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iSCSI Concept
Concept HAReplicator User Manual


Replication is popular technical method that involves sharing information so as to ensure consistency between redundant resources to improve reliability, fault-tolerance, accessibility and as while as real-time data backup.
Replication implemented in this software is block level. By this way, any file system and file system structure supported by the operating system can be replicated without modification, which will ensure all data consistency between two or more nodes

Synchronous Replication
Guarantees "zero data loss" by the means of atomic write operation, i.e. write either completes on both sides or not at all. Write is not considered complete until acknowledgement by both local and remote storage. Most applications wait for a write transaction to complete before proceeding with further work, hence overall performance decreases considerably. Inherently, performance drops proportionally to distance, as latency is caused by speed of light.
Then, by this way, all data on the two or more nodes will be consistent in any time, but performance will drop a little.

Synchronous Replication
Write is considered complete as soon as local storage acknowledges it. Remote storage is updated, but probably with a small lag. Performance is greatly increased, but in case of losing a local storage, the remote storage is not guaranteed to have the current copy of data and most recent data may be lost.
Then, by this way, it may not consistency between redundant nodes in some time point but it won't result in performance lost.

Disk image

Its a single file or storage device containing the complete contents and structure representing a data storage medium or device, such as a hard drive, volume, tape drive, optical disc, or USB flash drive. A disk image is usually created by creating a complete sector-by-sector copy of the source medium and thereby perfectly replicating the structure and contents of a storage device.


In computing, iSCSI is an abbreviation of Internet 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. The 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.

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 network card (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.

Host Bus Adapter
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 engine (TOE) technology and a SCSI bus adapter, which is how it appears to the operating system.

TCP Offload Engine
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 specification refers to a storage resource located on an iSCSI server (more generally, one of potentially many instances of iSCSI storage nodes running on that server) as a target. An iSCSI target usually represents hard disk storage that works over the IP or Ethernet networks.

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 file systems 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 file system as a task for the operating system.

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 better organize or label 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 Serial Attached SCSI (SAS) storage technologies.

Usually an iSCSI participant can be defined 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")

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. 


iSCSI initiators and targets prove their identity to each other using the CHAP protocol, which includes a mechanism to prevent clear text passwords from appearing on the wire. By itself, the CHAP protocol is vulnerable to dictionary attacks, spoofing, or reflection attacks. If followed carefully, the rules for using CHAP within iSCSI prevent most of these attacks.

Additionally, as with all IP-based protocols, IPsec can operate at the network layer. The iSCSI negotiation protocol is designed to accommodate other authentication schemes, though interoperability issues limit their deployment.

To ensure that only valid initiators connect to storage arrays, administrators most commonly run iSCSI only over logically-isolated backchannel networks. In this deployment architecture, only the management ports of storage arrays are exposed to the general-purpose internal network, and the iSCSI protocol itself is run over dedicated network segments or virtual LANs (VLAN). This mitigates authentication concerns; unauthorized users aren't physically provisioned for iSCSI, and thus can't talk to storage arrays. However, it also creates a transitive trust problem, in that a single compromised host with an iSCSI disk can be used to attack storage resources for other hosts.

Because iSCSI aims to consolidate storage for many servers into a single storage array, iSCSI deployments require strategies to prevent unrelated initiators from accessing storage resources. As a pathological example, a single enterprise storage array could hold data for servers variously regulated by the Sarbanes-Oxley Act for corporate accounting, HIPAA for health benefits information, and PCI DSS for credit card processing. During an audit, storage systems must demonstrate controls to ensure that a server under one regime cannot access the storage assets of a server under another.

Typically, iSCSI storage arrays explicitly map initiators to specific target LUNs; an initiator authenticates not to the storage array, but to the specific storage asset it intends to use. However, because the target LUNs for SCSI commands are expressed both in the iSCSI negotiation protocol and in the underlying SCSI protocol, care must be taken to ensure that access control is provided consistently.



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