Levels of nested RAID,[1] also known as hybrid RAID,[2] combine two or more of the standard levels of RAID (redundant array of independent disks) to gain performance, additional redundancy, or both.
RAID 0+1
A RAID 0+1 (also called RAID 01), is a RAID level used for both replicating and sharing data among disks.[3] 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. The minimum number of disks required to implement RAID 0+1 is three, where two independent volumes are striped across the three drives and mirror each other. But it is more common to use a minimum of four disks as the stripe pattern is simpler for an even number of drives.
, where is the total number of drives and is the capacity of the smallest drive in the array. The minimum number of disks required to implement RAID 0+1 is three, where two independent volumes are striped across the three drives and mirror each other. But it is more common to use a minimum of four disks as the stripe pattern is simpler for an even number of drives.RAID 1+0
A 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.[3]
Near versus far, advantages for bootable RAID[edit]
A nonstandard definition of "RAID 10" was created for the Linux MD driver;[4] 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) [5]and requires a minimum of 4 drives. Linux "RAID 10" can be implemented with as few as two disks. Implementations supporting two disks such as Linux RAID 10[4] offer a choice of layouts, including one in which copies of a block of data are "near" each other or at the same address on different devices or predictably offset: Each disk access is split into full-speed disk accesses to different drives, yielding read and write performance like RAID 0 but without necessarily guaranteeing that every stripe is on both drives. Another layout uses "a more RAID 0 like arrangement over the first half of all drives, and then a second copy in a similar layout over the second half of all drives - making sure that all copies of a block are on different drives." This has high read performance because only one of the two read locations must be found on each access, but writing requires more head seeking as two write locations must be found. Very predictable offsets minimize the seeking in either configuration. "Far" configurations may be exceptionally useful for Hybrid SSD with huge caches of 4 GB (compared to the more typical 64 MB of spinning platters in 2010) and by 2011 64 GB (as this level of storage exists now on one single chip). They may also be useful for those small pure SSD bootable RAIDs that are not reliably attached to network backup and so must maintain data for hours or days, but which are quite sensitive to the cost, power, and complexity of more than two disks. Write access for SSDs is extremely fast, so the multiple access becomes less of a problem with speed: At PCIe x4 SSD speeds, the theoretical maximum of 730 MB/s is already more than double the theoretical maximum of SATA-II at 300 MB/s.
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.[6] 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 (speed)
According to manufacturer specifications[7] and official independent benchmarks,[8][9] in most cases RAID 10 provides better throughput and latency than all other RAID levels except RAID 0 (which wins in throughput).
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.[10]
Implementation
The Linux kernel RAID 10 implementation (from version 2.6.9 and onwards) is not nested. The mirroring and striping is done in one process. Only certain layouts are standard RAID 10.[4] See also the Linux MD RAID 10 and RAID 1.5 sections in the Non-standard RAID article for details.
RAID 100 (RAID 1+0+0)
A 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".[11]
RAID 0+3 and 3+0
RAID 0+3
RAID level 0+3 or RAID level 03 is a dedicated parity array across striped disks. Each block of data at the RAID 3 level is broken up amongst RAID 0 arrays where the smaller pieces are striped across disks.
RAID 30
RAID level 30 is also known as striping of dedicated parity arrays. It is a combination of RAID level 3 and RAID level 0. RAID 30 provides high data transfer rates, combined with high data reliability. RAID 30 is best implemented on two RAID 3 disk arrays with data striped across both disk arrays. RAID 30 breaks up data into smaller blocks, and then stripes the blocks of data to each RAID 3 RAID set. RAID 3 breaks up data into smaller blocks, calculates parity by performing an Exclusive OR on the blocks, and then writes the blocks to all but one drive in the array. The parity bit created using the Exclusive OR is then written to the last drive in each RAID 3 array. The size of each block is determined by the stripe size parameter, which is set when the RAID is created.
One drive from each of the underlying RAID 3 sets can fail. Until the failed drives are replaced the other drives in the sets that suffered such a failure are a single point of failure for the entire RAID 30 array. In other words, if one of those drives fails, all data stored in the entire array is lost. The time spent in recovery (detecting and responding to a drive failure, and the rebuild process to the newly inserted drive) represents a period of vulnerability to the RAID set. RAID 30's strength over both RAID 03 and a large RAID 3 is that only the HDDs of one RAID 3 set (that is, at most half the HDDs) are affected during the rebuild process, which means both shorter rebuild times, and a less severe performance hit during the rebuild.
RAID 50 (RAID 5+0)
A RAID 50 combines the straight block-level striping of RAID 0 with the distributed parity of RAID 5.[3] This is a RAID 0 array striped across RAID 5 elements. It requires at least 6 drives.
On the left is an example where three collections of 240 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 (detecting and responding to a drive failure, and the rebuild process to the newly inserted drive) represents a period of vulnerability to the RAID set.
In the example below, datasets may be striped across both RAID sets. A dataset with 5 blocks would have 3 blocks written to the first RAID set, and the next 2 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 51
A RAID 51 or RAID 5+1 is an array that consists of two RAID 5's that are mirrors of each other. Generally this configuration is used so that each RAID 5 resides on a separate controller. In this configuration reads and writes are balanced across both RAID 5s. Some controllers support RAID 51 across multiple channels and cards with hinting to keep the different slices synchronized. However a RAID 51 can also be accomplished using a layered RAID technique. In this configuration, the two RAID 5's have no idea that they are mirrors of each other and the RAID 1 has no idea that its underlying disks are RAID 5's. However, if the configuration is layered, the RAID controllers cannot exploit the fact that each HDD is mirrored for a faster rebuild process, but has to go through the lengthier RAID 5 rebuild process. This configuration can sustain the failure of all disks in either of the arrays, plus up to one additional disk from the other array before suffering data loss. The maximum amount of space of a RAID 51 is (N - 1) where N is the number of drives making up each individual RAID 5 set.
RAID 05 (RAID 0+5)
A RAID 0 + 5 consists of several RAID 0's (a minimum of three) that are grouped into a single RAID 5 set. The total capacity is (N-1) where N is total number of RAID 0's that make up the RAID 5. This configuration is not generally used in production systems.
RAID 53
RAID 53 is typically used as a name for RAID 30 or 0+3.
RAID 60 (RAID 6+0)
A 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
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