Hi all!.. Half of the company has Vmware (about 50 physical servers) and the other half is considering going to Xen, but we are confuse about the concepts of the two solutions. What is the main difference in the virtualization technique of Xen using VT enabled hardware and VMWare with and w/o VT? If Xen+VT is full virtualization, what are the differences between the full virtualization of vmware?? Thanks a lot!. Cheers MRB _______________________________________________ Xen-users mailing list Xen-users@lists.xensource.com http://lists.xensource.com/xen-users
> -----Original Message----- > From: xen-users-bounces@lists.xensource.com > [mailto:xen-users-bounces@lists.xensource.com] On Behalf Of MaTT > Sent: 01 September 2006 10:40 > To: xen-users@lists.xensource.com > Subject: [Xen-users] Virtualization Technique > > Hi all!.. > > Half of the company has Vmware (about 50 physical servers) > and the other > half is considering going to Xen, but we are confuse about > the concepts > of the two solutions. > > What is the main difference in the virtualization technique > of Xen using > VT enabled hardware and VMWare with and w/o VT? > If Xen+VT is full virtualization, what are the differences > between the > full virtualization of vmware??Vmware uses something called "Binary translation" to support the same ideas as the hardware virtualization technologies. This means that priviliged code is scanned for special instructions, and the special instructions are replaced with a "trap" (intercept) code to get into the VMM (Hypervisor). Hardware virtuailization technologies doesn''t need this, as there is functionality in the processor to figure out when any special instruction needs to be "trapped" (intercepted). Both solutions are using similar technology to emulate hardware (intercept the write operations either directly through IO-mapping intercept setup or through memory mapped page-table access restrictions), and then pretend to perform the instruction through software that changes the state of the emulated hardware based on the operation intercepted. There isn''t MUCH difference between these two technologies, except it''s MUCH easier to write the code to handle the HW virtualization - but there''s still plenty of difficult-to-write code to handle the memory mapping (OS''s in general expect that memory starts around address zero and grows up, but in a virtual machine, there may will be more than a single OS''s running on the same machine, so the memory start address has to be "faked" to make it look like it''s at zero for the OS, but actually be located elsewhere from the machines standpoint). And IO devices need to be emulated in some way or another (at least for any device that is shared between multiple guests). I''m not sure if this is reducing your confusion... But essentially what I''m sayins is that there''s not a WHOLE LOT of difference... Performance may vary a little bit, with extremely bad cases existing for both technologies. One additional point: Xen is also supporting para-virtualization for Linux, which is a much better approach in many ways - it allows optimization of the work within the hypervisor on the guests behalf - say for instance a guest is performing 1000 writes to the page-table to map a 4MB block of memory. Since the hardware virtualization technology doesn''t actually know what is happening, it has to intercept each of these writes individually, causing 1000 intercepts. The para-virtualization technology will modify the source-code to call the hypervisor for the write to page-tables, and it can easily add an extra parameter to indicate that multiple writes are needed, so it reduces the number of transitions between the hypervisor and the OS. Unfortunately, the para-virtualization technology also requires that the source-code is available to make changes to the source code and recompile it. That''s why it''s not available for Windows and other OS''s that aren''t "open source". -- Mats> > Thanks a lot!. > > Cheers > MRB > > _______________________________________________ > Xen-users mailing list > Xen-users@lists.xensource.com > http://lists.xensource.com/xen-users > > >_______________________________________________ Xen-users mailing list Xen-users@lists.xensource.com http://lists.xensource.com/xen-users
Hei all!. I was reading about paravirtualization outperforming hardware enabled virtualization, and virtual iron''s posts about paravirtualization as a dead end. Regarding performance and reliability, is paravirtualization still better solutions than HW assisted virtualization? Would HW reach a point where paravirtualization won''t longer be necessary? What are your thoughts about this? Thanks ! MRB _______________________________________________ Xen-users mailing list Xen-users@lists.xensource.com http://lists.xensource.com/xen-users
> -----Original Message----- > From: xen-users-bounces@lists.xensource.com > [mailto:xen-users-bounces@lists.xensource.com] On Behalf Of MaTT > Sent: 04 September 2006 10:48 > To: xen-users@lists.xensource.com > Subject: [Xen-users] VT/Pacifica - Paravirtualization > > Hei all!. > > I was reading about paravirtualization outperforming hardware enabled > virtualization, and virtual iron''s posts about > paravirtualization as a > dead end.The big problem with para-virtualization is that the OS source code needs to be modified. This is both time-consuming and "delaying" (there will always be a lag behind the official release, unless the OS has para-virtualization support from the original source code). Since each new release of the base-OS needs an update of the para-virt work, it will require work every time a new release is made - even if the release is only a minor upgrade some work is probably necessary.> > Regarding performance and reliability, is paravirtualization still > better solutions than HW assisted virtualization?Reliability in this case is very much on the hypervisor components, and there shouldn''t be much difference, except perhaps the maturity of the code [para-virt being more mature - but it''s countered by the full virtualization solution having slightly less code to deal with...]> > Would HW reach a point where paravirtualization won''t longer > be necessary?Possibly, but almost certainly it would require a new set of drivers, and it would take some time before those are available for the more common OS''s, and the less common (legacy) OS''s will probably never have them (but of coruse, they will not have Paravirtualization either, unless they are Open Source and someone decides that they want to put the effort in to redo the OS for Para-virt usage). There is work already in progress to create para-virtual drivers for Xen + Linux, so that a unmodified Linux kernel can be used to run with para-virtual drivers. As it''s drivers that cause the biggest performance loss when running full virtualization, this is going to be a great feature. When there will be para-virtual Windows drivers is another interesting question... Note that para-virtual drivers will currently not work on "bare-metal", and each VMM/Hypervisor would have slightly different interfaces for their para-virt drivers, so a VMWare para-virt driver for Windows would be very unlikely to work with Xen, etc. -- Mats> > > What are your thoughts about this? > > Thanks ! > > MRB > > _______________________________________________ > Xen-users mailing list > Xen-users@lists.xensource.com > http://lists.xensource.com/xen-users > > >_______________________________________________ Xen-users mailing list Xen-users@lists.xensource.com http://lists.xensource.com/xen-users