Storage area Network Question Bank with Solutions(18CS822)

Module 3: IP SAN and FCoE

1. What is iSCSI? Explain the diagrammatic representation of iSCSI
implementation?

Sol: ISCSI is a transport layer protocol that describes how Small Computer
System Interface (SCSI) packets should be transported over
a TCP/IP network.

ISCSI, which stands for Internet Small Computer System Interface, works on
top of the Transport Control Protocol (TCP) and allows the SCSI command to
be sent end-to-end over local-area networks (LANs), wide-area networks
(WANs) or the internet.

iSCSI is an IP based protocol that establishes and manages connections

between host and storage over IP.

Figure 1 represents the implementation of iSCSI. The iSCSI encapsulates

SCSI commands and data into an IP packet and transports them using
TCP/IP. iSCSI is widely adopted for connecting servers to storage because it is
relatively inexpensive and easy to implement, especially in environments in
which an FC SAN does not exist.

Figure 1: iSCSI Implementation

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2. Explain the various components of iSCSI and host connectivity?

Sol: ISCSI is a transport layer protocol that describes how Small Computer
System Interface (SCSI) packets should be transported over
a TCP/IP network.

An initiator (host), target (storage or iSCSI gateway), and an IP-based network

are the key iSCSI components.

If an iSCSI-capable storage array is deployed, then a host with the iSCSI

initiator can directly communicate with the storage array over an IP network.
However, in an implementation that uses an existing FC array for iSCSI
communication, an iSCSI gateway is used. These devices perform the
translation of IP packets to FC frames and vice versa, thereby bridging the
connectivity between the IP and FC environments .

iSCSI Host Connectivity:

A standard NIC with a software iSCSI initiator is the simplest and least
expensive connectivity option. It is easy to implement because most servers
come with at least one, and in many cases two, embedded NICs. It requires
only a software initiator for iSCSI functionality. Because NICs provide

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standard IP function, encapsulation of SCSI into IP packets and

decapsulation are carried out by the host CPU. This places additional
overhead on the host CPU. If a standard NIC is used in heavy I/O load
situations, the host CPU might become a bottleneck.

TOE NIC helps alleviate this burden. A TOE NIC offloads TCP management
functions from the host and leaves only the iSCSI functionality to the host
processor.

The host passes the iSCSI information to the TOE card, and the TOE card
sends the information to the destination using TCP/IP. Although this solution
improves performance, the iSCSI functionality is still handled by a software
initiator that requires host CPU cycles.

An iSCSI HBA is capable of providing performance benefits because it offl

oads the entire iSCSI and TCP/IP processing from the host processor. The use
of an iSCSI HBA is also the simplest way to boot hosts from a SAN
environment via iSCSI.

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3. Discuss different iSCSI topologies with neat diagram?

Sol: Two topologies of iSCSI implementations are native and bridged

1. Native iSCSI Connectivity:

Native topology does not have FC components. The initiators may be either
directly attached to targets or connected through the IP network. For example,
the initiators can exist in an IP environment while the storage remains in an
FC environment.

FC components are not required for iSCSI connectivity if an iSCSI-enabled

array is deployed. In Figure 6-2 (a), the array has one or more iSCSI ports
configured with an IP address and is connected to a standard Ethernet switch.

After an initiator is logged on to the network, it can access the available LUNs
on the storage array. A single array port can service multiple hosts or initiators
as long as the array port can handle the amount of storage traffic that the
hosts generate.

2. Bridged iSCSI Connectivity:

Bridged topology enables the coexistence of FC with IP by providing iSCSI-to-

FC bridging functionality. A bridged iSCSI implementation includes FC
components in its configuration.

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The Figure (b) illustrates iSCSI host connectivity to an FC storage array. In this
case, the array does not have any iSCSI ports. Therefore, an external device,
called a gateway or a multiprotocol router, must be used to facilitate the
communication between the iSCSI host and FC storage.

