I want two virtual servers to share the same disk (one R/W, the other one R/O) Can I do this on VMWare ESX, setting up the same virutal disk on both servers?
You can share disks in VMware ESX by editing the properties of the SCSI controller for "virtual" or sharing mode. This will allow you to access the same LUN/disk from two different virtual machines.
This will not allow you to have one system using it R/W and the other R/O. It is designed for use in clusters with cluster aware filesystems.
Here is a way to do it, create a lightweight no gui install of linux, install samba. share out the drive as a file server. You can do this with any distro, 2 min setup after you have it installed, seriously.
ESX does not support this. Although you might be able to do something with a clustering file system and raw device mappings. Also as of 3.5 U2 (VC/vSphere 2.5 U2) you can use MS Cluster services to cluster the servers.