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RAID, NAS and iSCSI on Wikipedia

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Nested RAID https://en.wikipedia.org/wiki/Redundant_array_of_independent_disks

RAID 10 (RAID 0 on top and the redundant array at the bottom) is preferable to RAID 0+1, because fewer disks then need to be regenerated when a disk fails.

A RAID 0+1 (also called RAID 01) is a RAID used for both replicating and sharing data among disks. The difference between RAID 0+1 and RAID 1+0 is the location of each RAID system — it is a mirror of stripes. Consider an example of RAID 0+1: six 120 GB drives need to be set up on a RAID 0+1. Below is an example where two 360 GB level 0 arrays are mirrored, creating 360 GB of total storage space:

                        RAID 1
            /--------------------------\
            |                          |
          RAID 0                     RAID 0
   /-----------------\        /-----------------\
   |        |        |        |        |        |
120 GB   120 GB   120 GB   120 GB   120 GB   120 GB
  A1       A2       A3       A1       A2       A3
  A4       A5       A6       A4       A5       A6
  B1       B2       B3       B1       B2       B3
  B4       B5       B6       B4       B5       B6

A RAID 10, sometimes called RAID 1+0, or RAID 1&0, is similar to a RAID 0+1 with exception that the RAID levels used are reversed—RAID 10 is a stripe of mirrors. Below is an example where three collections of 120 GB level 1 arrays are striped together to add up to 360 GB of total storage space:

                        RAID 0
       /-----------------------------------\
       |                 |                 |
     RAID 1            RAID 1            RAID 1
   /--------\        /--------\        /--------\
   |        |        |        |        |        |
120 GB   120 GB   120 GB   120 GB   120 GB   120 GB
  A1       A1       A2       A2       A3       A3
  A4       A4       A5       A5       A6       A6
  B1       B1       B2       B2       B3       B3
  B4       B4       B5       B5       B6       B6

NAS https://en.wikipedia.org/wiki/Network-attached_storage

A class of systems that provide file services to host computers. A host system that uses network attached storage uses a file system device driver to access data using file access protocols such as NFS or SMB. NAS systems interpret these commands and perform the internal file and device I/O operations necessary to execute them. A NAS Storage Element consists of an engine, which implements the file services, and one or more devices, on which data is stored. NAS elements may be attached to any type of network. When attached to SANs, NAS elements may be considered to be members of the Server Attached Storage (SAS) class of storage elements.

iSCSI https://en.wikipedia.org/wiki/ISCSI

Building iSCSI-based Storage Area Networks (SAN) has become a less costly but worthy alternative to creating Fibre Channel-based SANs. The iSCSI protocol uses TCP/IP for its data transfer. Unlike other network storage protocols, such as Fibre Channel (which is the foundation of most SANs), it requires only the simple and ubiquitous Ethernet interface (or any other TCP/IP-capable network) to operate.

iSCSI host bus adapters (HBAs) implement the iSCSI protocol on the adapter itself. They expose themselves as a SCSI HBA to the operating system. Some of the iSCSI HBAs have TOE NIC and some also offload iSCSI processing. Some adapters use NVRAM for configuring bootable iSCSI targets and abstract themselves as SCSI host bus adapters in the OS driver system.