The gateway converts IP packets to FC frames and vice versa. The bridge
devices contain both FC and Ethernet ports to facilitate the communication
between the FC and IP environments.

3. Combining FC and Native iSCSI Connectivity

The most common topology is a combination of FC and native iSCSI.

Typically, a storage array comes with both FC and iSCSI ports that enable
iSCSI and FC connectivity in the same environment,

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4. With a neat diagram explain iSCSI Protocol stack?

Sol: Figure below displays a model of the iSCSI protocol layers and depicts the
encapsulation order of the SCSI commands for their delivery through a
physical carrier.

SCSI is the command protocol that works at the application layer of the Open
System Interconnection (OSI) model. The initiators and targets use SCSI
commands and responses to talk to each other.

The SCSI command descriptor blocks, data, and status messages are
encapsulated into TCP/IP and transmitted across the network between the
initiators and targets.

iSCSI is the session-layer protocol that initiates a reliable session between

devices that recognize SCSI commands and TCP/IP. The iSCSI session-layer
interface is responsible for handling login, authentication, target discovery, and
session management. TCP is used with iSCSI at the transport layer to provide
reliable transmission.

TCP controls message flow, windowing, error recovery, and retransmission. It

relies upon the network layer of the OSI model to provide global addressing and
connectivity. The Layer 2 protocols at the data link layer of this model enable
node-to-node communication through a physical network.

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5. Define FCIP? With a neat diagram explain FCIP protocol stack?

Sol: FC SAN provides a high-performance infrastructure for localized data

movement. Organizations are now looking for ways to transport data over a
long distance between their disparate SANs at multiple geographic locations.

FCIP is a tunneling protocol that enables distributed FC SAN islands to be

interconnected over the existing IP-based networks. The FCIP standard has
rapidly gained acceptance as a manageable, cost effective way to blend the
better of the two worlds FC SAN and the proven, widely deployed IP
infrastructure.

The FCIP protocol stack is shown in Figure below Applications generate SCSI
commands and data, which are processed by various layers of the protocol
stack.

Figure: FCIP Protocol Stack

The upper layer protocol SCSI includes the SCSI driver program that executes
the read-and-write commands. Below the SCSI layer is the Fibre Channel
Protocol (FCP) layer, which is simply a Fibre Channel frame whose payload is
SCSI. The FCP layer rides on top of the Fibre Channel transport layer. This
enables the FC frames to run natively within a SAN fabric environment. In

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addition, the FC frames can be encapsulated into the IP packet and sent to a
remote SAN over the IP.

The FCIP layer encapsulates the Fibre Channel frames onto the IP payload
and passes them to the TCP layer as shown in the figure below. TCP and IP
are used for transporting the encapsulated information across Ethernet,
wireless, or other media that support the TCP/IP traffic.

Figure: FCIP Encapsulation

6. With a neat diagram explain FCIP topologies?

Sol: In an FCIP environment, an FCIP gateway is connected to each fabric via

a standard FC connection as shown in figure below.

 The FCIP gateway at one end of the IP network encapsulates the FC

frames into IP packets.
 The gateway at the other end removes the IP wrapper and sends the FC
data to the layer 2 fabric.
 The fabric treats these gateways as layer 2 fabric switches. An IP
address is assigned to the port on the gateway, which is connected to
an IP network.
 After the IP connectivity is established, the nodes in the two
independent fabrics can communicate with each other

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Figure: FCIP Topology

7. Explain the difference between general purpose servers and NAS?

Explain the various benefits of NAS?

Sol: A NAS device is optimized for file-serving functions such as storing, retrieving,
and accessing files for applications and clients. As shown in Figure below, a general-
purpose server can be used to host any application because it runs a general-
purpose operating system.

Unlike a general-purpose server, a NAS device is dedicated to file-serving. It has

specialized operating system dedicated to file serving by using industry-standard
protocols. Some NAS vendors support features, such as native clustering for high
availability.

