Punya satu PC, tapi ingin simulasi jaringan? Ingin mencoba distro Linux baru, tapi tidak ingin mempartisi ulang? Ingin menjalankan program yang rawan crash? Atau ingin meng-install sistem operasi non-Intel? Sepertinya Anda butuh program virtual machine bernama Qemu
- Anda ingin mencoba distro Linux baru, tapi tidak ingin menghapus sistem Linux yang sudah ada dan juga tidak ingin mempartisi ulang hard disk.
- Anda ingin melakukan praktek konfigurasi jaringan tapi hanya punya satu PC. Anda pun tidak ingin memakai kabel UTP, hub, clamping dengan konektor RJ-45 dan perangkat lainnya bila ternyata butuh PC tam-bahan. Padahal Anda hanya ingin mencoba perintah jaringan sederhana seperti routing dan sejenisnya untuk lebih sekadar memahaminya.
- Kadang Anda ingin mencoba program ‘berbahaya’ seperti virus, trojan, atau program yang erat kaitannya dengan sistem seperti tool partisi dan boot manager.
- Anda ingin merasakan sistem PowerPC namun tidak punya komputer Macintosh.
Program virtual machine atau disebut juga emulator bekerja berdasarkan prinsip emulasi. Oleh karena cara kerjanya yang ‘menirukan’ aslinya tersebut, maka emulator tidak terlepas dari beberapa keterbatasan berikut ini.
- Anda tidak akan dapat menjalankannya secepat program atau hardware aslinya. Emulator bekerja dengan ‘meminjam’ resource komputer dan sistem operasi yang menjalankannya, sehingga kinerjanya terbagi dengan proses-proses lainnya yang juga sedang berjalan.
- Emulator tidak bebas bug. Suatu instruksi yang di platform aslinya (native) bekerja dengan baik, bisa saja menghasilkan output lain atau malah error di dalam emulator. Bug seperti ini bisanya secara bertahap dan kontinyu akan dikoreksi pada versi berikutnya.
- Hingga kini, emulasi belum menyediakan akses langsung ke beberapa periferal PC seperti modem atau joystick. Namun, tidak tertutup kemungkinan akses tersebut akan tersedia pada masa mendatang.
Virtual Machine(VM) adalah sebuah mesin yang mempunyai dasar logika yang menggunakan pendekatan lapisan-lapisan ( layers)dari sistem komputer.Sehingga sistem komputer dengan tersendiri dibangun atas lapisan-lapisan tersebut, dengan urutan lapisannya mulai dari lapisan terendah sampai lapisan teratas adalah sebagai berikut:
- Perangkat keras (semua bagian fisik komputer)
- Kernel (program untuk mengontrol disk dan sistem file, multi-tasking, load-balancing, networking dan security)
- Sistem program (program yang membantu general user)
Kekurangan Virtual Machine(VM)
Ada beberapa kesulitan utama dari konsep VM, diantaranya adalah:- Dalam sistem penyimpanan. Sebagai contoh kesulitan dalam sistem penyimpanan adalah sebagai berikut: Andaikan kita mempunyai suatu mesin yang memiliki 3 disk drive namun ingin mendukung 7 VM. Keadaan ini jelas tidak memungkinkan bagi kita untuk dapat mengalokasikan setiap disk drive untuk tiap VM, karena perangkat lunak untuk mesin virtual sendiri akan membutuhkan ruang disk secara substansi untuk menyediakan memori virtual dan spooling.Solusinya adalah dengan menyediakan disk virtual atau yang dikenal pula dengan minidisk, dimana ukuran daya penyimpanannya identik dengan ukuran sebenarnya. Dengan demikian, pendekatan VM juga menyediakan sebuah antarmuka yang identik dengan underlying bare hardware.
- Dalam hal pengimplementasian. Meski konsep VM cukup baik, namun VM sulit diimplementasikan.
Kelebihan Virtual Machine(VM)
Terlepas dari segala kekurangannya, VM memiliki beberapa keunggulan, antara lain:- Dalam hal keamanan. VM memiliki perlindungan yang lengkap pada berbagai sistem sumber daya, yaitu dengan meniadakan pembagian resources secara langsung, sehingga tidak ada masalah proteksi dalam VM. Sistem VM adalah kendaraan yang sempurna untuk penelitian dan pengembangan sistem operasi. Dengan VM, jika terdapat suatu perubahan pada satu bagian dari mesin, maka dijamin tidak akan mengubah komponen lainnya.
- Memungkinkan untuk mendefinisikan suatu jaringan dari Virtual Machine(VM). Tiap-tiap bagian mengirim informasi melalui jaringan komunikasi virtual. Sekali lagi, jaringan dimodelkan setelah komunikasi fisik jaringan diimplementasikan pada perangkat lunak.
