RAID configuration FAQ

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What is meant by “heavy I/O load”?

For the purposes of this paper, heavy I/O means sequentially reading files whose physical size is either close to or exceeds the amount of physical memory in the computer in size and where I/O rates need to be at least 100 MB/sec or preferably 50-75 MB/sec per CPU core in your system to process the data in the timeframe required by the SAS users.

In addition, this term can mean having multiple SAS sessions/users in which the total amount of data that is accessed simultaneously is close to or exceeds the amount of memory in the computer.

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.

What is the difference between RAID 5 and RAID 6?

RAID Level 5 requires a minimum of 3 drives to implement:

Each entire data block is written on a data disk; parity for blocks in the same rank is generated on Writes, recorded in a distributed location and checked on Reads.

RAID Level 6 requires a minimum of 4 drives to implement:

Two independent parity computations must be used in order to provide protection against double disk failure. Two different algorithms are employed to achieve this purpose.

Is there an option/site to calculate RAID capacity before proceeding?

Yes. You can visit http://www.raid-calculator.com to calculate the RAID capacity

What is the difference between RAID and NAS?

Redundant Array of Independent Disks can apply to any multi disk system designed to increase speed of access or security of data in a computer or externally.

NAS simply means Network Attached Storage which could be a single disk or several in an external enclosure.

What is RAID 10?

RAID 10 is a nested RAID system created by combining RAID 1 and RAID 0. The combination is known as a stripe of mirrors. In this arrangement, data is striped much as it is in a RAID 0 array. The difference is that each member of the striped set has its data mirrored. This ensures that if any single drive in the RAID 10 array fails, the data isn’t lost.

One way to think of a RAID 10 array is as a RAID 0 with an online backup of each RAID element ready to go, should a drive fail.

RAID 10 requires a minimum of four drives and can be expanded in pairs; you can have a RAID 10 array with 4, 6, 8, 10, or more drives. RAID 10 should be composed of equal-size drives.

The Big Issue with All RAID Setups

I’ve discussed the various pros and cons of each of the levels of RAID that can be used on personal computers but there is another issue that many people don’t realize when it comes to creating RAID drive setups. Before a RAID setup can be used, it first must be constructed either by the hardware controller software or within the software of the operating system. This essentially initializes the special formatting required to properly track how the data will be written and read on the drive.

This probably doesn’t sound like a problem but it is if you even need to change how you want your RAID array configured. For instance, say you are running low on data and want to add an extra drive for either a RAID 0 or RAID 5 array. In most cases, you won’t be able to without first reconfiguring the RAID array which will also remove any of the data that was stored in those drives. This means that you have to fully back up your data, add the new drive, reconfigure the drive array format that drive array and then restore your original data back to the drive. That can be an extremely painful process. As a result, make sure you really have the array setup up the way you want to the first time you do it.

What are the different RAID levels? Which one is recommended for file server and database server?

A Redundant Array of Independent Drives (or Disks), also known as Redundant Array of Inexpensive Drives (or Disks) (RAID) is an term for data storage schemes that divide and/or replicate data among multiple hard drives. RAID can be designed to provide increased data reliability or increased I/O performance, though one goal may compromise the other.

There are total 10 types of RAID levels:

  • RAID level 0
  • RAID level 1
  • RAID level 2
  • RAID level 3
  • RAID level 4
  • RAID level 5
  • RAID level 6
  • RAID level 10
  • RAID level 50
  • RAID level 0+1

What is a filesystem?

A file system is a method of storing and organizing files and where they are placed for storage and retrieval.

Some of the most common file systems are NTFS (for Windows), ZFS (for Solaris), FS2 (AIX), EXT4 (for Linux), XFS (XFS for Linux), JFS (for HP-UX) and VxFS from Veritas (available on many different operating systems).

A file system includes metadata that the operating system uses to identify where to

write files. An additional benefit of a file system is that it can exploit a system-level cache to help speed up access to data. Performance is enhanced because the cache accesses frequently used data, thus bypassing the latency that is associated with direct access to the physical disk drive.