Editing Diskless node (section)

==Principles of operation==

The operating system (OS) for a diskless node is loaded from a server, using [[network booting]]. In some cases, removable storage may be used to initiate the bootstrap process, such as a [[USB flash drive]], or other bootable media such as a [[floppy disk]], CD or DVD. However, the [[firmware]] in many modern computers can be configured to locate a server and begin the bootup process automatically, without the need to insert bootable media.

[[File:Carry-i-front-and-rear.jpg|thumb|center|600px|The ''[[Carry-I]]'' book-size LAN station was an early diskless system based on an [[Intel 80286]] processor and produced by Taiwan's [[Flytech Technology]] circa 1991.]]

For network auto-booting, the [[Preboot Execution Environment]] (PXE) or [[Bootstrap Protocol]] (BOOTP) network protocols are commonly used to find a server with files for booting the device. Standard full-size desktop PCs are able to be network-booted in this manner with an add-on network card that includes a [[Preboot Execution Environment|Universal Network Device Interface]] boot ROM. Diskless network booting is commonly a built-in feature of desktop and laptop PCs intended for business use, since it can be used on an otherwise disk-booted standard desktop computer to remotely run diagnostics, to install software, or to apply a [[disk image]] to the local hard drive.

After the bootstrapping process has been initiated, as described above, bootstrapping will take place according to one of three main approaches.

*In the first approach (used, for example, by the [[Linux Terminal Server Project]]), the [[kernel (operating system)|kernel]] is loaded into memory and then the rest of the operating system is accessed via a [[Clustered file system|network filesystem]] connection to the server. (A small [[RAM drive|RAM disk]] may be created to store temporary files locally.) This approach is sometimes called the "[[Network File System|NFS]] root" technique when used with Linux or Unix client operating systems.
*In the second approach, the kernel of the OS is loaded, and part of the system's memory is configured as a large RAM disk, and then the remainder of the OS image is fetched and loaded into the RAM disk. This is the implementation that [[Microsoft]] has chosen for its [[Windows XP Embedded]] remote boot feature.<ref name="microsoft">{{cite web|url=http://msdn2.microsoft.com/en-us/embedded/aa731381.aspx|title=Remote Boot Feature Overview|website=Windows Embedded Developer Center|archive-url=https://web.archive.org/web/20080423084418/http://msdn2.microsoft.com/en-us/embedded/aa731381.aspx|archive-date=2008-04-23}}</ref>
*In the third approach, disk operations are virtualized and are actually translated into a network protocol. The data that is usually stored in a disk drive are then stored in virtual disks files homed on a server. The disk operations such as requests to read/write disk sectors are translated into corresponding network requests and processed by a service or daemon running on the server side. This is the implementation that is used by [[Neoware]] Image Manager, Ardence, VHD Central Management System<ref name="vhdsoft">{{cite web|url=http://www.vhdsoft.com/index.php?menu_id=0&submenu_id=0|title=VHD Central Management System|website=Xtreaming Technology Inc.|access-date=2014-03-22|archive-url=https://web.archive.org/web/20140323032852/http://www.vhdsoft.com/index.php?menu_id=0&submenu_id=0|archive-date=2014-03-23}}</ref> and various "boot over iSCSI" products. This third approach differs from the first approach because what is remote is not a [[file system]] but actually a disk device (or [[raw device]]) and that the client OS is not aware that it is not running off a hard disk. This is why this approach is sometimes named "[[VHD (file format)|Virtual Hard Disk]]" or "Network Virtual Disk".

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This third approach makes it easier to use client OS than having a complete disk image in RAM or using a read-only file system. In this approach, the system uses some "write cache" that stores every data that a diskless node has written. This write cache is usually a file, stored on a server (or on the client storage if any). It can also be a portion of the client RAM. This write cache can be persistent or volatile. When volatile, all the data that has been written by a specific client to the virtual disk are dismissed when said client is rebooted, and yet, user data can remain persistent if recorded in user (roaming) profiles or home folders (that are stored on remote servers). The two major commercial products (the one from [[Hewlett-Packard]], and the other one from [[Citrix Systems]]) that allow the deployment of Diskless Nodes that can boot [[Microsoft Windows]] or [[Linux]] client OS use such write caches. The Citrix product cannot use persistent write cache, but VHD and HP product can.
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