SAN interview questions - PART 2

70. WHAT ARE THE TYPES OF PORTS IN FC?

Ø  N_port is a port on the node

Ø  F_port is a port on the switch that connects to a node point-to-point

Ø  E_port is the connection between two fibre channel switches.

Ø  NL_port is a port on the node used with an FC-AL topology. Also known as Node Loop port.

Ø  FL_port is a port on the switch that connects to a FC-AL loop (to NL_ports). Also known as fabric loop port.

 

71. WHAT IS AN ISL?

    E_port is the connection between two fibre channel switches. Also known as an Expansion port. When E_ports between two switches form a link, that link is referred to as an inter-switch link (ISL).

72. WHAT IS EXCHANGE IN FC?

    Exchange is to establish a relationship between 2 N_PORTs and then these two ports transfer data via one or more sequence within this relationship.
Example: Exchange exist to transfer the command, data and the status of one SCSI task

73. WHAT ARE THE FABRIC LOGIN PROCESSES?

    Fabric Login (FLOGI)
Port Login (PLOGI)
Process Login (PRLI)

74. EXPLAIN FABRIC LOGIN PROCESS IN DETAIL.

FLOGI: When you connect your server or storage, it will send a Fabric login (FLOGI) request. This FLOGI request serves two purposes:

·         Fibre Channel ID (the domain ID & the port ID)

·         Exchange buffer credits with the switch

To better understand the first function, it is exactly the same if I rewrite it as, “request an IP address for a MAC address from the DHCP server”.

PLOGI: once the Fabric login is done, then we need to login to the particular port.

Here the device gets registered in destination device’s name server.  The name server will expose this newly connected device & all the registered requests it receives this way can communicate with each other.

PRLI: As the two devices can now communicate with each other using the FC network SCSI, communication now takes place between the two devices. The SCSI has two main operations which are SCSI read or SCSI write.

75. WHAT ARE THE 3 TYPES OF CLUSTERS?

 a) High availability clusters
b) High Performance Clusters
c) Load Balancing Clusters.

76. WHAT ARE THE 3 LEVELS OF MANAGEMENT IN STORAGE?

 a) Storage Level Management
b) Network Level Management
c) Enterprise Level Management

77. WHAT ARE THE KEY ACTIVITIES IN SAN MANAGEMENT?

 a) Monitoring
b) Configuring
c) Controlling
d) Troubleshooting
e) Diagnosing

78. WHAT IS THE DIFFERENCE BETWEEN HBA & NIC?

    HBA=Host bus adapters are used in storage based traffic while

    NIC (Network Interface Cards) are used in IP based LAN traffic.

79. WHAT ?IS THE MEASURING UNIT OF DATA ACTIVITY

Gigabits per second (Gb/ps)

80. WHAT ARE THE BASIC STORAGE POLICIES?

 a) Security and authentication
b) Capacity, Content and quota management
c) Quality of Service

81. WHAT IS BY-PASS CIRCUITRY?

A circuit that automatically removes the storage device from the data path (FC device out of FC AL loop) when signaling is lost (this signal is called port by-pass signal).

82. HOW MANY CONNECTIONS ARE POSSIBLE IN FABRIC TOPOLOGY?

2^24 (24 bit address to the port), and the largest possible fabric will have 239 interconnected switches, out of 256.

87. APART FROM THE USES, IS THERE ANY DISADVANTAGE OF USING SWITCH ?

Latency

83. WHAT IS JITTER?

Jitter refers to any deviation in timing that a bit stream suffers as it traverses the physical medium and the circuitry on-board the end devices. Certain amount of deviation from the original signaling will occur naturally as serial bit stream propagates over fiber-optic or copper cabling.

(Mainly caused by electro-magnetic interference.)

84. WHAT IS BER?

BER is Bit Error Rate

Probability that a transmitted bit will be erroneously received is the measure of number of bits (erroneous) at the output of the receiver and dividing by the total number of bits in transmission.

85. WHAT IS WWPN?

WWPN is the 16bit character that is assigned to the port, SAN volume controller uses it to uniquely identify the fiber channel HBA that is installed in the host system.

