How Xsan storage is organized in macOS Server
Although an Xsan volume mounted on a client computer looks like a single disk, it consists of multiple physical disks combined on several levels using RAID techniques.
The following paragraphs describe these elements and how you combine them to create shared Xsan volumes.
The smallest storage element you work with in Xsan is called a SCSI logical unit number, or LUN. A LUN represents a group of drives combined into a RAID array.
You create a LUN when you create a RAID array on a RAID storage device. The RAID system combines physical drives into an array based on the RAID scheme you choose. Each array appears on the Fibre Channel network as a LUN.
If the standard RAID arrays on your RAID systems aren’t right for your application, you can use the RAID system management software to re-create arrays based on other RAID schemes or different numbers of drive modules.
Your RAID LUNs are labeled and initialized for use with the Xsan file system when you use the Xsan pane of the Server app to set up a volume.
LUNs are combined to form storage pools. A storage pool in a small volume might consist of a single RAID array, but a larger volume might consist of several storage pools each of which includes several arrays.
Xsan distributes file data in parallel across the LUNs in a storage pool using a RAID 0 (striping) scheme. So, you can improve a client’s access speed by distributing available storage over several LUNs in a storage pool.
You can set up storage pools that have different performance or recoverability characteristics based on the RAID level of their LUNs. Users can then select where to store files based on their need for speed or safety.
When you create or modify a storage pool, the number of assigned LUNs needs to be a power of 2.
You use the Xsan pane of the Server app to add available LUNs to storage pools.
Storage pools are combined to create the volumes that users see. From the user’s perspective, the SAN volume looks and behaves like a large local disk, except that:
The size of the volume can grow as you add underlying arrays or storage pools
Multiple users on the SAN can access files on the volume at the same time