Chapter 7: Technology Background

• Introduction to RAID (below)

•

Choosing a RAID Level (page 241)

•

Stripe Size (page 244)

•

Sector Size (page 244)

•

Cache Policy (page 245)

•

Capacity Coercion (page 246)

•

Initialization (page 247)

•

Hot Spare Drive(s) (page 247)

•

Partition and Format the Logical Drive (page 248)

•

RAID Level Migration (page 248)

•

Media Patrol (page 250)

•

Predictive Data Migration (PDM) (page 251)

•

Transition (page 252)

Introduction to RAID

RAID (Redundant Array of Independent Disks) allows multiple hard drives to be combined together in a disk array. Then all or a portion of the disk array is formed into a logical drive. The operating system sees the logical drive as a single storage device, and treats it as such. The RAID software and/or controller handle all of the individual drives on its own. The benefits of a RAID can include:

• Higher data transfer rates for increased server performance

• Increased overall storage capacity for a single drive designation (such as, C,

D, E, etc.)

• Data redundancy/fault tolerance for ensuring continuous system operation in the event of a hard drive failure

Different types of disk arrays use different organizational models and have

varying benefits. Also see Choosing RAID Level on page 241. The following

outline breaks down the properties for each type of RAID disk array:

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RAID 0 – Stripe

When a disk array is striped, the read and write blocks of data are interleaved between the sectors of multiple drives. Performance is increased, since the workload is balanced between drives or “members” that form the disk array.

Identical disk drives are recommended for performance as well as data storage efficiency. The disk array’s data capacity is equal to the number of disk drive members multiplied by the smallest drive's capacity.

Data

Stripe

Disk Drives

Figure 1. RAID 0 Striping interleaves data across multiple drives

For example, one 100GB and three 120GB drives will form a 400GB (4 x 100GB) disk array instead of 460 GB.

RAID 0 arrays require one or more physical drives.

Recommended applications: Image Editing, Pre-Press Applications, other applications requiring high bandwidth.

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RAID 1 – Mirror

When a disk array is mirrored, identical data is written to a pair of drives, while reads are performed in parallel. The reads are performed using elevator seek and load balancing techniques where the workload is distributed in the most efficient manner. Whichever drive is not busy and is positioned closer to the data will be accessed first. With RAID 1, if one drive fails or has errors, the other mirrored drive continues to function. This is called Fault Tolerance. Moreover, if a spare drive is present, the spare drive will be used as the replacement drive and data will begin to be mirrored to it from the remaining good drive.

Data Mirror

Disk Drives

Figure 2. RAID 1 Mirrors identical data to two drives

Due to the data redundancy of mirroring, the drive capacity of the disk array is only the size of the smallest drive. For example, two 100GB drives which have a combined capacity of 200GB instead would have 100GB of usable storage when set up in a mirrored disk array. Similar to RAID 0 striping, if drives of different capacities are used, there will also be unused capacity on the larger drive.

RAID 1 arrays use two physical drives. You can create multiple RAID 1 disk arrays on the same Promise product.

Recommended applications: Accounting, Payroll, Financial, other applications requiring very high availability.

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RAID 1E – Enhanced Mirror

RAID 1E offers the security of mirrored data provided by RAID 1 plus the added capacity of more than two disk drives. It also offers overall increased read/write performance plus the flexibility of using an odd number of disk drives. With RAID

1E, each data stripe is mirrored onto two disk drives. If one drive fails or has errors, the other drives continue to function, providing fault tolerance.

Enhanced Data Mirrors

Disk Drives

The advantage of RAID 1E is the ability to use an odd number of disk drives, unlike RAID 1 and RAID 10. You can also create a RAID 1E Logical Drive with an even number of disk drives. However, if you have an even number of disks, you will obtain greater security with comparable performance using RAID 10.

RAID 1E arrays consist of three or more physical drives. You can create an array with just two physical drives and specify RAID 1E. But the resulting array will actually be a RAID 1.

Recommended applications: Imaging Applications, Database Servers, General

Fileservers.

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RAID 5 – Block and Parity Stripe

RAID 5 organizes block data and parity data across the physical drives.

Generally, RAID Level 5 tends to exhibit lower random write performance due to the heavy workload of parity recalculation for each I/O. RAID 5 is generally considered to be the most versatile RAID level

Distributed Parity

Data

Blocks

Disk Drives

Figure 3. RAID 5 Stripes all drives with data and parity information

The capacity of a RAID 5 disk array is the smallest drive size multiplied by the number of drives less one. Hence, a RAID 5 disk array with (4) 100 GB hard drives will have a capacity of 300GB. A disk array with (8) 120GB hard drives and

(1) 100GB hard drive will have a capacity of 800GB.

RAID 5 arrays consist of three or more physical drives.

Recommended applications: File and Application Servers; WWW, E-mail, News servers, Intranet Servers

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RAID 10 – Mirror + Stripe

Mirroring/striping combines both of the previous RAID 1 and RAID 0 disk array types. RAID 10 is similar though not identical to RAID 0+1. RAID 10 can increase performance by reading and writing data in parallel while protecting data with duplication. At least four drives are needed for RAID 10 to be installed. With four disk drives, the drive pairs are striped together with one pair mirroring the first pair. The data capacity is similar to a RAID 1 disk array, with half of the total storage capacity used for redundancy. An added plus for using RAID 10 is that, in many situations, such a disk array offers double fault tolerance. Double fault tolerance may allow your logical drive to continue to operate depending on which two disk drives fail.

Data Stripe

Data

Mirror

Disk Drives

Figure 4. RAID 10 takes a data mirror on one drive pair and stripes it over two drive pairs

RAID 10 arrays require an even number of physical drives and a minimum of four.

For RAID 10 characteristics with an odd number of disk drives, use RAID 1E.

Recommended applications: Imaging Applications, Database Servers, General

Fileservers.

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RAID 50 – Striping of Distributed Parity

RAID 50 combines both RAID 5 and RAID 0 features. Data is striped across disks as in RAID 0, and it uses distributed parity as in RAID 5. RAID 50 provides data reliability, good overall performance and supports larger volume sizes.

Distributed Parity

Axle 1

Data

Stripes

Axle 2

Disk Drives

Figure 5. RAID 50 Striping of Distributed Parity disk arrays

RAID 50 also provides high reliability because data is still available even if multiple disk drives fail (one in each axle). The greater the number of axles, the greater the number of disk drives that can fail without the RAID 50 array going offline.

RAID 50 arrays consist of six or more physical drives.

Recommended applications: File and Application Servers, Transaction

Processing, Office applications with many users accessing small files.

RAID 50 Axles

When you create a RAID 50, you must specify the number of axles. An axle refers to a single RAID 5 array that is striped with other RAID 5 arrays to make

RAID 50. An axle can have from three to eight physical drives, depending on the number of physical drives in the array.

The chart below shows RAID 50 arrays with 6 to 15 disk drives, the available number of axles, and the resulting distribution of disk drives on each axle. VTrak

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No. of Drives in RAID 50

Array

No. of Axles in RAID 50

Array

8

9

6

7

10

11

12

13

14

15

4

2

2

3

3

4

3

2

3

2

3

2

2

2

2

2

2

3

3

4

4

5

No. of Drives per Axle

3,4,4

6,6

4,4,4

3,3,3,3

6,7

4,4,5

3,3,3,4

7,7

3,3

3,4

4,4

4,5

3,3,3

5,5

3,3,4

5,6

4,5,5

3,3,4,4

7,8

5,5,5

3,4,4,4

3,3,3,3,3

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