Nested RAID levels
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A RAID 0+1 (also called RAID 01), 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.
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.
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.
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.
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 (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.
RAID 100 (RAID 10+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".
RAID 50 (RAID 5+0)
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 (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, 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 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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