iSCSI

목요일 3월 26, 2009

iSCSI

In computing, iSCSI is Internet SCSI (Small Computer System Interface), an Internet Protocol (IP)-based storage networking standard for linking data storage facilities. By carrying SCSI commands over IP networks, iSCSI is used to facilitate data transfers over intranets and to manage storage over long distances. iSCSI can be used to transmit data over local area networks (LANs), wide area networks (WANs), or the Internet and can enable location-independent data storage and retrieval. (for "Internet SCSI") protocol allows clients (called initiators) to send SCSI commands (CDBs) to SCSI storage devices (targets) on remote servers. It is a popular Storage Area Network (SAN) protocol, allowing organizations to consolidate storage into data center storage arrays while providing hosts (such as database and web servers) with the illusion of locally-attached disks. Unlike traditional Fibre Channel, which requires special-purpose cabling, iSCSI can be run over long distances using existing network infrastructure.

1. Functionality

iSCSI uses TCP/IP (typically TCP ports 860 and 3260). In essence, iSCSI simply allows two hosts to negotiate and then exchange SCSI commands using IP networks. By doing this, iSCSI takes a popular high-performance local storage bus and emulates it over wide-area networks, creating a storage area network (SAN). Unlike some SAN protocols, iSCSI requires no dedicated cabling; it can be run over existing switching and IP infrastructure. As a result, iSCSI is often seen as a low-cost alternative to Fibre Channel, which requires dedicated infrastructure.

Although iSCSI can communicate with arbitrary types of SCSI devices, system administrators almost always use it to allow server computers (such as database servers) to access disk volumes on storage arrays. iSCSI SANs often have one of two objectives:

Storage consolidation

Organizations move disparate storage resources from servers around their network to central locations, often in data centers; this allows for more efficiency in the allocation of storage. In a SAN environment, a server can be allocated a new disk volume without any change to hardware or cabling.

Disaster recovery

Organizations mirror storage resources from one data center to a remote data center, which can serve as a hot standby in the event of a prolonged outage. In particular, iSCSI SANs allow entire disk arrays to be migrated across a WAN with minimal configuration changes, in effect making storage "routable" in the same manner as network traffic.

2. Storage Area Network (SAN)

In the context of computer storage, a SAN system allows a machine to use a network protocol to connect to remote storage resources such as disks and tape drives on an IP network for block level I/O. From the point of view of the class drivers and application software, the devices appear as locally attached devices.

One should distinguish a SAN device from a network-attached storage (NAS) device, where computers access resources through a file-based interface rather than through a low-level device interface. A NAS server arbitrates access from multiple clients, thus allowing the arbitrary addition of consumers for its resources. With iSCSI, the burden of synchronizing access to shared resources generally belongs to the initiator (network client) rather than with the target (network server). Sharing low-level device interfaces is a requirement of computer clusters which use specialized cluster software to manage the use of shared resources.

3. Concepts

3.1 Initiator

An initiator functions as an iSCSI client. An initiator typically serves the same purpose to a computer as a SCSI bus adapter would, except that instead of physically cabling SCSI devices (like hard drives and tape changers), an iSCSI initiator sends SCSI commands over an IP network. An initiator falls into two broad types:

Software initiator :

A software initiator uses code to implement iSCSI. Typically, this happens in a kernel-resident device driver that uses the existing NIC and network stack to emulate SCSI devices for a computer by speaking the iSCSI protocol. Software initiators are available for most mainstream operating systems, and this type is the most common mode of deploying iSCSI on computers.

Hardware initiator :

A hardware initiator uses dedicated hardware, typically in combination with software (firmware) running on that hardware, to implement iSCSI. A hardware initiator mitigates the overhead of iSCSI and TCP processing and Ethernet interrupts, and therefore may improve the performance of servers that use iSCSI.

3.1.1. Host Bus Adapter (HBA)

An iSCSI host bus adapter (more commonly, "HBA") implements a hardware initiator. A typical HBA is packaged as a combination of a Gigabit (or 10 Gigabit) Ethernet NIC, some kind of TCP/IP offload technology (TOE) and a SCSI bus adapter, which is how it appears to the operating system.

An iSCSI HBA can include PCI option ROM to allow booting from an iSCSI target.

3.1.2. TCP Offload Engine (TOE)

A TCP Offload Engine, or "TOE Card", offers an alternative to a full iSCSI HBA. A TOE "offloads" the TCP/IP operations for this particular network interface from the host processor, freeing up CPU cycles for the main host applications. When a TOE is used rather than an HBA, the host processor still has to perform the processing of the iSCSI protocol layer itself, but the CPU overhead for that task is low.

iSCSI HBAs or TOEs are used when the additional performance enhancement justifies the additional expense of using an HBA for iSCSI, rather than using a Software-based iSCSI Client (initiator).

3.2. Target

iSCSI refers to a storage resource located on an iSCSI server (more generally, one of potentially many instances of iSCSI running on that server) as a "target". An iSCSI target usually represents hard disk storage. As with initiators, software to provide an iSCSI target is available for most mainstream operating systems.