86. WHAT IS CONNECTION ALLEGIANCE?

Given multiple connections are established, individual command/response pair must flow over the same connection. This connection allegiance ensures that specific read or writes commands are fulfilled without any additional overhead of monitoring multiple connections and to see whether a particular request is completed.

87. WHAT IS BURST LENGTH?

The burst length is the number of bytes that the SCSI initiator sends to the SCSI target in the FCP_DATA sequence.

88. WHAT ARE JUMBO FRAMES?

MTU = maximum transfer unit

1 MTU is 1500 Byes

Jumbo Frame = 6 X1500 = 9000 byes

89.WHAT IS iSCSI?

iSCSI is a protocol defined by the Internet Engineering Task Force (IETF) which enables SCSI commands to be encapsulated in TCP/IP traffic, thus allowing access to remote storage over low cost IP networks.

90. ADVANTAGES OF iSCSI OVER DAS & FC?

 · iSCSI is cost effective, allowing use of low cost Ethernet rather than expensive Fibre architecture.
· Traditionally expensive SCSI controllers and SCSI disks no longer need to be used in each server, reducing overall cost.
· Many iSCSI arrays enable the use of cheaper SATA disks without losing hardware RAID functionality.
· The iSCSI storage protocol is endorsed by Microsoft, IBM and Cisco, therefore it is an industry standard.
· Administrative/Maintenance costs are reduced.
· Increased utilisation of storage resources.
· Expansion of storage space without downtime.· Easy server upgrades without the need for data migration.
· Improved data backup/redundancy.

91. WHAT IS UAS?

92. WHAT ARE THE DIFFERENT FIELDS FOUND IN IQN NUMBER?

IQN Format:

·         The word “IQN”

·         date (yyyy-mm) that the naming authority took ownership of the domain

·         reversed domain name of the authority (org.alpinelinux, com.example, to.yp.cr)

·         Optional ":" prefixing a storage target name specified by the naming authority.

93.WHAT ARE THE FC-3 LEVEL SERVICES?

94. EXPLAIN INITIATORS?

 95.WHAT IS FLOW CONTROL? WHAT ARE THE DIFFERENT FLOW CONTROL MECHANISMS USED BY THE DIFFERENT TYPES OF FRAMES?

 

96. WHAT IS DAS & WHAT ARE IT'S ADVANTAGES?

97. WHAT IS THE USE OF LUN?

98. WHAT IS WWPN & WWNN? CAN SAME WWNN BE ASSIGNED TO DIFFERENT PORTS?

99. WHAT IS SCSI & iSCSI?

100. HOW CAN YOU COMPARE FC WITH SCSI?

101.EXPLAIN FCoE.

102.WHAT ARE THE DIFFERENT TOPOLOGIES & NUMBER OF DEVICES?

 

103.WHAT IS THE CONCEPT OF STORAGE VIRTUALIZATION?

104. WHAT ARE SAN PROTOCOLS?

FCIP: Entire Fibre Channel Frame Over IP

FCP: Fibre Channel Protocol

iFCP: Internet Fibre Channel Protocol

iSCSI: Internet Small Computer System Interface

iSNS: Internet Storage Name Service

NDMP: Network Data Management Protocol

SAS: Serial Attached SCSI

SCSI: Small Computer System Interface

For description on every protocols, click on the link,

http://www.javvin.com/protocolSAN.html

105. WHAT IS F/W?

Low-level software for booting and operating an intelligent device. Firmware generally resides in read-only memory (ROM) on the device.

106. WHAT IS AN HBA?

Host bus adapter. An I/O adapter that connects a host I/O bus to a computer’s memory system. Abbreviated HBA. Host bus adapter is the preferred term in SCSI contexts. Adapter and NIC are the preferred terms in Fiber Channel contexts. The term NIC is used in networking contexts such as Ethernet and token ring.

107. WHAT IS SNS?

Simple name server. A service provided by a Fiber Channel switch that simplifies discovery of devices attached to the fabric.

