How Xsan uses available storage in macOS Server
Xsan stores user files and file system data on SAN volumes, and stripes data across the LUNs in a volume for better performance.
Metadata and journal data
Xsan records information about the files in an Xsan volume using metadata files and a file system journal. File system metadata includes information such as which specific parts of which disks are used to store a file and whether the file is being accessed. The journal data includes a record of file system transactions that help ensure the integrity of files in the event of a failure.
These files are managed by the Xsan metadata controller but are stored on LUNs, not on the controller itself. Metadata is stored on the first storage pool you add to a volume. Journal data can also be stored on the same storage pool as metadata, or you can use a separate storage pool for journal data. You must have journal data on only one storage pool.
Stripe at a higher level
When a RAID system writes a file using a RAID 0 (striping) scheme, it breaks the file into segments and spreads them across disk drives in the RAID array. This improves performance by writing parts of the file in parallel (instead of one part at a time) to disks in the array.
Xsan applies this same technique in the storage hierarchy. Within each storage pool in a volume, Xsan stripes file data across the LUNs that make up the storage pool. Performance is improved because data is written in parallel.
You can tune SAN performance by adjusting the type of data written to each LUN in a storage pool (mixing or separating metadata, journal data, and user data).