© 2009 EMC Corporation. All rights reserved. Why do we need RAID o Performance limitation of disk drive o An individual drive has a certain life expectancy oMeasured in MTBF (Mean Time Between Failure) oThe more the number of HDDs in a storage array, the larger the probability for disk failure. For example: oIf the MTBF of a drive is 750,000 hours, and there are 100 drives in the array, then the MTBF of the array becomes 750,000 / 100, or 7,500 hours o RAID was introduced to mitigate this problem o RAID provides: oIncrease capacity oHigher availability oIncreased performance

© 2009 EMC Corporation. All rights reserved. After completing this chapter, you will be able to: oDescribe what is RAID and the needs it addresses oDescribe the concepts upon which RAID is built oDefine and compare RAID levels oRecommend the use of the common RAID levels based on performance and availability considerations oExplain factors impacting disk drive performance

© 2009 EMC Corporation. All rights reserved. RAID Array Components RAID Controller Hard Disks Logical Array Physical Array RAID Array Host

© 2009 EMC Corporation. All rights reserved. RAID Implementations oHardware (usually a specialized disk controller card) oControls all drives attached to it oArray(s) appear to host operating system as a regular disk drive oProvided with administrative software oSoftware oRuns as part of the operating system oPerformance is dependent on CPU workload oDoes not support all RAID levels

© 2009 EMC Corporation. All rights reserved. RAID Levels o0 Striped array with no fault tolerance o1 Disk mirroring oNested RAID (i.e., 1 + 0, 0 + 1, etc.) o3 Parallel access array with dedicated parity disk o4 Striped array with independent disks and a dedicated parity disk o5 Striped array with independent disks and distributed parity o6 Striped array with independent disks and dual distributed parity

© 2009 EMC Corporation. All rights reserved. Data Organization: Striping Stripe 1 Stripe 2 Strips Strip 1Strip 2Strip 3 Stripe Strip Stripe

© 2009 EMC Corporation. All rights reserved. RAID 0 oData is distributed across the HDDs in the RAID set. oAllows multiple data to be read or written simultaneously, and therefore improves performance. oDoes not provide data protection and availability in the event of disk failures.

© 2009 EMC Corporation. All rights reserved. RAID Host RAID Controller

© 2009 EMC Corporation. All rights reserved. RAID 1 oData is stored on two different HDDs, yielding two copies of the same data. oProvides availability. oIn the event of HDD failure, access to data is still available from the surviving HDD. oWhen the failed disk is replaced with a new one, data is automatically copied from the surviving disk to the new disk. oDone automatically by RAID the controller. oDisadvantage: The amount of storage capacity is twice the amount of data stored. oMirroring is NOT the same as doing backup!

© 2009 EMC Corporation. All rights reserved. RAID 1 Block 1 Block 0 Host Block 0 RAID Controller

© 2009 EMC Corporation. All rights reserved. Nested RAID oCombines the performance benefits of RAID 0 with the redundancy benefit of RAID 1. oRAID 0+1 – Mirrored Stripe oData is striped across HDDs, then the entire stripe is mirrored. oIf one drive fails, the entire stripe is faulted. oRebuild operation requires data to be copied from each disk in the healthy stripe, causing increased load on the surviving disks. oRAID 1+0 – Striped Mirror oData is first mirrored, and then both copies are striped across multiple HDDs. oWhen a drive fails, data is still accessible from its mirror. oRebuild operation only requires data to be copied from the surviving disk into the replacement disk.

© 2009 EMC Corporation. All rights reserved. Nested RAID – 0+1 (Striping and Mirroring) Block 3 Block 2 Block 1 Host RAID 0 Block 0 Block 3 Block 2 Block 1 Block 0 RAID 1 RAID Controller

© 2009 EMC Corporation. All rights reserved. Nested RAID – 0+1 (Striping and Mirroring) RAID Controller Block 3 Block 2 Block 1 RAID 0 Block 0 RAID 1 Block 3 Block 2 Block 1 Block 0 Block 3 Block 2 Block 1 Block 0 Host

© 2009 EMC Corporation. All rights reserved. Host Nested RAID – 1+0 (Mirroring and Striping) Block 3 Block 1 RAID 1 Block 0 Block 1 RAID 0 Block 2 RAID Controller

© 2009 EMC Corporation. All rights reserved. Host Nested RAID – 1+0 (Mirroring and Striping) RAID Controller RAID 1 Block 0 RAID 0 Block 2 Block 3 Block 1 Block 0 Block 2

© 2009 EMC Corporation. All rights reserved. RAID Redundancy: Parity Parity Disk Host RAID Controller Parity calculation = 18 The middle drive fails: ? + 7 = 18 ? = 18 – 4 – 6 – 7 ? = 1 ?

© 2009 EMC Corporation. All rights reserved. RAID 3 and RAID 4 oStripes data for high performance and uses parity for improved fault tolerance. oOne drive is dedicated for parity information. oIf a drive fails, data can be reconstructed using data in the parity drive. oFor RAID 3, data read / write is done across the entire stripe. oProvide good bandwidth for large sequential data access such as video streaming. oFor RAID 4, data read/write can be independently on single disk.

© 2009 EMC Corporation. All rights reserved. Host RAID Controller Block 1 Block 2 Block 3 P Block 0 Block 3 Block 2 Block 1 Block 0 Parity Generated RAID 3

© 2009 EMC Corporation. All rights reserved. RAID 5 and RAID 6 oRAID 5 is similar to RAID 4, except that the parity is distributed across all disks instead of stored on a dedicated disk. oThis overcomes the write bottleneck on the parity disk. oRAID 6 is similar to RAID 5, except that it includes a second parity element to allow survival in the event of two disk failures. oThe probability for this to happen increases and the number of drives in the array increases. oCalculates both horizontal parity (as in RAID 5) and diagonal parity. oHas more write penalty than in RAID 5. oRebuild operation may take longer than on RAID 5.

© 2009 EMC Corporation. All rights reserved. Host Block 0 P Block 7 RAID Controller P Block 0Block 4Block 0 Block 1 Block 5 Block 2 Block 6 Block 3 Parity Generated Block 0 P Block 4 P Block 4 P Block 4 Parity Generated RAID 5

© 2009 EMC Corporation. All rights reserved. RAID Min Disks Storage Efficiency % CostRead PerformanceWrite Performance 02100Low Very good for both random and sequential read Very good 1250High Good Better than a single disk Good Slower than a single disk, as every write must be committed to two disks 33 (n-1)*100/n where n= number of disks Moderate Good for random reads and very good for sequential reads Poor to fair for small random writes Good for large, sequential writes 53 (n-1)*100/n where n= number of disks Moderate Very good for random reads Good for sequential reads Fair for random write Slower due to parity overhead Fair to good for sequential writes 64 (n-2)*100/n where n= number of disks Moderate but more than RAID 5 Very good for random reads Good for sequential reads Good for small, random writes (has write penalty) 1+0 and HighVery goodGood RAID Comparison

© 2009 EMC Corporation. All rights reserved. RAID Penalty Exercise oTotal IOPS at peak workload is 1200 oRead/Write ratio 2:1 oCalculate IOPS requirement at peak activity for oRAID 1/0 oRAID 5 Additional Task Discuss impact of sequential & Random I/O in different RAID Configuration

© 2009 EMC Corporation. All rights reserved. RAID Controller Hot Spares

© 2009 EMC Corporation. All rights reserved. Chapter Summary Key points covered in this chapter: oWhat RAID is and the needs it addresses oThe concepts upon which RAID is built oSome commonly implemented RAID levels