RayLopez99 said:
I am using VirtualBox by Oracle. works fine. But questions:
1) if you 'take a snapshot' do you need to physically backup your VM VirtualBox virtual folder as well? I don't think so. I've restored from a snapshot no problem, so why bother with backing up a virtual folder (other than to be extra safe)?
2) how much slower is a program when running under a virtual OS as opposed to the real OS? Assume the same OS (Windows 7). Reason: I compile complicated programs and do notice there's a performance hit when compiling the same program in Visual Studio when in the virtual OS. But I can't quantify it, other than it seems somewhat slower. Anybody else notice this?
RL
Maybe I can answer (2).
a) Using SuperPI as a benchmark, I can get 90% of the performance
of one core of my processor, when running in a Virtual Machine.
That's X86 guest instructions, running on an X86 host, so no
instruction translation of note is involved. Things running in Ring3
shouldn't need any assistance (except when virtual I/O is called for
and that is emulated). But compute-bound performance, staying in L1 or
L2, can get to the 90% level. The rest might be scheduler overheads and
context switching once in a while.
Using other tools in the past, running X86 on Mac or X86 on Sparc,
you might get anywhere between 1% to 10% of the CPU core, in terms
of performance. Even when translated instruction sets are cached,
just the emulation process itself is slow (ten instructions in
the native instruction set, to do the work of one instruction in the
foreign instruction set).
But if the virtual machine and the host use the same instruction set,
things can work very well indeed.
b) OK, you're compiling. You should be aware that disk I/O can be
painfully slow. Disk I/O doesn't run anywhere near native speed.
For a laugh, you could install HDTune, and see if it'll run in
a guest or not. Then see what kind of results it gets.
Same can happen with network operation. The host may be able to
access another computer at 117MB/sec, but a guest OS in a VM
gets around 1MB/sec. Now, that's slow. Especially when using
network shares, as a means to access information in some host
somewhere.
Virtually any storage I/O method that matters in there, is slow.
Many times, I've had to walk away from the computer, and prepare
a meal, while I'm waiting for some large file transfer to complete.
*******
With regard to your first question, I think VirtualBox has a manual,
which is more than can be said for some VM software. You should
browse that first. I've also read a riveting document that discussed
time keeping in the virtual environment, and how the time of day is
kept up to date. This would be important for a person compiling, because
if the date stamps applied to recently written files are wrong, your
compiler won't selectively update executables properly.
A virtual machine can be "frozen" and written out. That involves
something similar to hibernation, in that the virtual machine
memory is written to a disk file on the host. If you use the little
"X" in the upper right hand corner of a VM window, and select "power
off", that would do a dirty shutdown, similar to turning off the power
on a real PC in mid-session. If you select "shutdown" from within the
OS, that's an orderly controlled shutdown, and no hibernation type
file need be written in that case. The file system is stored on one
or more .vhd files or equivalent.
I have one set of VM files, which consists of just the .vhd stuff.
If that VM is corrupted by malware or some experiment goes bad,
I simply toss the existing .vhd files, and copy the "clean" set
into the appropriate place. That's an example of using a backup
copy, to clean up a mess.
I hardly ever archive the current bloated .vhd files from
virtual machines. I've lost a couple VMs due to corruption,
and the lesson I've learned from that, is it is best to do a full
shutdown on a VM from inside, rather than "hibernating" the thing
over and over again. In the past, that's lead to a disaster.
I have a couple Linux VMs at around the 30GB size, and it's just
too much work to archive a thing like that. Some of the other
VMs are a bit smaller, and if there was something valuable in there,
it might be more practical to back up. Most of my VMs aren't for
serious work, and are throw-aways. If they die, I won't be too
upset.
I would at least keep your source files for your software development,
somewhere other than inside that .vhd. Maybe ZIP up the source
tree once in a while, and copy it back to the host side for backup.
The rest of the environment can probably be bolted back together if
it broke.
Paul
