|Full name||Virtual Machine File System|
|Introduced||(ESX Server v1.x)|
|Partition identifier||0xfb (MBR)|
|Max. file size||2TB - 512 bytes|
|Max. number of files||~100,000 (VMFS-5) |
|Max. volume size||64 TB (VMFS-5) |
|Supported operating systems||VMware ESX|
VMware VMFS (Virtual Machine File System) is VMware, Inc.'s cluster file system. It is used by VMware ESX Server and the company's flagship server virtualization suite, vSphere (and predecessor VMware Infrastructure). It was developed and is used to store virtual machine disk images, including snapshots. Multiple servers can read/write the same filesystem simultaneously, while individual virtual machine files are locked. VMFS volumes can be logically "grown" (non-destructively increased in size) by spanning multiple VMFS volumes together. It is not mandatory to use VMFS with VMware; an alternative is NFS.
There are four versions of VMFS, corresponding with ESX Server product releases.
- VMFS version 1 was used by ESX Server v1.x, which is no longer sold. It didn't feature the cluster filesystem properties and was used only by a single server at a time. VMFS1 is a flat filesystem with no directory structure. -- Officially named "VMware File System"
- VMFS version 2 is used by ESX Server v2.x and (in a limited capacity) v3.x. VMFS2 is a flat filesystem with no directory structure. -- Officially named "VMware File System"
- VMFS version 3 is used by ESX Server v3.x and vSphere (4.x). As a most noticeable feature, it introduced directory structure in the filesystem. Older versions of ESX Server cannot read or write VMFS3 volumes. Beginning from ESX 3 and VMFS3, virtual machine configuration files are stored in the VMFS partition by default. -- Officially named "VMware Virtual Machine File System"
- VMFS version 5 is used by ESXi Server v5.x and vSphere (5.x). As a most noticeable feature, it introduced unified 1MB block-size (upgraded VMFS5 volumes will however inherit the VMFS3 block size value), and larger LUN-sizes than 2TB (64TB max), but the maximum VMDK-file size remain at 2TB (minus 512 bytes, irrespective of the file‐system block size), same as VMFS3. The VMDK size limit is increased in vSphere 5.5 to 64TB. Older versions of ESX/ESXi cannot read or write VMFS5 volumes. Beginning from ESXi 5.0 and VMFS5, virtual machine configuration files are stored in the VMFS5 partition by default.
- Allows access by multiple ESX servers at the same time by implementing per-file locking. SCSI reservations are only implemented when logical unit number (LUN) metadata is updated (e.g. file name change, file size change, etc.)
- Add or delete an ESX server from a VMware VMFS volume without disrupting other ESX server hosts.
- With ESX/ESXi4, VMFS volumes also can be expanded using LUN expansion
- Optimize virtual machine I/O with adjustable volume, disk, file and block sizes.
- Recover virtual machines faster and more reliably in the event of server failure with Distributed Journaling.
- Can be shared with up to 64 ESX Servers.
- Can support LUNs with max size of 2TB and a max VMFS size of 64 TB as of version 4 (vSphere), and 64 TB as of version 5.
- In VMFS-3 and VMFS-5 prior to vSphere 5.1, the maximum number of hosts which can share a read-only file is 8. This affects the scalability of linked clones sharing the same base image. In vSphere 5.1, this limit is increased to 32 with the introduction of a new locking mechanism.
- VMFS-3 limits files to 262,144 (218) blocks, which translates to 256 GB for 1 MB block sizes (the default) up to 2 TB for 8 MB block sizes.
- This restriction is lifted in VMFS-5 where a 1MB unified block size is now used for all files, with subblock support for very small files However the maximum file size is now 2 TB - 512 B.
- There is also a limit of approx 30720 files (using MBR) on a single VMFS-3 datastore. This has been raised to 130690 files(using GPT)on VMFS-5 
Open source implementation
An open source VMFS driver which enables read-only access to files and folders on partitions formatted with the Virtual Machine File System (VMFS) is developed and maintained by fluid Operations AG. It allows features like offloaded backups of virtual machines hosted on VMware ESX hosts up to VMFSv3.
- "vSphere 5.0 Storage Features Part 1 - VMFS-5". VMware. 2011-07-12. Retrieved 2012-01-05.
- "Deliver High-performance Storage for Virtual Machines". VMware. Retrieved 2007-09-26.
- http://www.vmware.com/pdf/vsphere5/r50/vsphere-50-configuration-maximums.pdf Configuration Maximums VMware® vSphere 5.0
- "Configuration Maximums for VMware vSphere 4.1" (PDF). VMware. 2010-07-13. Retrieved 2010-07-13.
- "vSphere 5.1 New Storage Features". VMware.