Storage Area Networking SAN NAS Fibre Channel iSCSI

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Monday, June 29, 2009

Interview questions most frequently asked for Storage pros but this is just the basics

Many Storage job interviewer ask one or other question about Luns, Lun management, Lun secure,Lun relocation, Lun replication. But the below one is the most frequently asked question in any storage technology interview. So better you get the basics right.

Interview question: What is Lun Mapping,Lun Masking,Persistent Binding,Fabric Zoning.All these are with respect to a FC SAN array. (Fibre Channel Storage Area Network Array)

LUN Mapping:
LUN Mapping is host-centric method of storage LUN visibility management. LUN Mapping selectively allows a system administrator to scan for specified SCSI targets and LUNs at storage-driver boot time and to ignore selectively non-specified SCSI targets and LUNs.

The advantage of LUN Mapping is that it provides a level of security management in SANs where LUN Masking is not an option, perhaps because it is not supported
on the storage hardware.

The disadvantage is that LUN Mapping is configured and enabled on a host-by-host basis. It requires good coordination among the administrators of the systems sharing the storage, which ensures that only one host sees certain storage unless planned, as in a clustered server configuration.

LUN Masking:
LUN Masking is RAID storage subsystem-centric method of LUN visibility management to
hosts or servers.

LUN Masking makes sure that each RAID controller is configured to allow each host to see only a subset of the actual LUN's & hide all other LUNs from it. so all connected hosts will see only those LUNs that are assigned to them.

The advantage of LUN Masking is easy to implement in storage subsystems and no changes needed in Hosts or servers that are connected to them.
The disadvantage is multi-vendor devices might be present in the real SAN networks & not all the vendors implemented in same way.

Persistent Binding:
It is another host-centric method. This can be implemented in Operating systems (OS) by assigning specific SCSI-Target IDs to the Storage LUNs.

Fabric Zoning:
It is Network-centric method & basically Switch (or Fabric) present in the SAN networks have the capability to allow grouping of Ports from Storage Sub-systems to Connected Hosts in the form of Zoning.
The disadvantage is that Fabric zoning cannot mask individual LUNs that sit behind a port. All hosts connected to the same port will see all the LUNs addressed through that port.

Each of these Host-centric, Storage centric & switch-centric methods provide their own benifits & drawbacks. Usually to provide very good solution, combination of any two methods will be implemented in real SAN networks.

Storage Area Network and Network Attached Storage

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