VMware vCenter

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ESX is what is called a hypervisor. The hypervisor—ESX’s VMkernel—sits between the hardware and the virtual machine. Its primary task is to run virtual machines, as well as carry out other important ancillary tasks such as monitoring the performance of the processes it manages. This VMkernel is incredibly slim, leaving less of a “surface” area for vulnerabilities that could make it unstable or open to attack by intruders. So, ESX is like an appliance that sits in your rack. Indeed, VMware could have taken the original equipment manufacturer (OEM) model of buying servers with truckloads of CPU and RAM, and then yanked the bezels off the front, attached a VMware bezel, and sold ESX as a VMware appliance. Fortunately, VMware didn’t use this hardware route to market. Instead, the company partnered with existing OEMs and leveraged the OEMs pre-sale, support, and access to clients. For example, ESX is shipped on a USB stick inside certain IBM servers. To boot from the ESX USB stick, you merely enable it in the BIOS. This is done with all servers of this type, despite the fact that the customer might not use ESX. It’s a good indication of how ESX has become the de facto platform in the corporate datacenter for running virtual machines.

VMware Server (formerly VMware GSX Server) is a free-of-charge virtualization-software server suite. VMware Server has fewer features than VMware ESX, but can create, edit, and play virtual machines. It uses a client–server model, allowing remote access to virtual machines, at the cost of some graphical performance (and 3D support). It can run virtual machines created by other VMware products and by Microsoft Virtual PC.
In January 2010 VMware Server was declared End Of Availability; general support will end on June 30, 2011[3].

VMware vCenter Server (formerly VMware VirtualCenter) is the most common method of managing many ESX (and GSX) servers and the virtual machines.
vCenter provides:
- centralized control and visibility at every level of virtual infrastructure
- proactive management
- scalable and extensible platform that forms the foundation for virtualization management
- open plug-in architecturewith vCenter Server APIs (>300 Vmware partners directly integrate with vCenter Server)
VMware vCenter Server position and role (source: www.vmware.com)

vCenter Server allows you to centrally manage hosts from either a physical or virtual Windows machine (Linux version in tests), and enables the use of advanced features such as:
- VMware Distributed Resource Scheduler (DRS; continuously monitors utilization across a resource pool and intelligently allocates available resources among virtual machines e.g. Increase energy efficiency by running fewer servers and dynamically powering down unused servers),
- VMware High Availability (HA; Monitors virtual machines to detect operating system and hardware failures. Restarts virtual machines on other physical servers in the resource pool without manual intervention when server failure is detected),
- VMware Vmotion (move running virtual machines from one physical server to another with no impact to end users).
Linked mode allows the administrator to consolidate multiple vCenter instances together into a single view and log in once via the vSphere Client
- allows for multiple vCenter servers to share information between them
- up to 1,000 ESX physical servers and 10,000 VMs (vs 300 and 3000 without)
- uses a ADAM (Active Directory Application Mode) that allows vCenter instances to share:
-- User roles definitions
-- Connection information (IP addresses and ports)
-- Certificates and licenses (No more license server)
Minimum Requirements for vCenter Server:
- Microsoft SQL Server 2005 Express disk requirements – Up to 2GB free disk space to decompress the installation archive. Approximately 1.5GB of these files are deleted after the installation is complete.
- CPU – 2 CPUs
- Processor – 2.0GHz or faster Intel or AMD processor. Processor requirements might be higher if the database runs on the same machine.
- Memory – 3GB RAM. Memory requirements might be higher if the database runs on the same machine. vCenter Server includes a service called VMware VirtualCenter Management Webservices. This service requires 128MB to 1.5GB of additional memory.
- Disk storage – 2GB. Disk requirements might be higher if the database runs on the same machine.
- Networking – Gigabit connection recommended.

vCenter Server used ports:
80 - for direct HTTP connections; redirects requests to HTTPS port 443; useful if accidentally use http://server instead of https://server.
389 - must be open on the local and all remote instances of vCenter Server; LDAP port number for the Directory Services for the vCenter Server group. The vCenter Server system needs to bind to port 389, even if you are not joining this vCenter Server instance to a Linked Mode group. If needed (eg the instance is serving as the Microsoft Windows Active Directory), you can run the  LDAP service on any port from 1025 through 65535.
443 - The default port that the vCenter Server system uses to listen for connections from the vSphere Client, the vSphere Web Access Client and other SDK clients.
636 - For vCenter Linked Mode.
902 - The default port that the vCenter Server system uses to send data to managed hosts. Managed hosts also send a regular heartbeat over UDP port 902 to the vCenter Server system.
902/903 – used by the vSphere Client to display virtual machine consoles.
8080 Web Services HTTP. Used for the VirtualCenter Management Webservices.
8443 Web Services HTTPS. Used for the VirtualCenter Management Webservices.

Taking a snapshot saves the current state of the virtual machine, so you can return to it at any time – where current state means:
- the virtual machine’s power state (powered-on, powered-off, suspended, etc).
- data - all the files that make-up the virtual machine, including disks, memory, and other devices, such as virtual network interface cards.
The child disk which is created with a snapshot, is a sparse disk. Sparse disks employ the copy-on-write (COW) mechanism, in which the virtual disk contains no data in places, until copied there by a write:
- If a virtual machine is running off of a snapshot, it is making changes to a child disk. The more write operations made to this disk, the larger it grows.
- The space requirements of the child disk are in addition to the parent disk on which it depends.  If a virtual machine has a 10 GB disk with a child disk, the space being used will be 10 GB + the child disk size. 
- Child disks have been known to grow large enough to fill an entire datastore.
- The speed at which child disks grow are directly dependent on the amount of I/O being done to the disk.
- The size of the child disk has a direct impact on the length of time it takes to delete the snapshot associated to the child disk.http://www.vmware.com/
How snapshots work (source: www.vmware.com)

A clone is a copy of an existing virtual machine. The existing virtual machine is called the parent of the clone. A clone's MAC address are different from those of the parent virtual machine.
Two kind of clones are possible:
- Full Clones
- A linked clone - is made from a snapshot of the parent. All files available on the parent at the moment of the snapshot continue to remain available to the linked clone. Ongoing changes to the virtual disk of the parent do not affect the linked clone, and changes to the disk of the linked clone do not affect the parent. A linked clone must access the parent. Without access to the parent, a linked clone is disabled.

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