Levels of nested RAID, also known as hybrid RAID, combine two or more of the standard
levels of RAID to gain performance, additional redundancy, or both.
RAID 0+1
RAID 0+1, is a RAID level used for both replicating and sharing data among disks. RAID 0+1 is
a mirror of stripes. The usable capacity of a RAID 0+1 array is the same as a RAID 1
array, where half of the total capacity is used to mirror the other half. , where is
the total number of drives and is the capacity of the smallest drive in the array.
RAID 1+0
RAID 1+0, sometimes called RAID 1&0 or RAID 10, is similar to a RAID 0+1 with exception that
the RAID levels used are reversed – RAID 10 is a stripe of mirrors.
RAID 10, as recognized by the storage industry association and as generally implemented by
RAID controllers, is a RAID 0 array of mirrors, which may be two-way or three way-mirrors,
and requires a minimum of four drives. However, a nonstandard definition of "RAID 10" was
created for the Linux MD driver; Linux "RAID 10" can be implemented with as few as two disks.
Implementations supporting two disks such as Linux RAID 10 offer a choice of layouts.
Examples
More typically, larger arrays of disks are combined for professional applications. In
high end configurations, enterprise storage experts expected PCIe and SAS storage to dominate
and eventually replace interfaces designed for spinning metal and for these interfaces
to become further integrated with Ethernet and network storage suggesting that rarely
accessed data stripes could often be located over networks and that very large arrays using
protocols like iSCSI would become more common. Pictured is an example where three collections
of 120 GB level 1 arrays are striped together to make 360 GB of total storage space.
Performance According to manufacturer specifications and
official independent benchmarks, in most cases RAID 10 provides better throughput and latency
than all other RAID levels except RAID 0. It is the preferable RAID level for I/O-intensive
applications such as database, email, and web servers, as well as for any other use
requiring high disk performance. Implementation
The Linux kernel's RAID 10 implementation performs mirroring and striping at the same
time, providing various layouts. Only some of the available layouts are functioning as
standard RAID 10. RAID 100
RAID 100, sometimes also called RAID 10+0, is a stripe of RAID 10s. This is logically
equivalent to a wider RAID 10 array, but is generally implemented using software RAID
0 over hardware RAID 10. Being "striped two ways", RAID 100 is described as a "plaid RAID".
RAID 50
RAID 50 combines the straight block-level striping of RAID 0 with the distributed parity
of RAID 5. This is a RAID 0 array striped across RAID 5 elements. It requires at least
six drives. On the left is an example where three collections
of 120 GB RAID 5s are striped together to make 720 GB of total storage space:
One drive from each of the RAID 5 sets could fail without loss of data. However, if the
failed drive is not replaced, the remaining drives in that set then become a single point
of failure for the entire array. If one of those drives fails, all data stored in the
entire array is lost. The time spent in recovery represents a period of vulnerability to the
RAID set. In the example below, data sets may be striped
across both RAID sets. A data set with five blocks would have three blocks written to
the first RAID set, and the next two blocks written to RAID set 2.
The configuration of the RAID sets will impact the overall fault tolerance. A construction
of three seven-drive RAID 5 sets has higher capacity and storage efficiency, but can only
tolerate three maximum potential drive failures. Because the reliability of the system depends
on quick replacement of the bad drive so the array can rebuild, it is common to construct
three six-drive RAID 5 sets each with a hot spare that can immediately start rebuilding
the array on failure. This does not address the issue that the array is put under maximum
strain reading every bit to rebuild the array precisely at the time when it is most vulnerable.
A construction of seven three-drive RAID 5 sets can handle up to seven drive failures,
if they are in different RAID 5 sets, but has lower capacity and storage efficiency.
RAID 50 improves upon the performance of RAID 5 particularly during writes, and provides better
fault tolerance than a single RAID level does. This level is recommended for applications
that require high fault tolerance, capacity and random positioning performance.
As the number of drives in a RAID set increases, and the capacity of the drives increase, this
impacts the fault-recovery time correspondingly as the interval for rebuilding the RAID set
increases. RAID 53
RAID 53 is typically used as a name for RAID 30 or 0+3.
RAID 60
RAID 60 combines the straight block-level striping of RAID 0 with the distributed double
parity of RAID 6. That is, a RAID 0 array striped across RAID 6 elements. It requires
at least eight disks. See also
Non-RAID drive architectures Non-standard RAID levels
Standard RAID levels References