SAN INTERVIEW QUESTION AND ANSWER

Can you describe SAN in your won word?

A storage area network (SAN) is a high-speed special-purpose network (or subnetwork) that interconnects different kinds of data storage devices with associated data servers on behalf of a larger network of users. Typically, a storage area network is part of the overall network of computing resources for an enterprise. A storage area network is usually clustered in close proximity to other computing resources such as IBM Power5 boxes but may also extend to remote locations for backup and archival storage, using wide area network carrier technologies such as ATM or SONET .

A storage area network can use existing communication technology such as IBM’s optical fiber ESCON or it may use the newer Fibre Channel technology. Some SAN system integrators liken it to the common storage bus (flow of data) in a personal computer that is shared by different kinds of storage devices such as a hard disk or a CD-ROM player.

SANs support disk mirroring, backup and restore, archival and retrieval of archived data, data migration from one storage device to another, and the sharing of data among different servers in a network. SANs can incorporate subnetworks with network-attached storage (NAS) systems.

So you mention NAS, but What is NAS?

Network-attached storage (NAS) is hard disk storage that is set up with its own network address rather than being attached to the department computer that is serving applications to a network’s workstation users. By removing storage access and its management from the department server, both application programming and files can be served faster because they are not competing for the same processor resources. The network-attached storage device is attached to a local area network (typically, an Ethernet network) and assigned an IP address. File requests are mapped by the main server to the NAS file server.

Network-attached storage consists of hard disk storage, including multi-disk RAID systems, and software for configuring and mapping file locations to the network-attached device. Network-attached storage can be a step toward and included as part of a more sophisticated storage system known as a storage area network (SAN).

NAS software can usually handle a number of network protocols, including Microsoft’s Internetwork Packet Exchange and NetBEUI, Novell’s Netware Internetwork Packet Exchange, and Sun Microsystems’ Network File System. Configuration, including the setting of user access priorities, is usually possible using a Web browser.

What is SMTP and how it works?

SMTP (Simple Mail Transfer Protocol) is a TCP/IP protocol used in sending and receiving e-mail. However, since it is limited in its ability to queue messages at the receiving end, it is usually used with one of two other protocols, POP3 or IMAP, that let the user save messages in a server mailbox and download them periodically from the server. In other words, users typically use a program that uses SMTP for sending e-mail and either POP3 or IMAP for receiving e-mail. On Unix-based systems, sendmail is the most widely-used SMTP server for e-mail. A commercial package, Sendmail, includes a POP3 server. Microsoft Exchange includes an SMTP server and can also be set up to include POP3 support.

SMTP usually is implemented to operate over Internet port 25.

Do you have any idea about NAT?

Short for Network Address Translation, an Internet standard that enables a local-area network (LAN) to use one set of IP addresses for internal traffic and a second set of addresses for external traffic. A NAT box located where the LAN meets the Internet makes all necessary IP address translations.

NAT serves three main purposes:

• Provides a type of firewall by hiding internal IP addresses
• Enables a company to use more internal IP addresses. Since they’re used internally only, there’s no possibility of conflict with IP addresses used by other companies and organizations.
• Allows a company to combine multiple ISDN connections into a single Internet connection.

Explain DHCP and its uses to an environment?

Short for Dynamic Host Configuration Protocol, a protocol for assigning dynamic IP addresses to devices on a network. With dynamic addressing, a device can have a different IP address every time it connects to the network. In some systems, the device’s IP address can even change while it is still connected. DHCP also supports a mix of static and dynamic IP addresses.

Dynamic addressing simplifies network administration because the software keeps track of IP addresses rather than requiring an administrator to manage the task. This means that a new computer can be added to a network without the hassle of manually assigning it a unique IP address. Many ISPs use dynamic IP addressing for dial-up users.

What does SNMP stands for?

Short for Simple Network Management Protocol, a set of protocols for managing complex networks. SNMP works by sending messages, called Protocol Data Units, to different parts of a network. SNMP-compliant devices, called Agents, store data about themselves in Management Information Bases and return this data to the SNMP requesters.

What do you know about TCPDump?

TCPdump is a common computer network debugging tool that runs under the command line. It allows the user to intercept and display TCP/IP and other packets being transmitted or received over a network to which the computer is attached. Tcpdump works on most Unix-like platforms: Linux, Solaris, BSD, Mac OS X, HP-UX and AIX among others. On Windows, WinDump can be used; it’s a port of tcpdump to Windows.

You must have a root or super user authority to use TCPdumps in UNIX like envrionment.

