In my previous article, I talked about the “single
digit” RAID levels. That is, RAID 0, RAID 1, RAID 3, RAID 4 and RAID
5. While these RAID levels can provide a great level of redundancy, and it
might not seem like there’s a need for other levels, a number of additional
levels have been created to address other redundancy concerns. These multilevel
RAID sets are fairly standard and widely used. In my next article, I will talk
about some other, less common RAID levels that you may run across.

RAID 0+1 (Mirror of Stripes, RAID 01, or RAID 0 then RAID 1)

  • Drives required (minimum): 4 (requires an even number
    of disks)
  • Max capacity: Number of disks x Disk capacity / 2
  • Description: RAID 0+1 is a mirror (RAID 1) of a
    stripe set (RAID 0). For example, suppose you have six hard disks. To
    create a RAID 0+1 scenario, you would take three of the disks and create a
    RAID 0 stripe set with a total capacity of three times the size of each
    disk (number of disks x capacity of disks). Now, to the other three disks,
    you would mirror the contents of this stripe set.
  • Pros: A
    RAID 0+1 set could theoretically withstand the loss of all of the drives
    in one of the RAID 0 arrays and remain functional since all of the data
    would be mirrored to the second RAID 1 array. In most cases, the failure
    of two drives will compromise the array since many RAID controllers will
    take one of the RAID 0 mirrors offline if one of the disks in the RAID set
    fails (after all, a RAID 0 array does not provide any kind of redundancy),
    thus, leaving just the other RAID 0 set active, which has no redundancy. In
    short, a total array failure requires the loss of a single drive from each
    RAID 0 set. Provides very good sequential and random read and write
    performance.
  • Cons: Requires
    50% of the total disk capacity to operate. Not as fault-tolerant as RAID
    10. Can withstand loss of only a single drive with most controllers. Scalability
    is limited and expensive.

Notes: Please take
note of the RAID 0 then RAID 1 notation above. This is exactly how the RAID set
is created and is an important differentiator with RAID 10.

Figure A

RAID 10 (Stripe of Mirrors, RAID 1+0, or RAID 1 then RAID 0)

  • Drives required (minimum): 4 (requires an even number
    of disks)
  • Max capacity: Number of disks x Disk capacity / 2
  • Description: RAID 10 is a stripe (RAID 0) of multiple
    mirror sets (RAID 1). Again, suppose you have six hard disks. To create a RAID
    10 array, take two of the disks and create a RAID 1 mirror set with a
    total capacity of one disk in the array. Repeat the same procedure twice
    for the other four disks. Finally, create a RAID 0 array that houses each
    of these mirror sets.
  • Pros: A
    RAID 10 set can withstand the loss of one disk in every RAID 1 array, but
    cannot withstand the loss of both disks in one RAID 1 array. As with RAID
    0+1, RAID 10 provides very good sequential and random read and write
    performance. These multilevel RAID arrays can often perform better than
    their single-digit counterparts due to the ability to read from and write
    to multiple disks at once.
  • Cons:
    Requires 50% of the total disk capacity to operate. Scalability is limited
    and expensive.

Notes: Again, take
note of the RAID 1 then RAID 0 notation.

Figure B

RAID 50 (Stripe of Parity Set, RAID 5+0, or RAID 5 then RAID 0)

  • Drives required (minimum): 6
  • Max capacity: (Drives in each RAID 5 set – 1) x
    Number of RAID 5 sets x Disk capacity
  • Description: RAID 50 is a stripe (RAID 0) of multiple
    parity sets (RAID 5). This time, suppose you have twelve hard disks. To
    create a RAID 50 array, take four of the disks and create a RAID 5 stripe
    with parity set with a total capacity of three times the size of each disk
    (remember, in RAID 5, you “lose” one disk’s worth of capacity). Repeat
    the same procedure twice for the other eight disks. Finally, create a RAID
    0 array that houses each of these RAID 5 sets.
  • Pros: A
    RAID 50 set can withstand the loss of one disk in every RAID 5 array, but
    cannot withstand the loss of multiple disks in one of the RAID 5 arrays. RAID
    50 provides good sequential and random read and write performance. These
    multilevel RAID arrays can often perform better than their single-digit
    counterparts due to the ability to read from and write to multiple disks
    at once.
  • Cons: RAID
    50 is somewhat complex and can be expensive to implement. A rebuild after
    a drive failure can seriously hamper overall array performance.

Figure C

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