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iSCSI storage: Common misconceptions

Used to facilitate data transfers and improve data storage management, iSCSI storage has become an increasingly popular SAN protocol. But despite iSCSI

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proving to be a worthy alternative to Fibre Channel in enterprise data storage environments, there are still a lot of misconceptions surrounding the technology.

Greg Schulz, founder and senior analyst at Stillwater, Minn.-based StorageIO Group, discusses some of the common misconceptions surrounding iSCSI. Learn how iSCSI functions with network-attached storage (NAS), if iSCSI disk drives actually exist, and whether or not iSCSI has the ability to be partitioned as RAID.

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Clearing up the iSCSI storage confusion in network storage
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SearchStorage.com: There are storage systems that support both iSCSI and NAS, but is there really such a thing as an iSCSI NAS?

Schulz: That's where the word mixing results in confusion. Are there NAS systems that support iSCSI? Yes. Are there iSCSI storage systems that support NAS? Yes, we call them multiprotocol or unified storage systems. But is there really such a thing as a NAS device that does iSCSI underneath? No, that's a bit of a misnomer.

SearchStorage.com: Is there such a thing as an iSCSI disk, then?

Schulz: That's another misconception that comes up again and again. Is the disk the individual drive, or is that disk the storage solution that you attach to the server? If it is in the context of the individual disk drive, then no, there is no such thing as an iSCSI disk drive. The native disk drives existing today are Fibre Channel [FC], SAS, SATA and USB drives.

However, there are disk drives that go into iSCSI systems. In other words, a SAS drive, a SATA drive, a Fibre Channel or any other type of drive that plugs into that storage system, will appear as if the drive is an iSCSI disk.

SearchStorage.com: Does iSCSI have the ability to be partitioned as RAID?

Schulz: Whether or not iSCSI has the ability to be partitioned as RAID is going to be vendor specific. There should not be anything within the iSCSI protocol or the iSCSI specs that limits that. If there is any limitation with that it's going to stem from a vendor-specific implementation.

Another way of looking at that is with an iSCSI storage system, you should have the same functionality that you would have with a shared SAS, a shared Fibre Channel over Ethernet [FCoE] or even a traditional NAS system. In other words, you should have the ability to select specific RAID levels, apply the different RAID levels to different groups of drives, create different volumes and LUNs, and then be able to map all of those on the iSCSI.

Where it gets interesting is that when you have a unified storage device -- a storage system that supports iSCSI and NAS concurrently -- the iSCSI storage may be set up as RAID 1 or RAID 10, with some number of the drives created as a LUN and mapped as SCSI targets to the iSCSI. However, your NAS device might be set up as RAID 5 or RAID 6, or maybe even dual parity RAID if it is a NetApp device. In other words, your NAS device might be set up for higher capacity and resiliency, but it is mapped out through that file system.

So it is possible to have some of the capacity mapped out through iSCSI as a LUN via RAID 1 and then mapped out through the NFS as a file system. But again, the details are really going to be in how a particular vendor deploys it with their feature functionality and the different buzzwords for their functionality that they provide you.


This was first published in May 2010

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