1. A virtual disk is a file that encapsulates the file system of a virtual machine. In the case of VMware ESX Server, a virtual disk is a .vmdk file - and it may be the C: drive or C: and D: drive in one big file. It cannot span LUNs unless the LUN has been extended by an underlying technology (such as EMC CLARiiON metaLUNs or VMware VMFS3 disc extents.

2. LUN - Logical Unit Number. A logical disk presented by SAN attached storage. You'd not normally consider a direct attached RAID array a LUN.

3. VMFS = Virtual Machine File System. It's a VMware-proprietary file system that allows concurrent access from multiple servers. It uses a system of file-level and SCSI locks to ensure that ESX servers do not write to the same file (or virtual machine) at the same time. It's one of the bits of technology that's at teh heart of why ESX Server works so well.

from what I have been reading I also understood that VDisk can span more than one LUN, otherwise they should just say VDisk is another name for LUN.

regarding a VMFS, is this file belongs to each ESX computer and is stored in the SAN?

in our network we have 5 ESX servers and some of them have 16 Virtual machines and some 18 , etc...
does that mean each ESX server has information about its VM in the VMFS file? if so, is this file can be accessed by any other ESX server if the one that owns VMFS is down?

Read through these:
http://www.vmware.com/pdf/vi3_35/esx_3/r35/vi3_35_25_prim.pdf

http://www.vmware.com/pdf/vi3_35/esx_3/r35/vi3_35_25_quickstart.pdf

They should be helpful.

Hi Disorganise -

I just want to clarify one of your statements:
>my understanding is that a ESX *can* create a volume that spans multiple LUNs

Absolutely correct. Here's what I said:

(such as EMC CLARiiON metaLUNs or VMware VMFS3 disc extents.

VMware do not rceommend expanding a VMFS3 file system using disk extents as it can create disk I/O "hot spots". Better practice is to create a new LUN and put your new VMs on that.

>does that mean each ESX server has information about its VM in the VMFS file?
Yes - each ESX server knows about the VMs that it is running - it is stored on local disk, though. The VMFS file system is shared between all the ESX servers - sort of like a Microsoft cluster. VIrtualCenter - the VMware managemet tool - knows about all the VMs running and will move VMs around to balance the load.

> if so, is this file can be accessed by any other ESX server if the one that owns VMFS is down?
Yes - as the VMFS partition should be on a SAN and shared between ESX servers, there's no problem in other ESX Servers accessing the data. In fact, VMware HA (High Availability) will restart virtual machines automatically on other ESX Servers if one server fails. Dynamic Resource Scheduling (DRS) then takes care of balancing the load amongst the remaining ESX Servers.

so vmfs is stored on the local disk of the ESX server or on the SAN?
Do we manually create the vmfs, or it gets created by itself after installing a certain software

>so vmfs is stored on the local disk of the ESX server or on the SAN?
Can be on local disk, but it is typically on a SAN. A virtual machine on a local disk obviously can't be shared amongst ESX Servers.

>Do we manually create the vmfs, or it gets created by itself after installing a certain software
You create the VMFS partition through VirtualCenter, the management console. You can also create it from the service console command line or via the management tool pointed directly to the ESX server.

each time you create a new virtual machine from Virtual Center and you assign the VM to a host(ESX server), the VMFS file gets updated?
is that how it works?

any updates ????