108. WHAT ARE SOME OF THE BROCADE NAME SERVER COMMANDS?

# nsshow – displays information of the name server

# nsallshow – displays the 24 bit address of all the devices in the fabric

# nscamshow – displays detailed information of all the name servers (devices) connected to all the switches in the fabric, including the remote devices.

PS: It is recommended to use the command # nscamshow for troubleshooting purpose.

109.  WHAT IS ACL? EXPLAIN.

If you are adding a new switch to the fabric, then Access Control Lists (ACLs) are used to allow or deny their addition. Host-to-fabric security technologies use ACLs at the port-level of the fabric to allow or deny HBA of a specific host from attaching to certain port. So an intruder host cannot just attach to any port on the fabric and access data without authority. ACLs are also used to filter network traffic, i.e. they can be used to allow or block routed packets from passing at the router interface. PKI can be used for authentication here. PKI and other encryption technologies like md5 can also be used on some of the switches for managing the entire fabric. All management and configuration changes are then passed to all the switches on the SAN from them.

This will also result into a SAN with a minimal number of security control points. Finally, configuration integrity is also very important. It ensures that configuration changes in the fabric only come from one location at a time, and are correctly propagated to all switches in the fabric with integrity. The use of a distributed lock manager is one way in which you can ensure that a serial and valid configuration change is enabled on the fabric.

110.       EXPLAIN ARBITRATED-LOOP.

A Fibre Channel interconnect topology in which each port is connected to the next, forming a loop. At any instant, only one port in a Fibre Channel Arbitrated Loop can transmit data.  Before transmitting data, a port in a Fibre Channel .

Arbitrated Loop must participate with all other ports in the loop in an arbitration to gain the right to transmit data. The arbitration logic is distributed among all of a loop's ports. The version of the Fibre Channel protocol used with the arbitrated loop physical topology. Arbitrated loop physical address.An 8-bit value used to identify a participating device in an Arbitrated Loop.

111.       WHAT IS BLOCK VIRTUALIZATION?

The act of applying virtualization (q.v.), to one or more block based (storage) services for the purpose of providing a new aggregated, higher level, richer, simpler, secure etc. block service to clients. cf. file virtualization. Block virtualization functions can be nested. A disk drive, RAID system or volume manager all perform some form of block address to (different) block address mapping or aggregation.

112.       WHAT IS THE PURPOSE OF USING CACHE?

To store data temporarily for expedited access. The location in which data is stored temporarily. There are a variety of cache types. Read cache holds data in anticipation that it will be requested. Write cache holds data written by a client until it can be stored on other (typically slower) storage media such as disk or tape. cf. buffer, disk cache, write back cache, write through cache.

113.       WHAT IS CASCADING?

The process of connecting two or more Fiber Channel hubs or switches together to increase the number of ports or extend distances.

114.       WHAT IS A CLUSTER?

A collection of computers that are interconnected (typically at high-speeds) for the purpose of improving reliability, availability, serviceability and/or performance (via load balancing). Often, clustered computers have access to a common pool of storage, and run special software to coordinate the component computers' activities.

115.       EXPLAIN THE FUNCTIONALITY OF STORAGE CONTROLLERS.

The control logic in a disk or tape that performs command decoding and execution, host data transfer, serialization and deserialization of data, error detection and correction, and overall management of device operations

The control logic in a storage subsystem that performs command transformation and routing, aggregation (RAID, mirroring, striping, or other), high-level error recovery, and performance optimization for multiple storage devices

A subclass of CIM_LogicalDevice. A CIM_Controller represents a device having a single protocol stack whose primary purpose is to communicate with, control, and reset connected devices. There are many subclasses of CIM_Controller, addressing SCSI, PCI, USB, serial, parallel, and video controllers.

116.       WHY IS DATA ENCRYPTION PREFERRED?

What if despite having all the security measures in place to prevent anybody from entering your SAN, somebody manages to get in? If all the data is sitting in plain text, then it’s all available to the hacker. In such a case, it becomes important to consider data encryption techniques. It may not be feasible to encrypt all the data sitting on the SAN, so you need to figure out which is the most sensitive data that needs to be encrypted. You might also need to encrypt certain data due to regulatory requirements.