What is software RAID Levels do?Redundant Arrays of Independent Disks (RAID) is formally defined as a method to store data on any type of disk medium.

LDAP

The Light Directory Access Protocol (LDAP) defines a standard method for accessing and updating information in a directory (a database) either locally or remotely in a client-server model.

What are the benefits of fibre channel SANs?

Fibre Channel SANs are the de facto standard for storage networking in the corporate data center because they provide exceptional reliability, scalability, consolidation, and performance. Fibre Channel SANs provide significant advantages over direct-attached storage through improved storage utilization, higher data availability, reduced management costs, and highly scalable capacity and performance.

What environment is most suitable for fibre channel SANs?

Typically, Fibre Channel SANs are most suitable for large data centers running business-critical data, as well as applications that require high-bandwidth performance such as medical imaging, streaming media, and large databases. Fibre Channel SAN solutions can easily scale to meet the most demanding performance and availability requirements.

What customer problems do fibre channel SANs solve?

The increased performance of Fibre Channel enables a highly effective backup and recovery approach, including LAN-free and server-free backup models. The result is a faster, more scalable, and more reliable backup and recovery solution. By providing flexible connectivity options and resource sharing, Fibre Channel SANs also greatly reduce the number of physical devices and disparate systems that must be purchased and managed, which can dramatically lower capital expenditures. Heterogeneous SAN management provides a single point of control for all devices on the SAN, lowering costs and freeing personnel to do other tasks.

How long has fibre channel been around?

Development started in 1988, ANSI standard approval occurred in 1994, and large deployments began in 1998. Fibre Channel is a mature, safe, and widely deployed solution for high-speed (1 GB, 2 GB, 4 GB) communications and is the foundation for the majority of SAN installations throughout the world.

What is the future of fibre channel SANs?

Fibre Channel is a well-established, widely deployed technology with a proven track record and a very large installed base, particularly in high-performance, business-critical data center environments. Fibre Channel SANs continue to grow and will be enhanced for a long time to come. The reduced costs of Fibre Channel components, the availability of SAN kits, and the next generation of Fibre Channel (4 GB) are helping to fuel that growth. In addition, the Fibre Channel roadmap includes plans to double performance every three years.

What are the benefits of 4gb fibre channel?

Benefits include twice the performance with little or no price increase, investment protection with backward compatibility to 2 GB, higher reliability due to fewer SAN components (switch and HBA ports) required, and the ability to replicate, back up, and restore data more quickly. 4 GB Fibre Channel systems are ideally suited for applications that need to quickly transfer large amounts of data such as remote replication across a SAN, streaming video on demand, modeling and rendering, and large databases. 4 GB technology is shipping today.

How is fibre channel different from ISCSI?

Fibre Channel and iSCSI each have a distinct place in the IT infrastructure as SAN alternatives to DAS. Fibre Channel generally provides high performance and high availability for business-critical applications, usually in the corporate data center. In contrast, iSCSI is generally used to provide SANs for business applications in smaller regional or departmental data centers.

When should I deploy fibre channel instead of ISCSI?

For environments consisting of high-end servers that require high bandwidth or data center environments with business-critical data, Fibre Channel is a better fit than iSCSI. For environments consisting of many midrange or low-end servers, an IP SAN solution often delivers the most appropriate price/performance.

Name some of the SAN topologies ?

Point-to-point, arbitrated loop, and switched fabric topologies.

What’s the need for separate network for storage why LAN cannot be used?

LAN hardware and operating systems are geared to user traffic, and LANs are tuned for a fast user response to messaging requests.

With a SAN, the storage units can be secured separately from the servers and totally apart from the user network enhancing storage access in data blocks (bulk data transfers), advantageous for server-less backups.

What are the advantages of RAID?

“Redundant Array of Inexpensive Disks”

Depending on how we configure the array, we can have the
- data mirrored [RAID 1] (duplicate copies on separate drives)
- striped [RAID 0] (interleaved across several drives), or
- parity protected [RAID 5](extra data written to identify errors).

These can be used in combination to deliver the balance of performance and reliability that the user requires.

Define RAID? Which one you feel is good choice?

RAID (Redundant array of Independent Disks) is a technology to achieve redundancy with faster I/O. There are Many Levels of RAID to meet different needs of the customer which are: R0, R1, R3, R4, R5, R10, R6.

