Section 1 Storage System Data Protection RAID Chapter

Section 1 : Storage System Data Protection: RAID Chapter 3 EMC Proven Professional The #1 Certification Program in the information storage and management industry © 2009 EMC Corporation. All rights reserved.

Why RAID o Performance limitation of disk drive o An individual drive has a certain life expectancy o Measured in MTBF (Mean Time Between Failure) o The more the number of HDDs in a storage array, the larger the probability for disk failure. For example: o If 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: o Increase capacity o Higher availability o Increased performance © 2009 EMC Corporation. All rights reserved.

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

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

RAID Implementations o Hardware (usually a specialized disk controller card) o Controls all drives attached to it o Array(s) appear to host operating system as a regular disk drive o Provided with administrative software o Software o Runs as part of the operating system o Performance is dependent on CPU workload o Does not support all RAID levels © 2009 EMC Corporation. All rights reserved.

RAID Levels o 0 Striped array with no fault tolerance o 1 Disk mirroring o Nested RAID (i. e. , 1 + 0, 0 + 1, etc. ) o 3 Parallel access array with dedicated parity disk o 4 Striped array with independent disks and a dedicated parity disk o 5 Striped array with independent disks and distributed parity o 6 Striped array with independent disks and dual distributed parity © 2009 EMC Corporation. All rights reserved.

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

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

RAID 0 0 1 5 9 RAID Controller Host © 2009 EMC Corporation. All rights reserved. 2 6 10 3 7 11

RAID 1 o Data is stored on two different HDDs, yielding two copies of the same data. o Provides availability. o In the event of HDD failure, access to data is still available from the surviving HDD. o When the failed disk is replaced with a new one, data is automatically copied from the surviving disk to the new disk. o Done automatically by RAID the controller. o Disadvantage: The amount of storage capacity is twice the amount of data stored. o Mirroring is NOT the same as doing backup! © 2009 EMC Corporation. All rights reserved.

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

Nested RAID o Combines the performance benefits of RAID 0 with the redundancy benefit of RAID 1. o RAID 0+1 – Mirrored Stripe o Data is striped across HDDs, then the entire stripe is mirrored. o If one drive fails, the entire stripe is faulted. o Rebuild operation requires data to be copied from each disk in the healthy stripe, causing increased load on the surviving disks. o RAID 1+0 – Striped Mirror o Data is first mirrored, and then both copies are striped across multiple HDDs. o When a drive fails, data is still accessible from its mirror. o Rebuild 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) RAID 1 Block 0 Block 2 Block 0 3 2 1 RAID Controller RAID 0 Block 1 Host © 2009 EMC Corporation. All rights reserved. Block 3

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

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

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

RAID Redundancy: Parity 0 4 1 6 5 9 RAID Controller Host The middle drive fails: Parity calculation 4 + 6 + 1 + 7 = 18 4 + 6 + ? + 7 = 18 ? = 18 – 4 – 6 – 7 ? =1 © 2009 EMC Corporation. All rights reserved. Parity Disk 1 ? 3 7 7 11 0123 4 518 67

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

RAID 3 Block 0 3 2 1 Host RAID 0 Block Controller Block Parity 1 Generated Block 2 Block 3 P 0123 © 2009 EMC Corporation. All rights reserved.

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

RAID 5 Block 0 Block 4 Block 1 Block 5 Block 0 4 Parity RAID 4 Block 0 Generated Controller Block 2 Block 6 P 4 05 16 27 3 Block 3 Host P 4567 P 0123 Block 7 © 2009 EMC Corporation. All rights reserved.

RAID Comparison RAID 0 1 3 5 Min Disks 2 2 3 3 Storage Efficiency % 100 50 (n-1)*100/n where n= number of disks 6 4 (n-2)*100/n where n= number of disks 1+0 and 0+1 4 50 © 2009 EMC Corporation. All rights reserved. Cost Low High Moderate Read Performance Write Performance Very good for both random and sequential read Very good Good Better than a single disk Good Slower than a single disk, as every write must be committed to two disks Good for random reads and very good for sequential reads Poor to fair for small random writes Good for large, sequential writes Very good for random reads Good for sequential reads Fair for random write Slower due to parity overhead Fair to good for sequential writes Moderate but more than RAID 5 Very good for random reads Good for sequential reads Good for small, random writes (has write penalty) High Very good Good

RAID Impacts on Performance RAID Controller Ep new = Ep old E 4 old - + E 4 new 2 XOR Ep new Ep old P 0 D 1 E 4 old D 2 D 3 o Small (less than element size) write on RAID 3 & 5 o Ep = E 1 + E 2 + E 3 + E 4 (XOR operations) o If parity is valid, then: Ep new = Ep old – E 4 old + E 4 new (XOR operations) o 2 disk reads and 2 disk writes o Parity Vs Mirroring o Reading, calculating and writing parity segment introduces penalty to every write operation o Parity RAID penalty manifests due to slower cache flushes o Increased load in writes can cause contention and can cause slower read response times © 2009 EMC Corporation. All rights reserved. E 4 new D 4

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

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

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

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