While SAN vendors bolster their security, several companies are betting there's a market for storage encryption. Many vendors have also introduced security appliances to encrypt data between the application server and the RAID. But, these products are new and have little or no track record in the real world. So, better wait for reviews to come.

117.  WHAT IS DISK ARRAY?

A set of disks from one or more commonly accessible disk subsystems, combined with a body of control software. The control software presents the disks' storage capacity to hosts as one or more virtual disks. Control software is often called firmware or microcode when it runs in a disk controller. Control software that runs in a host computer is usually called a volume manager.

119.   EXPLAIN PORTS, IN GENERAL.

A port can be an entrance to or exit from a storage network.  It can be a connection point for a peripheral device or an application program.  It can be logical, physical or both.  Examples include Fibre Channel Port, Internet Protocol Suite Port and SCSI Port.

CONTEXT [Fiber Channel]

A Fibre Channel port provides physical interface attachment to other Fibre Channel ports.  A Fibre Channel port includes the transmitter, receiver and associated logic at either end of a link within a Node.  There may be multiple Ports per Node.  Each Port is assigned  a unique Port_ID, which is the Fibre Channel address used for routing.  Each port is identified by a unique World Wide Port Name (WW Port Name) Ports can be implemented on Host Bus Adapters (HBAs), Storage Adapters (SAs), routers, switches, bridges, gateways, etc. 

Fibre Channel ports may have many different logical operating modes, such as N_Port, NL_Port, F_Port, FL_Port, E_Port and B_Port.

120.   WHAT IS FABRIC ADDRESSING?

A unique 24 bit address used for frame routing and assigned to an N_Port or NL_Ported.   The Port_ID hierarchicy includes an 8-bit Domain ID (typically a switch number), an 8-bit Area ID (a port or group of switch ports) and an 8-bit Device ID (typically 00 for N_Ports or for NL_Ports, the Loop Initialization assigned Arbitrated Loop Physical Address (ALPA).  The Port_ID of the Source Port (S_ID) and the Port_ID of the Destination Port  (D_ID) is used in the Fibre Channel frame header for routing.

121.       WHAT ARE THE TYPES OF SWITCH PORTS? EXPLAIN EACH & EVERYONE'S FUNCTIONALITY.

G_Port :

A “Generic” Port can operate as either an E_Port or an F_Port.  A G_Port can determine operating mode at switch port initialization, F_Port when an N_Port attachment is determined, E_Port when an E_Port attachment is determined. cf. E_Port, F_Port, FL_Port, L_Port, N_Port, NL_Port.

E_Port:

The “Expansion” port within a Fiber Channel switch connects to another Fiber Channel switch or bridge device via an inter-switch link.  E_Ports are used to link Fiber Channel switches to form a multi-switch fabric. The “Expansion” port within a Fibre Channel switch connects to another Fiber Channel switch or bridge device via an inter-switch link.  E_Ports are used to link Fiber Channel switches to form a multi-switch fabric.

F_Port:

The “Fabric” port within a Fibre Channel fabric switch provides a point-to-point link attachment to a single N_Port.  F_Ports are intermediate ports in virtual point-to-point links between end ports, for example N_Port to F_Port to F_Port to N_Port using a single Fibre Channel fabric switch.

N_Port :

A “Node” port connects via a point-to-point link to either a single N_Port or a single F_Port.  . N_Ports handle creation, detection, and flow of message units to and from the connected systems.  N_Ports are end ports in virtual point-to-point links through a fabric, for example N_Port to F_Port to F_Port to N_Port using a single Fibre Channel fabric switch. cf. E_Port, F_Port, FL_Port, G_Port, L_Port, NL_Port

L_Port:

A “Loop” port is capable of performing arbitrated loop functions and protocols.  NL_Ports and FL_Ports are examples of loop-capable ports. cf. E_Port, F_Port, FL_Port, G_Port, N_Port, NL_Port

FL_Port :

A “Fabric Loop” port within a Fibre Channel fabric switch is capable of Fibre Channel Arbitrated Loop operations and is connected to one or more NL_Ports via a Fibre Channel Arbitrated Loop.  An FL_Port becomes a shared entry point for public NL_Port devices to a Fibre Channel fabric.  FL_Ports are intermediate ports in virtual point-to-point links between end ports that do not reside on the same loop, for example NL_Port to FL_Port to F_Port to N_Port through a single Fibre Channel fabric switch.