Generally customer chooses R5 to achieve better redundancy and speed and it is cost effective.
R0 – Striped set without parity/[Non-Redundant Array].
Provides improved performance and additional storage but no fault tolerance. Any disk failure destroys the array, which becomes more likely with more disks in the array. A single disk failure destroys the entire array because when data is written to a RAID 0 drive, the data is broken into fragments. The number of fragments is dictated by the number of disks in the drive. The fragments are written to their respective disks simultaneously on the same sector. This allows smaller sections of the entire chunk of data to be read off the drive in parallel, giving this type of arrangement huge bandwidth. RAID 0 does not implement error checking so any error is unrecoverable. More disks in the array means higher bandwidth, but greater risk of data loss

R1 – Mirrored set without parity.
Provides fault tolerance from disk errors and failure of all but one of the drives. Increased read performance occurs when using a multi-threaded operating system that supports split seeks, very small performance reduction when writing. Array continues to operate so long as at least one drive is functioning. Using RAID 1 with a separate controller for each disk is sometimes called duplexing.

R3 – Striped set with dedicated parity/Bit interleaved parity.
This mechanism provides an improved performance and fault tolerance similar to RAID 5, but with a dedicated parity disk rather than rotated parity stripes. The single parity disk is a bottle-neck for writing since every write requires updating the parity data. One minor benefit is the dedicated parity disk allows the parity drive to fail and operation will continue without parity or performance penalty.

R4 – Block level parity.
Identical to RAID 3, but does block-level striping instead of byte-level striping. In this setup, files can be distributed between multiple disks. Each disk operates independently which allows I/O requests to be performed in parallel, though data transfer speeds can suffer due to the type of parity. The error detection is achieved through dedicated parity and is stored in a separate, single disk unit.

R5 – Striped set with distributed parity.
Distributed parity requires all drives but one to be present to operate; drive failure requires replacement, but the array is not destroyed by a single drive failure. Upon drive failure, any subsequent reads can be calculated from the distributed parity such that the drive failure is masked from the end user. The array will have data loss in the event of a second drive failure and is vulnerable until the data that was on the failed drive is rebuilt onto a replacement drive.

R6 – Striped set with dual distributed Parity.
Provides fault tolerance from two drive failures; array continues to operate with up to two failed drives. This makes larger RAID groups more practical, especially for high availability systems. This becomes increasingly important because large-capacity drives lengthen the time needed to recover from the failure of a single drive. Single parity RAID levels are vulnerable to data loss until the failed drive is rebuilt: the larger the drive, the longer the rebuild will take. Dual parity gives time to rebuild the array without the data being at risk if one drive, but no more, fails before the rebuild is complete.

What is the difference between RAID 0+1 and RAID 1+0?

RAID 0+1 (Mirrored Stripped)

In this RAID level all the data is saved on stripped volumes which are in turn mirrored, so any disk failure saves the data loss but it makes whole stripe unavailable. The key difference from RAID 1+0 is that RAID 0+1 creates a second striped set to mirror a primary striped set. The array continues to operate with one or more drives failed in the same mirror set, but if drives fail on both sides of the mirror the data on the RAID system is lost. In this RAID level if one disk is failed full mirror is marked as inactive and data is saved only one stripped volume.

RAID 1+0 (Stripped Mirrored)
In this RAID level all the data is saved on mirrored volumes which are in turn stripped, so any disk failure saves data loss. The key difference from RAID 0+1 is that RAID 1+0 creates a striped set from a series of mirrored drives. In a failed disk situation RAID 1+0 performs better because all the remaining disks continue to be used. The array can sustain multiple drive losses so long as no mirror loses both its drives.
This RAID level is most preferred for high performance and high data protection because rebuilding of RAID 1+0 is less time consuming in comparison to RAID 0+1.

When JBOD’s are used?

“Just a Bunch of Disks”

It is a collection of disks that share a common connection to the server, but don’t include the mirroring,
striping, or parity facilities that RAID systems do, but these capabilities are available with host-based software.

Differentiate RAID & JBOD?

RAID: “Redundant Array of Inexpensive Disks”
Fault-tolerant grouping of disks that server sees as a single disk volume

Combination of parity-checking, mirroring, striping

Self-contained, manageable unit of storage

JBOD: “Just a Bunch of Disks”

Drives independently attached to the I/O channel
Scalable, but requires server to manage multiple volumes

Do not provide protection in case of drive failure

What is a HBA?

Host bus adapters (HBAs) are needed to connect the server (host) to the storage.

What are the advantages of SAN?

Massively extended scalability.

Greatly enhanced device connectivity.
Storage consolidation.
LAN-free backup.
Server-less (active-fabric) backup.
Server clustering.
Heterogeneous data sharing.
Disaster recovery – Remote mirroring.
While answering people do NOT portray clearly what they mean & what advantages each of them have, which are cost effective & which are to be used for the client’s requirements.

