Storage division according to data access
The basic division of data storage is based on how they access stored data. They can retrieve files (address files) using file-based protocols such as NFS or SMB/CIFS. Or they can access the medium directly and read disk blocks, which are converted into files, which is common for locally attached disks or SAN networks.
Storage connection architectures
DAS - Directly Attached Storage
Disks (devices) directly connected to a computer. These are disks that are connected directly to an internal or external disk controller. So either disks in the computer, or a simple disk array that is connected to the computer via a Point to Point connection. In practice, SCSI, SAS or SATA interfaces are used.
NAS - Network Attached Storage
Network-attached data storage, based on the idea of file servers. It is represented by a server or special hardware to which a disk array is connected (often DAS). Data is connected to various computers using sharing. So if we create a shared directory on one PC and connect it to another computer, we are using NAS in its simple form. In addition, special storage devices have been created (consisting of a NAS head, which creates an interface for the user, and a disk array) that operate on the NAS principle and bring a number of advantages for larger environments.
When sharing data to the network, several (file-based) protocols are used. The original was Server Message Block (SMB) developed by IBM and used in DOS and early Windows. Microsoft modified this protocol some time ago and created Common Internet File System (CIFS), which is used today. On the other hand, alongside SMB, Sun created the Network File System (NFS) protocol, which is now used in Linux and other OS environments. Other possible protocols are FTP, HTTP, etc.
SAN - Storage Area Network
The most powerful solution for remote storage connection. It uses separate dedicated networks for storage communication. Protocols that work directly with disk blocks are used. The basis is the SCSI protocol (we're not talking about the SCSI interface), which is encapsulated for transmission over Fibre Channel (FC) using Fibre Channel Protocol (FCP) or for transmission over TCP/IP using Internet Small Computer System Protocol (iSCSI). Or some other protocols that are not currently very widespread such as Fibre Channel over Ethernet (FCoE), SCSI over Ethernet or others. From the above, it follows that a disk (device) connected via SAN appears in the system as if it were directly connected.
SAN disk array consists of a controller and disk shelves. Certain disk spaces are created on the controller - virtual disks (identified by Logical Unit Number - LUN), which are assigned to certain servers. If we want to share the same LUN among multiple servers, it must be treated with some special method, otherwise collisions (data corruption) occur.
SAN over Fibre Channel (FC)
Originally the only type of SAN network. It uses Fibre Optics (currently at 4Gbps speed) and FCP protocol. Special FC switches are needed (if we don't have just a Point-to-Point connection) and servers must have a Fibre Channel Host Bus Adapter (HBA), which replaces the SCSI controller. Each HBA has its unique address World Wide Name - WWN (similar to MAC address for a network card), HBA can have one or more ports, each port has its port WWN (WWNP) address and the whole HBA has a node WWN (WWNN) address. HBA ensures communication over the network and encapsulates SCSI into FCP (and vice versa).
SAN over TCP/IP - iSCSI
SAN over FC is quite expensive, so when gigabit ethernet started to be used (at that time FC with 2Gbps speed was used), a standardized protocol Internet Small Computer System Protocol (iSCSI) appeared. It encapsulates SCSI into TCP/IP protocol, so it can work on a regular ethernet network (the minimum speed of 1 Gbps is a condition). Theoretically, it can work on an existing network (switches and network cards), but in practice, it is definitely recommended to build a separate network for SAN (for data traffic, it's not a problem to utilize a 1Gbps network).
Although the TCP protocol is not as good for data transfer as the FCP protocol, which was specially developed for this purpose, it is sufficient. Nowadays, SAN networks based on iSCSI are widely spread. However, there is a question of the network card. iSCSI can be operated over a classic network card, but then all processing (encapsulation of SCSI into iSCSI) is done by the computer's processor. The second option is to purchase a special iSCSI HBA, which performs these tasks and passes normal SCSI to the computer.
Disk interfaces
PATA - Parallel Advance Technology Attachment
Standard interface for connecting storage devices (such as hard drives and CD-ROM drives) inside a computer. It uses parallel communication. Previously, it was referred to only as ATA. There are (almost) synonyms such as IDE (Integrated Drive Electronics), EIDE (Enhanced IDE) and ATAPI (AT Attachment Packet Interface).
SATA - Serial ATA
Bus technology for computers, primarily designed for data transfer to and from hard drives and optical drives. A more modern serial version of classic ATA. SATA 2 theoretically 3Gb/s = 375 MBps.
SCSI - Small Computer Systems Interface
This is a group of standards for physical connection and data transfer between a computer and peripheral devices. It uses a parallel bus port. Ultra640 SCSI theoretically 640 MBps
SAS - Serial Attached SCSI
SAS was created, like SATA, as a modern extension of bus technology for connecting hard drives and optical drives. It is a serial bus based on SCSI, which is backward compatible with SATA drives. Theoretically 3Gbps. Internally, drives are either SATA 2 or SCSI.
Fiber Channel - FC
This is a network technology primarily designed for data networks (storage networking), it has become the standard for SAN storage. Traditionally, optical cables are used, which can reach speeds of up to 20 Gbps. It is used for connections between servers and SAN controllers, but also as an interface on disks inside SAN disk shelves.
FATA
A cheaper variant where a regular SATA disk is equipped with an FC connector, so it can be connected to enterprise disk shelves.
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