroring: Improves fault tolerance byredundancy.
Parity: Provides data integrity RAID provides:
Increase capacity Higher availability Increased performance
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RAIDController
Hard Disks
LogicalArray
PhysicalArray
RAID Array
Host
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RAIDController
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Hardware (usually a specialized diskcontroller card) Controls all drives attached to it
Array(s) appear to host operating system as a
regular disk drive Provided with administrative software
Software Runs as part of the operating system
Performance is dependent on CPU workload Does not support all RAID levels
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0 Striped array with no fault tolerance 1 Disk mirroring with fault tolerance Nested RAID (i.e., 1 + 0, 0 + 1) 3 Parallel access array with dedicated parity
disk 4 Striped array with independent disks and a
dedicated parity disk 5 Striped array with independent disks and
distributed parity 6 Striped array with independent disks and
dual distributed parity
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Intelligent Disk Subsystem-
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Stripe 1
Stripe 2
Strips
Strip 1=64KB
Stripe=192KB
Strip
Stripe
Strip 2=64KB Strip 3=64KB
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1
9
5
2
10
6
3
117
0
Host
RAIDController
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Block 1 Block 1Block 1Block 0Block 0
Host
Block 0RAID
Controller
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Host
Block 5Block 4Block 2Block 1
Block 5
Block 4
Block 2
RAID 0
Block 1
RAID 1
Block 0Block 3RAID
Controller
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RAIDController
Block 5
Block 4
Block 2
RAID 0
Block 1
RAID 1
Block 5
Block 4
Block 2
Block 1
Block 5
Block 4
Block 2
Block 1
Host
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Host
Block 5
Block 5
Block 2
RAID 1Block 0Block 0
Block 2
RAID 0
Block 3Block 3RAID
Controller
Block 1
Block 1
Block 4
Block 4
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Host
RAIDController
RAID 1
Block 1
Block 1
RAID 0
Block 4
Block 4 Block 5
Block 5
Block 2
Block 2Block 1
Block 4
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1
9
5
3
117
0
0 1 2 3
4 5 6 7
4
6
1
7
18
Host
RAIDController
Parity calculation 4 + 6 + 1 + 7 = 18The middle drive fails:
4 + 6 + ? + 7 = 18
?= 18 4 6 7
? = 1
?
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Host
RAIDController
Block 1
Block 2
Block 3
P 0 1 2 3
Block 0Block
ParityGenerated
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Host
Block 0
P 0 1 2 3
P 4 5 6 7
RAIDController
P 0 1 2 3
Block 0Block 0
Block 1
Block 5
Block 2
Block 6
Block 3
ParityGenerated
Block 0
P 0 1 2 3
Block 4
Block 7
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Host
Block 0
P 0 1 2 3
Block 7
RAIDController
P 0 1 2 3
Block 0Block 4Block 0
Block 1
Block 5
Block 2
Block 6
Block 3
ParityGenerated
Block 0
P 0 1 2 3
Block 4
P 4 5 6 7P 4 5 6 7
Block 4
P 4 5 6 7
Block 4Parity
Generated
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Two disk failures in a RAID set leads to dataunavailability and data loss in single-parityschemes, such as RAID-3, 4, and 5
Increasing number of drives in an array andincreasing drive capacity leads to a higher
probability of two disks failing in a RAID set RAID-6 protects against two disk failures by
maintaining two parities Horizontal parity which is the same as RAID-5 parity Diagonal parity is calculated by taking diagonal sets of
data blocks from the RAID set members Even-Odd, and Reed-Solomon are two commonly
used algorithms for calculating parity in RAID-6
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RAIDMin
Disks
Storage
Efficiency %
Cost Read Performance Write Performance
0 2 100 Low
Very good for bothrandom and sequential
readVery good
1 2 50 HighGood
Better than a single disk
GoodSlower than a single
disk, as every write mustbe committed to two
disks
3 3
(n-1)*100/nwhere n=number of
disksModerate
Good for random readsand very good forsequential reads
Poor to fair for smallrandom writesGood for large,
sequential writes
45 3
(n-1)*100/nwhere n=number of
disks
Moderate
Very good for randomreads
Good for sequential
reads
Fair for random writeSlower due to parity
overhead
Fair to good forsequential writes
6 4
(n-2)*100/nwhere n=number of
disks
Moderatebut more
than RAID 5
Very good for randomreads
Good for sequentialreads
Good for small, randomwrites
(has write penalty)
1+0and
0+1
4 50 High Very good Good
Intelligent Disk Subsystem
RAID Comparison
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Intelligent Disk Subsystem
RAID Comparison
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Small (less than element size) write on RAID 5 Ep = E1 + E2 + E3 + E4 (XOR operations)
If parity is valid, then: Ep new = Ep old E4 old + E4 new (XOR operations) 2 disk reads and 2 disk writes
Parity Vs Mirroring Reading, calculating and writing parity segment introduces penalty to every write operation Parity RAID penalty manifests due to slower cache flushes Increased load in writes can cause contention and can cause slower read response times
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Ep new
RAID Controller
2 XOR
+-= E4 oldEp old E4 new
P0 D1 D2 D3 D4
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Total IOPS at peak workload is 1200 Read/Write ratio 2:1
Calculate IOPS requirement at peak activityfor RAID 1/0
RAID 5
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Key points covered in this chapter: What RAID is and the needs it addresses
The concepts upon which RAID is built
Some commonly implemented RAID levels
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Click the attached file
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Click the attached file
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What is a RAID array? What benefits do RAID arrays provide?
What methods can be used to provide higherdata availability in a RAID array?
What is the primary difference between RAID3 and RAID 5?
What is advantage of using RAID 6?
What is a hot spare?