A process VM, sometimes called an application virtual machine, runs as a normal application inside an OS and supports a single process. It is created when that process is started and destroyed when it exits. Its purpose is to provide a platform-independent programming environment that abstracts away details of the underlying hardware or operating system, and allows a program to execute in the same way on any platform.
A process VM provides a high-level abstraction — that of a high level programming (compared to the low-level ISA abstraction of the system VM). Process VMs are implemented using an intrepreter; performance comparable to compiled programming languages is achieved by the use of just inter environment.
This type of VM has become popular with the Java language programming, which is implemented using the Java virtual programming. Another example is the Net framework, which runs on a VM called the common language comment.
A special case of process VMs are systems that abstract over the communication mechanisms of a (potentially heterogeneous) computer cluster. Such a VM does not consist of a single process, but one process per physical machine in the cluster. They are designed to ease the task of programming parallel applications by letting the programmer focus on algorithms rather than the communication mechanisms provided by the interconnect and the OS. They do not hide the fact that communication takes place, and as such do not attempt to present the cluster as a single parallel machine.
Unlike other process VMs, these systems do not provide a specific programming language, but are embedded in an existing language; typically such a system provides bindings for several languages (e.g., C and Fortran). Examples are PVM ( Parallel Virtual Machine) and MPI ( Messange Passing Interface) . They are not strictly virtual machines, as the applications running on top still have access to all OS services, and are therefore not confined to the system model provided by the "VM".
System virtual machines (sometimes called hardware virtual machines) allow the sharing of the underlying physical machine resources between different virtual machines, each running its own operating system. The software layer providing the virtualization is called a virtual machine monitor or hypervisor. A hypervisor can run on bare hardware (Type 1 or native VM) or on top of an operating system (Type 2 or hosted VM).
The main advantages of system VMs are:
- multiple OS environments can co-exist on the same computer, in strong isolation from each other
- the virtual machine can provide an architecture (ISA) that is somewhat different from that of the real machine
- application provisioning, maintenance, high availability and disaster recovery
Multiple VMs each running their own operating system (called guest operating system) are frequently used in server consolidation, where different services that used to run on individual machines in order to avoid interference are instead run in separate VMs on the same physical machine. This use is frequently called quality-of-service isolation (QoS isolation).
The desire to run multiple operating systems was the original motivation for virtual machines, as it allowed time-sharing a single computer between several single-tasking OSes. This technique requires a process to share the CPU resources between guest operating systems and memory virtualization to share the memory on the host.
The guest OSes do not have to be all the same, making it possible to run different OSes on the same computer (e.g.,microsoft windows and Linux, or older versions of an OS in order to support software that has not yet been ported to the latest version). The use of virtual machines to support different guest OSes is becoming popular in embedded system; a typical use is to support a real time operating system at the same time as a high-level OS such as Linux or Windows.
Another use is to sandbox an OS that is not trusted, possibly because it is a system under development. Virtual machines have other advantages for OS development, including better debugging access and faster reboots.
Alternate techniques such as solaris zone provides a level of isolation within a single operating system. This does not have isolation as complete as a VM. A kernel exploit in a system with multiple zones will affect all zones. Achieving the same goal in a virtual machine implementation would require exploiting a weakness in the hypervisor. A hypervisor typically has a smaller "attack surface" than a complete operating system, making this more challenging. Further, a kernel exploit in a VM guest would not affect other VMs on the host, just as a successful intrusion into one zone would not necessarily affect other zones. Zones are not virtual machines, but an example of "operating system virtualization". This includes other "virtual environments" (also called "virtual servers") such as virtuozzo, Free Bsd Jails, Linux V-server , Chroot Jails and Open Jails. These provide some form of encapsulation of processes within an operating system. These technologies have the advantages of being more resource-efficient than full virtualization and having better observability into multiple guests simultaneously; the disadvantage is that, generally, they can only run a single operating system and a single version/patch level of that operating system - so, for example, they cannot be used to run two applications, one of which only supports a newer OS version and the other only supporting an older OS version on the same hardware. However, Sun Microsystems has enhanced Solaris Zones to allow some zones to behave like Solaris 8 or Solaris 9 systems by adding a system call translator.
Thin client system Virtual PC Center operates in the same way as a traditional desktop PC, enabling the user to access all typical desktop PC applications.
The only difference to the user is that they no longer need to worry about issues such as backing up their data, maintaining software, and so on.
You are able to access your desktop environment, your applications, and your data using any Virtual PC Center thin client terminal in the world.
You can stop working, disconnect your PC, and then reconnect later on, and still pick up from where you left off.