Note: This article is now out of date. While the Apple Xserve RAID is still found in legacy installations, it has been replaced by the Promise RAID for new installations.
The most typical storage found in Xsan setups is the Apple XRAID. Any XRAID will work within an Xsan deployment. XRAID systems being used in an Xsan should have the maximum amount of RAM available, which is 512MB.
The Apple XRAID comes with 14 drive modules. Drive modules in an XRAID are split into two channels, each with 7 modules. Drive modules are just UltraATA or SATA enclosures for the Hard Drives used in an XRAID. XRAIDs are available in sizes up to 7TB. Due to the large amount of media, it can take 30 or more hours to format the RAID completely. While there are options that allow writing to the XRAID while it is formatting this is often going to cause a performance decrease on the XRAID.
Each controller of the XRAID has a serial port, a Gigabit Ethernet port and a Gigabit Fibre channel port. For best performance, both controllers should be used when using an Xserve RAID with Xsan.
This brings up the two main factors when planning the storage needs for an Xsan: speed and space. Space is typically the biggest choice made when purchasing storage devices. Since all of the Xserve RAIDs get their space aggregated to create two gigabits per Xserve RAID, the speed requirements can often mean a need for purchasing more Xserve RAIDs even when the extra space is not a requirement.