Data is the lifeblood of many organizations and fast, reliable
access to that data is critical in order to remain in business in today’s “Internet-time”
world. As such, most organizations make use of some level of RAID (Redundant
Array of Independent/Inexpensive Disks) in order to maintain data integrity. Now—show
of hands—how many of you are running your servers in a RAID 5 configuration
because it’s easy and seems like the best bet for your needs? While RAID 5 is a
good bet in many instances, if you’re looking for raw write performance, you
may benefit from some other RAID level. Now, how many of you, off the top of
your head, can explain what RAID levels 10 and 50 are for? These newer,
somewhat “invented” RAID levels can help to overcome some of RAID 5’s
drawbacks and still provide you with an ultra-redundant storage system. In this
article, I’ll go over the pros and cons of the various levels of typical RAID
levels. In my next article, I will discuss what are sometimes called “nested”
RAID levels (i.e., RAID 10).

RAID 0 (Disk striping)

  • Drives required (minimum): 2
  • Max capacity: Number of disks x disk capacity
  • Description: Data to be written to the disk is broken
    down into blocks with each block written to a separate disk.
  • Pros:
    Very, very fast since data is written to and read from storage over
    multiple “spindles”, meaning that the I/O load is distributed. The
    more disks that are added, the better the performance (in theory). As
    always, if you’re looking for huge performance gains, use a tool such as
    IOmeter to test your storage performance as the gains may not be that
    great.
  • Cons: When
    a single drive fails, the entire array can be compromised since this RAID
    level does not include any safeguards. As disks are added, the risk of
    failure increases.

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RAID 1 (Disk mirroring)

  • Drives required (minimum): 2 (or multiples of 2)
  • Max capacity: Total array capacity divided by 2
  • Description: All data that is written to the storage
    system is replicated to two physical disks, providing a high level of
    redundancy.
  • Pros: Very
    reliable, assuming only a single disk per pair fails. RAID 1 tends to
    provide good read performance (equal to or better than a single drive).
  • Cons:
    Because each drive is mirrored to another, requires 100% disk overhead to
    operate. Write performance can sometimes suffer due to the need to write
    the data to two drives, but is often still better than write performance
    for other RAID levels.

RAID 2: This RAID level is no longer used.

RAID 3 (Parallel transfer disks with parity)

  • Drives required (minimum): 3
  • Max capacity: (Number of disks minus 1) x capacity of
    each disk
  • Description: Data is broken down to the byte level
    and evenly striped across all of the data disks until complete. All parity
    information is written to a separate, dedicated disk.
  • Pros:
    Tolerates the loss of a single drive. Reasonable sequential write
    performance. Good sequential read performance.
  • Cons:
    Rarely used, so troubleshooting information could be sparse. Requires
    hardware RAID to be truly viable. RAID 3 is generally considered to be
    very efficient. Poor random write performance. Fair random read
    performance.

RAID 4 (Independent data disks with shared parity blocks)

  • Max capacity: (Number of disks minus 1) x capacity of
    each disk
  • Description: A file is broken down into blocks and
    each block is written across multiple disks, but not necessarily evenly. Like
    RAID 3, RAID 4 uses a separate physical disk to handle parity. Excellent
    choice for environments in which read rate is critical for heavy transaction
    volume.
  • Drives required (minimum): 3
  • Pros: Very
    good read rate. Tolerates the loss of a single drive.
  • Cons:
    Write performance is poor. Block read performance is okay.

RAID 5 (Independent access array without rotating parity)

  • Max capacity: (Number of disks – 1) x capacity of
    each disk
  • Description: Like RAID 4, blocks of data are written
    across the entire set of disks (sometimes unevenly), but in this case, the
    parity information is interspersed with the rest of the data.
  • Drives required (minimum): 3
  • Pros: Well
    supported. Tolerates the loss of a single drive.
  • Cons:
    Performance during a rebuild can be quite poor. Write performance is
    sometimes only fair due to the need to constantly update parity
    information.

RAID 6 (Independent Data disks with two independent distributed parity
schemes)

  • Max capacity: (Number of disks – 2) x capacity of
    each disk
  • Description: Like RAID 4, blocks of data are written
    across the entire set of disks (sometimes unevenly), but in this case, the
    parity information is interspersed with the rest of the data.
  • Drives required (minimum): 3
  • Pros:
    Tolerates the loss of up to two drives. Read performance is good. Excellent
    for absolutely critical applications.
  • Cons:
    Write performance is not very good. Write performance is worse than RAID 5
    due to the need to update multiple parity sets. Performance can heavily
    degrade during a rebuild.