The VMFS (Virtual Machine File System) is a clustered file system developed by VMware for storing virtual machines and their associated files. Understanding the on-disk structure of VMFS is crucial for managing and maintaining VMware virtualized environments effectively.

1. Magic Numbers:
   • VMFS uses two magic numbers to identify its file system type:
     • The first magic number is “4B 44 4D 56” in hexadecimal format, which appears at the beginning of the volume header.
     • The second magic number is “43 4F 52 50” in hexadecimal format, which appears at the end of the volume header.
   • These magic numbers serve as signatures that indicate to the VMware hypervisor that the storage device or partition contains VMFS-formatted data.
2. Volume Header:
   • At the beginning of a VMFS volume, there is a volume header containing essential metadata about the file system.
   • The volume header includes information such as the version of VMFS, the block size, the number of blocks, and the UUID (Universally Unique Identifier) of the volume.
   • Additionally, the volume header contains pointers to other important structures within the file system, such as the file descriptor block (FDB) and the pointer block.
3. File Descriptor Block (FDB):
   • The file descriptor block (FDB) is a critical component of the VMFS file system structure.
   • It contains metadata about files and directories stored on the volume, including file names, sizes, permissions, and pointers to data blocks.
   • The FDB is organized into a B-tree structure, allowing for efficient lookup and retrieval of file metadata.
4. Pointer Block:
   • The pointer block is another essential structure within VMFS that facilitates data storage.
   • It contains pointers to data blocks where file data is stored on disk.
   • Pointer blocks are organized into a tree-like structure, with each level pointing to lower-level pointer blocks or directly to data blocks.
5. Data Blocks:
   • Data blocks are the actual units of storage within VMFS where file data is stored.
   • These blocks can vary in size, depending on the configuration of the VMFS volume.
   • File data is stored in these blocks in a contiguous manner, with pointers in the FDB and pointer blocks indicating the location of each file’s data blocks.
6. Metadata Structures:
   • VMFS maintains various metadata structures to manage the file system efficiently, including allocation bitmaps, journal logs, and lock structures.
   • These metadata structures help ensure data consistency, integrity, and reliability, even in the event of system failures or crashes.

Overall, the on-disk structure of VMFS is designed to provide high performance, scalability, and reliability for storing virtual machines in VMware environments. By understanding how VMFS organizes and stores data on disk, administrators can effectively manage and troubleshoot their VMware infrastructure.