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There are pros and cons to running multiple protocols through a single array controller.
Prior to SANs, storage resources were dedicated to a single server. It was only in the mid-1990s that FC SANs enabled the separation of server and storage, making a single storage pool accessible to more than one server. While a huge leap forward, FC SANs were--and mostly still are--complex and expensive, and they left a huge gap for a simpler and less-expensive technology.
NAS filled this void by offering storage services, and incorporating and controlling the file system within the storage system, presenting files and directories to hosts in the NFS and CIFS file-system protocols. A large percentage of installed storage stores files.
File-system protocols like CIFS and NFS treat files as a single entity. For example, when a user opens a file, it's opened in
| its entirety and then locked to prevent others from modifying it. This is ill-suited for apps where data is accessed at a sub-file level, such as databases where small pieces of information are continuously read and updated within a larger file. Without question, block-based storage protocols like SCSI, iSCSI and FC are superior to the file-based access methods of NAS when it comes to non-file-based data access.
You need to remember that information is stored in blocks--not as files--on the disk subsystem. Data is written in blocks to the disk volume, and block size is determined when the volume is formatted, typically ranging from 512 bytes to 2,048 bytes in size. A file, on the other hand, is a concatenation of data blocks managed by the file system. While the disk subsystem manages data in terms of blocks, the OS and humans manage information in terms of files and directories.
Block-based storage access protocols (SCSI, iSCSI, FC) are indispensable because they're native to storage systems and can deal with all types of data efficiently. NAS lets you do away with standard file servers by incorporating file-system protocols into the storage system, providing simpler management, a higher degree of scalability, and added storage services like snapshots and replication. Here's how multiprotocol arrays differ from single-protocol arrays in terms of performance, price, ease of management and other features.
This was first published in March 2008