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Figure: General Purpose servers V/S NAS Devices

Benefits of NAS:

 Comprehensive access to information: Enables efficient file sharing

and supports many-to-one and one-to-many configurations.
o The many-to-one configuration enables a NAS device to serve
many clients simultaneously.
o The one-to-many configuration enables one client to connect with
many NAS devices simultaneously.
 Improved efficiency: NAS delivers better performance compared to a
general-purpose fi le server because NAS uses an operating system
specialized for file serving.
 Improved flexibility: Compatible with clients on both UNIX and
Windows platforms using industry-standard protocols. NAS is flexible
and can serve requests from different types of clients from the same
source.
 Centralized storage: Centralizes data storage to minimize data
duplication on client workstations, and ensure greater data protection n
 Simplified management: Provides a centralized console that makes it
possible to manage fi le systems efficiently

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 Scalability: Scales well with different utilization profiles and types of

business applications because of the high-performance and low-latency
design.
 High availability: Offers efficient replication and recovery options,
enabling high data availability. NAS uses redundant components that
provide maximum connectivity options. A NAS device supports
clustering technology for failover.
 Security: Ensures security, user authentication, and file locking with
industry-standard security schemas
 Low cost: NAS uses commonly available and inexpensive Ethernet
components.
 Ease of deployment: Configuration at the client is minimal, because
the clients have required NAS connection software built in.
8. What is NAS? Explain NAS implementation in detail?

Sol: There are three different types of NAS implementation they are

1. Unified NAS & its Connectivity: Unified NAS performs file serving and
storing of file data, along with providing access to block-level data. It
supports both CIFS and NFS protocols for file access and iSCSI and FC
protocols for block level access. Due to consolidation of NAS-based and
SAN-based access on a single storage platform, unified NAS reduces an
organization’s infrastructure and management costs.
A unified NAS contains one or more NAS heads and storage in a single
system. NAS heads are connected to the storage controllers (SCs), which
provide access to the storage. These storage controllers also provide
connectivity to iSCSI and FC hosts. The storage may consist of different
drive types, such as SAS, ATA, FC, and flash drives, to meet different
workload requirements.
In Unified NAS Connectivity, Each NAS head in a unified NAS has front-
end Ethernet ports, which connect to the IP network. The front-end ports

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provide connectivity to the clients and service the file I/O requests. Each
NAS head has back-end ports, to provide connectivity to the storage
controllers.

Figure: Unified NAS Connectivity

2. Gateway NAS and its Connectivity: A gateway NAS device consists of one
or more NAS heads and uses external and independently managed storage.
Similar to unified NAS, the storage is shared with other applications that
use block-level I/O. Management functions in this type of solution are
more complex than those in a unified NAS environment because there are
separate administrative tasks for the NAS head and the storage.
A gateway solution can use the FC infrastructure, such as switches and
directors for accessing SAN-attached storage arrays or direct  attached
storage arrays.
The gateway NAS is more scalable compared to unified NAS because NAS
heads and storage arrays can be independently scaled up when required.
In a gateway solution, the front-end connectivity is similar to that in a
unified storage solution. Communication between the NAS gateway and the
storage system in a gateway solution is achieved through a traditional FC
SAN. To deploy a gateway NAS solution, factors, such as multiple paths for
data, redundant fabrics, and load distribution, must be considered.

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Figure: Gateway NAS Connectivity

3. Scale out NAS: Scale-out NAS enables grouping multiple nodes together to
construct a clustered NAS system. A scale-out NAS provides the capability
to scale its resources by simply adding nodes to a clustered NAS
architecture. The cluster works as a single NAS device and is managed
centrally. Nodes can be added to the cluster, when more performance or
more capacity is needed, without causing any downtime. Scale-out NAS
provides the flexibility to use many nodes of moderate performance and
availability characteristics to produce a total system that has better
aggregate performance and availability. It also provides ease of use, low
cost, and theoretically unlimited scalability.
Scale-out NAS clusters use separate internal and external networks for
back-end and front-end connectivity, respectively. An internal network
provides connections for intra-cluster communication, and an external
network connection enables clients to access and share file data. Each
node in the cluster connects to the internal network. The internal network
offers high throughput and low latency and uses high-speed networking
technology, such as InfiniBand or Gigabit Ethernet. To enable clients to
access a node, the node must be connected to the external Ethernet
network. Redundant internal or external networks may be used for high
availability.

