Storage Area Networking SAN NAS Fibre Channel iSCSI

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Saturday, December 16, 2006

Storage area networking Interview Questions

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Some more interview questions contributed by Jai
51) What is one of the constrains of using storage switch?

52) What is the difference between NAS and SAN?

Cables used in the n/w
n/w protocols (TCP/IP, IPx) and file sharing protocols (IFS & NFS)
Lower TCO
Support heterogeneous clients

High-speed connectivity such as FC
Do not use n/w protocols because data request are not made over LAN
Higher TCO
Requires special s/w to provide access to heterogeneous clients

53) 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. A certain amount of deviation from the original signaling will occur naturally as serial bit stream propagates over fibre-optic or copper cabling.

Mainly caused by electro magnetic interference

54) What is BER/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.

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

56) 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 the additional overhead of monitoring multiple connections and to see whether a particular request is completed.

57) 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.

58) Explain different types of RAID?
RAID 0: data striping blocks are written sequentially, no redundancy, high performance than single disk access.

RAID 1: Mirroring, data blocks written to both disk at once, 100% redundancy, 100% additional capacity required. Read can be distributed across both the disks to increase performance.

RAID 3: Striping with byte parity, adds parity information to rebuild data in the event of disk failure, high transfer rate and availability with lower capacity required than RAID 1. Transactions performance low because all disks operate in lock step.

RAID 4: striping with block parity, independently accessible disks, data blocks written sequentially to each disk failure. Dedicated parity disk is write bottleneck and leads to poor performance.

RAID 5: Striping with rotational parity, parity blocks written per row and distributed across all disks, parity distribution eliminates single write bottleneck overhead for parity calculation on write supplemented with parallel microprocessors or caching.

59) What is NAS in detail ?
NAS or Network Attached Storage
“NAS is used to refer to storage elements that connect to a network and provide file
access services to computer systems. Abbreviated NAS. A NAS Storage Element consists of an interface or engine, which implements the file services, and one or more devices, on which data is stored.
NAS elements may be attached to any type of network. When attached to SANs, NAS elements may be considered to be members of the SAS (SAN Attached Storage) class of storage elements.

A class of systems that provide file services to host computers. A host system that uses network attached storage uses a file system device driver to access data using file access protocols such as NFS or CIFS. NAS systems interpret these commands and perform the internal file and device I/O operations necessary to execute them.”

Though the NAS does speed up bulk transfers, it does not offload the LAN like a SAN does. Most storage devices cannot just plug into gigabit Ethernet and be shared - this requires a specialized file server the variety of supported devices is more limited.NAS has various protocols established for such needed features as discovery, access control, and name services.

60)Briefly list the advantages of SAN ?
SANs fully exploit high-performance, highconnectivity network technologies
SANs expand easily to keep pace with fast growing storage needs
SANs allow any server to access any data
SANs help centralize management of storage resources
SANs reduce total cost of ownership (TCO)


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