Apple Xsan for high-speed storage access on Mac OS X

Apple Xsan allows you to set up customer shops with shared, high-speed storage access on Mac OS X. In the second part of his Apple Xserve storage series, Larry Loeb offers an overview of this technology.

Apple Xsan is a storage area network (SAN) file system and management application (Xsan Admin) that enables you to provide your customers with shared, high-speed storage access to users or applications on client computers.

An Xsan architecture, which uses Xserve hardware, contains:

  • Volumes of shared storage, stored on Xserve RAID systems, available to clients as mounted volumes that may be used as local disks.
  • At least one computer acting as a metadata controller that coordinates access to shared volumes.
  • Client computers that access storage in accordance with established permissions and quotas.
  • Underlying Fibre Channel and Ethernet networks that control the data and perform the actual data serving.
Apple XServe storage series
Part 1: Apple XServe storage benefits and functionality

Part 2: Apple Xsan for high-speed storage access on Mac OS X

Part 3: Apple Xserve RAID for new storage installations

When you add a computer to an Xsan SAN, you specify whether it will play the role of client, controller or both. You must always have at least one controller present in the SAN. The controller's function is to manage the SAN volume metadata, maintain a file system journal and control concurrent file access. Metadata includes information as to where files are actually stored and what portions of available storage are allocated to new files. Since controllers can also act as clients, you may use a standby controller as a working client while the primary controller is operational.

Independent networks, consisting of a Fibre Channel network as well as one or two Ethernet networks, connect the storage devices, metadata controllers and clients. User data is transferred over high-speed Fibre Channel. Controllers also use a Fibre Channel connection to move metadata to and from the volume. To eliminate unnecessary traffic on the Fibre Channel connections, controllers and clients use an Ethernet network to exchange file system metadata. This is different from the controller writing volume data through Fibre Channel, as mentioned before. The Xsan Admin application also uses the Ethernet connection to manage the SAN.

Xsan can take advantage of multiple Fibre Channel connections between clients and storage. It can alternate between connections for each read and write, or assign each logical unit number (LUN) in a volume to one of the connections when the volume is mounted.

In most SANs a LUN represents a group of drives, such as a RAID array. In Xsan, LUNs are Xserve RAID arrays or slices. You can create a LUN when using the RAID Admin application to create an Xserve RAID array. The controller hardware and software in the Xserve RAID system combine individual drive modules into an array based on the RAID scheme chosen. Each array appears on the network as a separate LUN. If an array is sliced, each slice appears as a LUN.

Xsan distributes file data in parallel across the LUNs in a storage pool using a RAID 0 (striping) scheme. As a VAR, you may improve a client's access speed by distributing available storage over several LUNs in a storage pool.

Apple's Bonjour discovery technology takes the manual effort out of finding LUNs in a network. The LUNs will show up automatically in the admin program once it is connected to a storage network.

Overall, you'll find that Xsan can turn a complex storage management process into a manageable one.

About the author: Larry Loeb has been online since the world revolved around {!decvax}. He's been in many of last century's dead tree magazines about computers, having been a consulting editor to the late, lamented BYTE magazine, among other things. You can reach him at larryloeb@larryloeb.com.

This was first published in December 2006

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