Common deployment scenarios for an iSCSI target include:

Storage array

In a data center or enterprise environment, an iSCSI target often resides in a large storage array, such as a NetApp filer or an EMC Corporation NS-series computer appliance. A storage array usually provides distinct iSCSI targets for numerous clients.^[1]

Software target

In a smaller or more specialized setting, mainstream server operating systems (like Linux, Solaris or Windows Server 2008) and some specific-purpose operating systems (like NexentaStor, StarWind iSCSI SAN, FreeNAS, iStorage Server, OpenFiler or FreeSiOS) can provide iSCSI target's functionality.

[edit] Logical Unit Number (LUN)

3.3. Logical Unit Number

In SCSI terminology, LUN stands for logical unit number. A LUN represents an individually addressable (logical) SCSI device that is part of a physical SCSI device (target). In an iSCSI environment, LUNs are essentially numbered disk drives. An initiator negotiates with a target to establish connectivity to a LUN; the result is an iSCSI connection that emulates a connection to a SCSI hard disk. Initiators treat iSCSI LUNs the same way as they would a raw SCSI or IDE hard drive; for instance, rather than mounting remote directories as would be done in NFS or CIFS environments, iSCSI systems format and directly manage filesystems on iSCSI LUNs.

In enterprise deployments, LUNs usually represent slices of large RAID disk arrays, often allocated one per client. iSCSI imposes no rules or restrictions on multiple computers sharing individual LUNs; it leaves shared access to a single underlying filesystem as a task for the operating system.

3.4. Addressing

Special names refer to both iSCSI initiators and targets. iSCSI provides three name-formats:

iSCSI Qualified Name (IQN)

Format: iqn.yyyy-mm.{reversed domain name} (e.g. iqn.2001-04.com.acme:storage.tape.sys1.xyz) (Note: there is an optional colon with arbitrary text afterwards. This text is there to help you better organize/label your resources.)

Extended Unique Identifier (EUI)

Format: eui.{EUI-64 bit address} (e.g. eui.02004567A425678D)

T11 Network Address Authority (NAA)

Format: naa.{NAA 64 or 128 bit identifier} (e.g. naa.52004567BA64678D)

IQN format addresses occur most commonly. They are qualified by a date (yyyy-mm) because domain names can expire or be acquired by another entity.

The IEEE Registration authority provides EUI in accordance with the EUI-64 standard. NAA is part OUI which is provided by the IEEE Registration Authority. NAA name formats were added to iSCSI in RFC 3980, to provide compatibility with naming conventions used in Fibre Channel and SAS storage technologies.

One can usually define an iSCSI participant by three or four fields:

Hostname or IP Address (e.g., "iscsi.example.com")

Port Number (e.g., 3260)

iSCSI Name (e.g., the IQN "iqn.2003-01.com.ibm:00.fcd0ab21.shark128")

An optional CHAP Secret (e.g., "secretsarefun")

3.5. iSNS

iSCSI initiators can locate appropriate storage resources using the Internet Storage Name Service (iSNS) protocol. In theory, iSNS provides iSCSI SANs with the same management model as dedicated Fibre Channel SANs. In practice, administrators can satisfy many deployment goals for iSCSI without using iSNS.

Character file & Block file

수요일 6월 25, 2008

By definition, device nodes correspond to resources that an operating-system kernel has already allocated. Unix identified those resources by a major number and a minor number, both stored as part of the structure of a node. The assignment of these numbers occurs uniquely in different operating systems and on different computer platforms. Generally, the major number identifies the device driver and the minor number identifies a particular device (possibly out of many) that the driver controls: in this case the system may pass the minor number to a driver as an argument.

As with other special file types, the computer system accesses device nodes using standard system calls and treats them like regular computer files. Two standard types of device files exist, differentiated by the type of hardware with which they interface and the way the operating system processes input and output operations.

Devices

Character devices

Character special files or character devices relate to devices through which the system transmits data one character at a time. These device nodes often serve for stream communication with devices such as teletype machines, virtual terminals, and serial modems, and usually do not support random access to data.

In most implementations, character devices use unbuffered input and output routines. The system reads each character from the device immediately or writes each character to the device immediately.

Block devices

Block special files or block devices correspond to devices through which the system moves data in the form of blocks. These device nodes often represent addressable devices such as hard disks, CD-ROM drives, or memory-regions.

Block devices often support random access and seeking, and generally use buffered input and output routines. The operating system allocates a data buffer to hold a single block each for input and output. When a program sends a request to read data from or to write data to the device, the system stores each character of that data in the appropriate buffer. When the buffer fills up, the appropriate operation takes place (data transfer) and the system clears the buffer.

Pseudo-devices

Device nodes on Unix-like systems do not necessarily have to correspond to physical devices. Nodes that lack this correspondence form the group of pseudo-devices. They provide various functions handled by the operating system. Some of the most commonly-used (character-based) pseudo-devices include:

/dev/null
    Accepts and discards all input; produces no output.
/dev/full
    Always full device.
/dev/loop
    loopback disk device
/dev/zero
    Produces a continuous stream of NUL (zero value) characters.
/dev/random
    Produces a variable-length stream of pseudo-random characters. (Blocking)
/dev/urandom
    Produces a variable-length stream of pseudo-random characters. (Non-Blocking)

- wikipedia.org