Saturday, 23 June 2012


SAN Interview Questions and Answers


 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.


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.


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.


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.


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.


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.


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.


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.

9. Name some of the SAN topologies?

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

10. 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.

11. 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.

12. 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.

13. 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.

14. 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.

15. 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.

16. What is a HBA?

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

17. 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.

18. 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.

19. 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

20. 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.



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