What is the difference b/w SAN and NAS?

The basic difference between SAN and NAS, SAN is Fabric based and NAS is Ethernet based.
SAN – Storage Area Network
It accesses data on block level and produces space to host in form of disk.

NAS – Network attached Storage
It accesses data on file level and produces space to host in form of shared network folder.

What is a typical storage area network consists of – if we consider it for implementation in a small business setup?

If we consider any small business following are essentials components of SAN:

Fabric Switch.
FC Controllers.
JBOD’s.

Can you briefly explain each of these Storage area components?

Fabric Switch: It’s a device which interconnects multiple network devices .There are switches starting from 16 port to 32 ports which connect 16 or 32 machine nodes etc. vendors who manufacture these kind of switches are Brocade, McData.

What is the most critical component in SAN?

Each component has its own criticality with respect to business needs of a company.

How is a SAN managed?

There are many management software’s used for managing SAN’s to name a few:

Santricity.
IBM Tivoli Storage Manager.
CA Unicenter.
Veritas Volumemanger.

Which one is the Default ID for SCSI HBA?

Generally the default ID for SCSI HBA is 7.

SCSI- Small Computer System Interface.

HBA – Host Bus Adaptor.

What is the highest and lowest priority of SCSI?

There are 16 different ID’s which can be assigned to SCSI device 7, 6, 5, 4, 3, 2, 1, 0, 15, 14, 13, 12, 11, 10, 9, 8.

Highest priority of SCSI is ID 7 and lowest ID is 8.

How do you install device drivers for the HBA first time during OS installation?

In some scenarios you are supposed to install Operating System on the drives connected thru SCSI HBA or SCSI RAID Controllers, but most of the OS will not be updated with drivers for those controllers, that time you need to supply drivers externally, if you are installing windows, you need to press F6 during the installation of OS and provide the driver disk or CD which came along with HBA.

If you are installing Linux you need to type “linux dd” for installing any driver.

What is Array?

Array is a group of Independent physical disks to configure any Volumes or RAID volumes.

Can you describe at-least 3 troubleshooting scenarios which you have come across in detail?

SCENARIO 1: How do you find/debug when there is error while working SCSI devices?

In our daily SAN troubleshooting there are many management and configuration tools we use them to see when there is a failure with target device or initiator device.

Some time it is even hard to troubleshoot some of the things such as media errors in the drives, or some of the drives taking long time to spin-up. In such cases these utilities will not come to help. To debug this kind of information most of the controller will be implemented with 3-pin serial debug port. With serial port debug connector cable you can collect the debug information with hyper terminal software.

SCENARIO 2: I am having an issue with a controller its taking lot of time to boot and detect all the drives connected how can I solve this.?
There are many possibilities that might cause this problem. One of the reason might be you are using bad drives that cannot be repaired. In those cases you replace the disks with working ones.

Another reason might be slots you connected your controller to a slot which might not be supported.
Try to connect with other types of slots.

One more probable reason is if you have flashed the firmware for different OEM’s on the same hardware.
To get rid of this the flash utilities will be having option to erase all the previous and EEPROM and boot block entry option. Use that option to rectify the problem.

SCENARIO 3: I am using tape drive series 700X, even the vendor information on the Tape drive says 700X, but the POST information while booting the server is showing as 500X what could be the problem?

First you should make sure your hardware is of which series, you can find out this in the product website.
Generally you can see this because in most of the testing companies they use same hardware to test different series of same hardware type. What they do is they flash the different series firmware. You can always flash back to exact hardware type.

Which are the 4 types of SAN architecture types ?

Core-edge.

Full-Mesh.

Partial-Mesh.

Cascade.

Which command is used in linux to know the driver version of any hardware device?

dmesg.

How many minimum drives are required to create R5 (RAID 5)?

You need to have at least 3 disk drives to create R5.

Can you name some of the states of RAID array?

There are states of RAID arrays that represent the status of the RAID arrays which are given below:

Online.
Degraded.
Rebuilding.
Failed.

What are the protocols used in physical/datalink and network layer of SAN?

Ethernet.

SCSI.

Fibre Channel.

What is storage virtualization?

Storage virtualization is amalgamation of multiple n/w storage devices into single storage unit.

Describe in brief the composition of FC Frame?

Start of the Frame locator

Frame header (includes destination id and source id, 24 bytes/6 words).
Data Payload (encapsulate SCSI instruction can be 0-2112 bytes in length).
CRC (error checking, 4 bytes).
End of Frame (1 byte).

What is virtualization?

