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disk_types.md

Digesting VMDK spec

Particularly, virtual disks types are very confusing. This document is an attempt to disentangle that and make easier the design of an implementation.

Flat or preallocated disk

  • Monolithic or one-file: Single flat extent with separate descriptor file. createType: monolithicFlat
  • Split or multiple files: 2gb preallocated extents or smaller, to account for file system limits. createType: 2GbMaxExtentFlat
  • VMFS (managed):
    • Zeroed: Thick (flat) disk on VMFS, with blocks zeroed on first use. createType: vmfsPreallocated
    • Eager zeroed: Pre‐allocated (flat) disk on VMFS, with all blocks zeroed when created. createType: vmfsEagerZeroedThick

Sparse or growable disk

  • Monolithic or one-file: Single sparse extent with embedded descriptor file. createType: monolithicSparse
    • Stream-optimized: Compressed sparse extents with embedded LBA, useful for OVF streaming. createType: streamOptimized
  • Split or multiple files: Sparse extents 2GB or smaller to account for file system limits. createType: 2GbMaxExtentSparse
  • VMFS (managed):
    • Thin: Thin‐provisioned VMFS disks that consume only as much space as needed. createType: vmfsThin
    • Sparse: Sparse disk on VMFS, often a redo log, not to be confused with thin‐provisioned disk. createType: vmfsSparse

Other VMFS types

  • Virtual raw device map (RDM): Virtual compatibility raw device map (RDM) acts like a symbolic link to physical disk. createType: vmfsRDM
  • Physical raw device map (RDMP): Physical compabibility RDM, similar but sends SCSI commands to underlying hardware.createType: vmfsRDMP
  • Special raw disk: Special raw disk for ESXi hosts, passthrough only mode. createType: vmfsRaw

Full device

Disk that takes the properties of, and is backed by, physical disk on the host. createType: fullDevice

Partitioned device

Disk backed by some partitions of physical disk, with other partitions hidden. createType: partitionedDevice

Custom

Descriptor file with arbitrary extents. createType: custom

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