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SAN Technology Overview Storage Area Networks, SAN's, are becoming more and more prevelant in both large and small IT shops today. There are a number of reasons but probably the primary reason is that a SAN can greatly simplify the management of constantly growing storage requirements faced by many organizations today. You can imagine that managing 50 TB of storage all in one place is much simpler than managing the same amount of storage in the form of 100 500 GB arrays attached to a myriad of computers from different vendors running different OSes. A typical modern SAN consists of 2 or more fibre channel switches and a high performance, high bandwidth array. The array will usually consist of 2 array controllers attached to dual ported fibre channel disks on several fibre channel loops. This provides the controllers with multiple paths to the disks for redundancy. Usually the disks and loops that they are attached to are referred to as the back-end connections. The controllers also provide multiple host connections. The connections are usually connected to fibre channel switches and are usually referred to as front-end connections. Hosts are connected to the SAN via host bus adapters, HBAs. The HBA looks like a scsi controller to the OS and accepts scsi commands just like a scsi controller. The HBA along with the host's FCP software layer will then encapsulate the scsi commands and data into fibre channel packets and transport them via fibre channel cable. The HBA's in the hosts are attached to the fibre channel switches which are in turn attached to the array controllers. Typically the hosts are dual attached to the switches via 2 HBAs to provide redundant paths to the storage. Modern fibre channel switches form what is referred to as a fabric. A fabric is made up of 1 or more switches and functions similar to multiple IP switches connected together in that devices on one switch can be configured to see devices on another switch. Typical SAN design is to have dual everything; dual fabrics each connected to both array controllers and dual HBAs in the hosts each connected to different fabrics. This way the host storage is very fault tolerant in that storage operations could continue through HBA, switch, cable, disk and array controller failures. Most SAN array controllers provide the ability to create logical devices from the physical disks and present the logical disks to the hosts. When creating the logical devices it is possible to enhance performance and fault tolerance by using various combinations of striping with or without parity and mirroring. Usually in modern arrays RAID-5 is used. In the past RAID-5 was not considered appropriate for write intensive applications because every write required a parity calculation. Modern arrays have addressed this potential performance hit by putting several GB of ram cache in front of the disks and developing specialized hardware for parity calculations. The cache allows the write operation to complete immediately meanwhile the controller is calculating parity and flushing cache to disk. Resources |
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