Virtualization is logical representation of physical devices. It is the technique of managing and presenting storage devices and resources functionally, regardless of their physical layout or location. Virtualization is the pooling of physical storage from multiple network storage devices into what appears to be a single storage device that is managed from a central console. Storage virtualization is commonly used in a storage area network (SAN). The management of storage devices can be tedious and time-consuming. Storage virtualization helps the storage administrator perform the tasks of backup, archiving, and recovery more easily, and in less time, by disguising the actual complexity of the SAN.

What is HA?

HA High Availability is a technology to achieve failover with very less latency. Its a practical requirement of data centers these days when customers expect the servers to be running 24 hours on all 7 days around the whole 365 days a year – usually referred as 24x7x365. So to achieve this, a redundant infrastructure is created to make sure if one database server or if one app server fails there is a replica Database or Appserver ready to take-over the operations. End customer never experiences any outage when there is a HA network infrastructure.

Can you name some of the available tape media types?

There are many types of tape media available to back up the data, some of them are:

DLT: Digital Linear Tape – technology for tape backup/archive of networks and servers; DLT technology addresses midrange to high-end tape backup requirements.

LTO: Linear Tape Open; a new standard tape format developed by HP, IBM, and Seagate.

AIT: Advanced Intelligent Tape; a helical scan technology developed by Sony for tape backup/archive of networks and servers, specifically addressing midrange to high-end backup requirements.

Can we assign a hot spare to R0 (RAID 0) array?

No, since R0 is not redundant array, failure of any disks results in failure of the entire array so we cannot rebuild the hot spare for the R0 array.

Name the features of SCSI-3 standard?

QAS: Quick arbitration and selection.
Domain Validation.
CRC: Cyclic redundancy check.

What is Multipath I/O?

Fault tolerant technique where, there is more than one physical path between the CPU in the computer systems and its main storage devices through the buses, controllers, switches and other bridge devices connecting them.

What is disk array?

Set of high performance storage disks that can store several terabytes of data. Single disk array can support multiple points of connection to the network.

What are different types of protocols used in transportation and session layers of SAN?

Fibre Channel Protocol (FCP).

Internet SCSI (iSCSI).

Fibre Channel IP (FCIP).

What is the type of Encoding used in Fibre Channel?

8b/10b, as the encoding technique is able to detect all most all the bit errors

What are the main constrains of SCSI in storage networking?

Deployment distance (max. of 25 mts).

Number of devices that can be interconnected (16).

What is a Fabric?

Interconnection of Fibre Channel Switches.

What are the services provided by Fabric to all the nodes?

1.  Fabric Login.
2. SNS.
3.  Fabric Address Notification.
4. Registered state change notification.
5. Broadcast Servers.

What is the difference between LUN and WWN?

LUN: Unique number that is assigned to each storage device or partition of the storage that the storage can support.

WWN: 64bit address that is hard coded into a fibre channel HBA and this is used to identify individual port (N_Port or F_Port) in the fabric.

What are the different topologies in Fibre Channel?

1.  Point-to-Point.
2. Arbitrary Loop.
3. Switched Fabric Loop.

What are the layers of Fibre Channel Protocol?

1.  FC Physical Media.
2. FC Encoder and Decoder.
3. FC Framing and Flow control.
4. FC Common Services.
5. FC Upper Level Protocol Mapping.

What is zoning?

Fabric management service that can be used to create logical subsets of devices within a SAN. This enables portioning of resources for management and access control purpose.

What is the purpose of disk array?

Probability of unavailability of data stored on the disk array due to single point failure is totally eliminated.

How does FC Switch maintain the addresses?

FC Switch uses simple name server (SNS) to maintain the mapping table.

What are the two major classification of zoning?

Two types of zoning are:

1. Software Zoning.
2. Hardware Zoning.

What are different levels of zoning?

1.  Port Level zoning.
2. WWN Level zoning.
3. Device Level zoning.
4. Protocol Level zoning.
5. LUN Level zoning.

What are the 3 prominent characteristics of SAS Protocol?

1. Native Command Queuing (NCQ.)
2. Port Multiplier.
3. Port Selector.

What are the 5 states of Arbitrary Loop in FC?

1. Loop Initialization.
2. Loop Monitoring.
3. Loop arbitration.
4. Open Loop.
5. Close Loop.

What is LUN Masking?

A method used to create an exclusive storage area and access control. And this can be achieved by storage device control program.

What is snapshot?

A snapshot of data object contains an image of data at a particular point of time.

What is hot-swapping?

Devices are allowed to be removed and inserted into a system without turning off the system.