NL_Port:

A “Node Loop” port is capable of arbitrated loop functions and protocols.  An NL_Port connects via an arbitrated loop to other NL_Port and at most a single FL_Port.  . NL_Ports handle creation, detection, and flow of message units to and from the connected systems.  NL_Ports are end ports in virtual point-to-point links through a fabric, for example NL_Port to F_Port to F_Port to N_Port using a single Fibre Channel fabric switch.  In the absence of a fabric switch FL_Port, NL_Ports can communicate with other NL_Ports in virtual point-to-point links through a FC_AL open loop circuit often through FC_AL (Arbitrated Loop) hub or loop switch devices. cf. E_Port, F_Port, FL_Port, G_Port, N_Port

GL_Port:

A “Generic Loop” Port can operate as an E_Port, F_Port or FL_Port. A G_Port can determine operating mode at switch port initialization, FL_Port when an NL_Port attachment is determined, F_Port when an N_Port attachment is determined, E_Port when an E_Port attachment is determined.

122.       EXPLAIN FC-AL.

Fibre Channel Arbitrated Loop

A form of Fibre Channel network in which up to 126 nodes are connected in a loop topology, with each node's L_Port transmitter connecting to the L_Port receiver of the node to its logical right. Nodes connected to a Fibre Channel Arbitrated Loop arbitrate for the single transmission that can occur on the loop at any instant using a Fibre Channel Arbitrated Loop protocol that is different from Fibre Channel switched and point to point protocols. An arbitrated loop may be private (no fabric connection) or public (attached to a fabric by an FL_Port).

123.       WHAT IS AN INITIATOR?

The system component that originates an I/O command over an I/O bus or network. I/O adapters, network interface cards, and intelligent controller device I/O bus control ASICs are typical initiators. cf. LUN, originator, target, target ID

124.       WHAT IS PERSISTENT BINDING?

Persistent binding is a host-centric enforced way of directing an operating system to assign certain SCSI target IDs and LUNs. For example, where a specific host will always assign SCSI ID 3 to the first router it finds, and LUNs 0, 1, and 2 to the three-tape drives attached to the router.

Operating systems and upper-level applications (such as backup software) typically require a static or predictable SCSI target ID for their storage reliability and persistent binding affords that happening.

125.       WHAT ARE PUBLIC & PRIVATE LOOPS?

Private loop: A Fiber Channel arbitrated loop with no fabric attachment.

Public loop: A Fiber Channel arbitrated loop with an attachment to a fabric.

126.       WHAT ARE S_PORTS?

A logical port inside a switch addressable by external N_Ports for service functions. An S_Port may be an implicit switch port or a separate entity such as a name server connected to and controlled by the switch. S_Ports have well known port names to facilitate early discovery by N_Ports.

127.       EXPLAIN SCSI ADDRESSING.

The full address used by a computer to communicate with a SCSI device, including an adapter number (required with computers configured with multiple SCSI adapters), and the target ID of the device. SCSI addresses do not include logical unit number, because those are not used for communication.

128.       HOW IS SERVER-LESS BACKUP DONE?

A backup methodology that utilizes a device other than the server to copy data without using the LAN. The copy may be performed by a network-attached controller (e.g., utilizing SCSI Extended Copy), by an appliance within the SAN, or by a Backup Server.

