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Frequently Asked Questions on RAID

Questions

  1. What is RAID?
  2. What is meant by redundancy, parity or parity data?
  3. What are the different RAID levels and what do they support?
  4. What's the minimum amount of disks I need to run a RAID set
  5. How many SCSI ID's will I need for each RAID set?
  6. What is the maximum partition sizes for SunOS and Solaris?


Answers

(1) What is RAID?

A general RAID concept of  was first defined by David A. Patterson, Garth Gibson and Randy H. Katz of the University of California, Berkeley in 1987. Read their original paper - A Case for Redundant Arrays of Inexpensive Disks (RAID).

Described in that paper, "a disk array combines the capabilities of a number of small, inexpensive disk drives to exceed the performance of a single, large, expensive disk drive". In addition, since RAIDs use a number of small drives, features can be added to protect against the loss of data when a single drive fails. This redundancy is why Raids have become so popular in high-availability applications.

RAID is an acronym for Redundant Array of Independent (or inexpensive) Disks. There are six levels of RAID: level 0 - level 5. Each level supports a different storage layout scheme on the disk drives, from mirroring to parity striping.

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(2) What is meant by redundancy, parity or parity data?

Redundancy means that there is protection against any single disk failure. Parity data is information used by a RAID system to rebuild the data on a disk in the event of a failure. Parity data is created by using a logical exclusive-OR (XOR) on actual user data and storing the result on disk. Example: If an array of 5 drives exists, the 4 drives are used as the storage devices and the 5th as the parity drive. Data on the first sector of each of the 4 data drives is XORed creating parity data that is stored on the first sector of the parity drive. The same holds true for the second sector.

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(3) What are the different RAID levels and what do they support?

Level 0: Disk Striping - data is transferred in parallel across an array of disks. Redundancy is not provided in this level.

Level 1: Disk Mirroring - duplicate contents of one disk are written onto another disk.

Level 0+1: Disk Striping and Mirroring - this level combined the performance of striping with the reliability of of mirroring. This results in very high I/O performance and high data availability.

Level 2: Bit interleaving data across multiple disks with parity information created using a Hamming code. A Hamming code detects errors that occur and determine which part is in error. RAID level 2 specifies 39 disks with 32 disks of user storage and 7 disks of error recovery coding.

Level 3: Data is striped across multiple drives and parity is written to a dedicated drive. Level 3 is typically implemented at the BYTE level.

Level 4: Data is striped across multiple drives and parity is written to a dedicated drive. Level 4 is typically implemented at the BLOCK level.

Level 5: Error correction data is striped at the block level across all the drives in the array. Reads and writes may be performed concurrently.

JBOD: "Just a Bunch Of Drives" - performance without data redundancy. Use where loss of data is not critical.

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(4) What is the minimum amount of disks I need to run a RAID set?

If you are implementing RAID level 0 in Z Microsystems' SmartStor RAID solution, you will need a minimum of 2 disk drives to create a rank. If you are implementing RAID level 5, then you will need a minimum of 3 disks to create a rank.

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(5) How many SCSIs will I need for each RAID rank?

The host channel uses one SCSI id. All the drives in a rank must be set to the exact same SCSI id. Each rank is differentiated by a unique logical unit number (LUN). Example: 2 ranks of 5 drives in each rank on a host channel with SCSI id 1. All the drives in both ranks will be seen as SCSI id 1, but rank 1 will be recognized as LUN 0, and rank 2 as LUN 1.

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(6) What is the maximum partition sizes for SunOS and Solaris?

On SunOS the maximum partition size is 2 GB and the maximum number of partitions is 7 so the largest RAID set is 14 GB. However, in reality, if the customer uses 4 GB drives, then the maximum will be 3x4GB = 12GB. On Solaris, the maximum number of partitions is still 7. The largest disk size Z Microsystems currently sells is 72 GB so the largest RAID set is 432 GB (6x72S GB) plus 1 drive for parity RAID 4 or RAID 5.

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