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Scale-out NAS is suitable to solve the “Big Data” challenges that

enterprises and customers face today. It provides the capability to manage
and store large, high-growth data in a single place with the flexibility to
meet a broad range of performance requirements.

Figure: Scale-Out NAS Connectivity

9. Explain with neat diagram NAS I/O operations and its components?

Sol: NAS provides file-level data access to its clients. File I/O is a high-level
request that specifies the file to be accessed. The process of handling I/O’s in
a NAS environment is as follows:

1. The requestor (client) packages an I/O request into TCP/IP and

forwards it through the network stack. The NAS device receives this
request from the network.
2. The NAS device converts the I/O request into an appropriate physical
storage request, which is a block-level I/O, and then performs the
operation on the physical storage.
3. When the NAS device receives data from the storage, it processes and
repackages the data into an appropriate file protocol response.
4. The NAS device packages this response into TCP/IP again and forwards
it to the client through the network

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Figure: NAS I/O Operations

The NAS head includes the following components:

 CPU and memory

 One or more network interface cards (NICs), which provide connectivity
to the client network. Examples of network protocols supported by NIC
include Gigabit Ethernet, Fast Ethernet, ATM, and Fiber Distributed
Data Interface (FDDI).
 An optimized operating system for managing the NAS functionality. It
translates file-level requests into block-storage requests and further
converts the data supplied at the block level to file data.
 NFS, CIFS, and other protocols for file sharing n Industry-standard
storage protocols and ports to connect and manage physical disk
resources

Figure: NAS Components

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10. Explain the various factors affecting NAS performance?

Sol: NAS uses IP network; therefore, bandwidth and latency issues associated
with IP affect NAS performance. The Various factors that affect NAS
performance at different levels follow:

1. Number of hops: A large number of hops can increase latency because IP

processing is required at each hop, adding to the delay caused at the
router.
2. Authentication with a directory service such as Active Directory or
NIS: The authentication service must be available on the network with
enough resources to accommodate the authentication load. Otherwise, a
large number of authentication requests can increase latency.
3. Retransmission: Link errors and buffer overflows can result in
retransmission. This causes packets that have not reached the specified
destination to be re-sent. Care must be taken to match both speed and
duplex settings on the network devices and the NAS heads. Improper
configuration might result in errors and retransmission, adding to latency.
4. Over utilized routers and switches: The amount of time that an
over  utilized device in a network takes to respond is always more than the
response time of an optimally utilized or underutilized device. Network
administrators can view utilization statistics to determine the optimum
utilization of switches and routers in a network. Additional devices should
be added if the current devices are over utilized
5. File system lookup and metadata requests: NAS clients access fi les on
NAS devices. The processing required to reach the appropriate fi le or
directory can cause delays. Sometimes a delay is caused by deep directory
structures and can be resolved by flattening the directory structure. Poor
file system layout and an over utilized disk system can also degrade
performance.
6. Over utilized NAS devices: Clients accessing multiple files can cause high
utilization levels on a NAS device, which can be determined by viewing
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utilization statistics. High memory, CPU, or disk subsystem utilization

levels can be caused by a poor file system structure or insufficient
resources in a storage subsystem.
7. Over utilized clients: The client accessing CIFS or NFS data might also be
over utilized. An overutilized client requires a longer time to process the
requests and responses. Specific performance-monitoring tools are
available for various operating systems to help determine the utilization of
client resources

Figure: Causes of Latency

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