129.       WHAT IS SHADOW COPY?

Shadow Copy (also called Volume Snapshot Service or VSS) is a feature introduced with Windows Server 2003, and available in all releases of Microsoft Windows thereafter, that allows taking manual or automatic backup copies or snapshots of a file or folder on a specific volume at a specific point in time. It is used by NTBackup and the Volume Shadow Copy service to backup files. Snapshots have two primary purposes. They allow the creation of consistent backups of a volume, ensuring that the contents cannot change while the backup is being made, avoiding problems with file locking; because the backup software is using a read-only copy of the volume, it is able to access every file without interfering with other programs writing to those same files. Microsoft's NTBackup utility (included in Windows 2000, Windows XP Professional and MCE) makes use of this mechanism. Also, users can access their files as they existed at the time of the snapshot, thus retrieving an earlier version of a file or recovering a file deleted by mistake.

10.   WHAT ARE VSANS?

Thanks to the developments taking place in this direction, we have now something called VSANs. A virtual SAN (VSAN) is a logical partition of a SAN. It allows the traffic to be isolated within specific sections of the network. So it becomes easier to isolate and rectify a problem with minimum disruption. The use of multiple VSANs is said to make a system easier to configure and also more scalable. You can add ports and switches at your will. You can also try different permutations and combinations of ports, because it is all logically done, giving you more flexibility. VSANs can also be configured separately and independently, making them more secure. They also offer the possibility of data redundancy, thereby reducing the risk of catastrophic data loss.

130.   WHAT IS A VOLUME?

Synonym for virtual disk. Used to denote virtual disks created by volume manager control software. Can function as a container for a file system.

A piece of removable media that has been prepared for use by a backup manager (e.g., by the writing of a media ID).

131.   WHAT ARE THE TYPES OF WRITE-POLICIES?

We have write-back & wrIte-through policies.

write back cache:

A caching technique in which the completion of a write request is signaled as soon as the data is in cache, and actual writing to non-volatile media occurs at a later time. Write-back cache includes an inherent risk that an application will take some action predicated on the write completion signal, and a system failure before the data is written to non-volatile media will cause media contents to be inconsistent with that subsequent action. For this reason, good write-back cache implementations include mechanisms to preserve cache contents across system failures (including power failures) and to flush the cache at system restart time. cf. write through cache.

Write through cache:

A caching technique in which the completion of a write request is not signaled until data is safely stored on non-volatile media. Write performance with a write-through cache is approximately that of a non-cached system, but if the data written is also held in cache, subsequent read performance may be dramatically improved. cf. write back cache.

132.   EXPLAIN 'ZONING' IN DETAIL.

zoning is a method of creating barriers in the SAN fabric to prevent any-to-any connectivity. In zoning, you have to create different groups of servers and storage devices that are connected to the SAN fabric. Only devices within a particular zone can talk to each other through managed port-to-port connections. So if a server wants to access data from a storage device located in a different zone, the latter must be configured for multi-zone access.

SANs provide port-to-port pathways from servers to storage devices and back through bridges, switches and hubs. Zoning lets you efficiently manage, partition and control these pathways. Additionally, with zoning, heterogeneous devices can be grouped by operating systems, and further demarcation done based on applications, functions, or departments. 

Zoning is of two types>

Soft zoning

Soft zoning, which as the name suggests, uses software to enforce zoning. It uses a name server database connected to the FC switch. This stores port numbers and WWN (World Wide Names) to identify devices during a zoning process. If a device is put in a different zone, it gets a record of Registered State Change Notification (RSCN) in the database. Each device must correctly address the RSCN after a zone-change else all its communications with storage devices in the previous zone will be blocked.

Hard zoning

You can also have hard zoning, which only uses WWNs to tag each device. Here, the SAN switches have to regulate data transfers between verified zones. Due to this, hard zoning requires that each device pass through the switches’ routing tables. For example, if two ports are not authorized to communicate with each other, their route tables are disabled and hence, the communication between those ports gets blocked.

PS: While zoning is a good way to control access between various devices on a SAN, it cannot mask individual tape or disk LUNs that sit behind a device port. This can be done through LUN masking.

 
133. HOW DIFFERENT IS STANDBY POWER SUPPLY FROM NORMAL PS?
With normal power supply, when there is power issue, read & write policies get disabled in cache and write cache goes to write-through mode. This in turn creates performance issues.
SPS keeps the cache enabled in write-back mode even during power brown-out.
(SPS has to be used along with sense cable.)