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Re: Pointers on LVM and Partitioning.

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On Wednesday, April 18, 2012 2:06:33 AM UTC+5:30, David Brown wrote:
> On 17/04/12 16:38, Harry wrote:
> > On Tuesday, April 17, 2012 12:53:12 PM UTC+5:30, David Brown wrote:
> >> On 17/04/2012 09:01, Harry wrote:
> >>> Hello,
> >>>
> >>> I know next to nothing about this topic as I write this, but
> >>> would like to learn *everything* about it. By 'everything', I
> >>> mean: I should be able to not just install and set up systems but
> >>> also troubleshoot and repair, should it become necessary.
> >>>
> >>> I don't have loads of hardware (CPUs or disks) with me to play
> >>> with; just a dual-core Fedora 16 desktop with a 250 GB disk, 4 GB
> >>> RAM, and may be a couple of USB 2GB+ sticks. Thus, for the most
> >>> part, I'll have to rely on some virtual machine environment to
> >>> make up for the necessary physical hardware.
> >>>
> >>> This self-learning exercise could last several months and, so,
> >>> before I proceeded with it, I thought I could use some helpful,
> >>> Getting Started tips from you forum members, like: * if there are
> >>> any good books that cover this topic thoroughly; * what kind of
> >>> virtual machine env I should setup (VirtualBox, Xen, or...) ? *
> >>> how did *you* learn this topic yourself (any helpful tips?)
> >>>
> >>> Would the virtual env be enough to learn and try out all
> >>> concepts that fall under the related topics of Partitioning, LVM&
> >>> (software) Raid? If not, can I get a list of things that I will
> >>> *not* be able to learn in such a virtual setup no matter what...
> >>> so that I know about them upfront?
> >>>
> >>> I have only a very vague idea right now of what all I will
> >>> possibly be doing. A partial list would be: setting up virtual
> >>> machines, creating lots of small-sized  virtual disks (say,
> >>> each<= 1MB), losetup's, dmsetup's, and other man Section 8
> >>> commands.
> >>>
> >>> Many thanks in advance. /HS
> >>
> >> Use VirtualBox - it's the easiest, fastest, and most powerful
> >> virtualisation software for desktops for general use.  KVM and Xen
> >> have some other benefits, and are perhaps a better choice for
> >> servers, but VBox is definitely my first choice for desktops.
> >
> > Would you like to elaborate just a little bit more on the benefits of
> > KVM and Xen, esp on servers? It's possible that, on my own, I may not
> > immediately notice such benefits, or any server vs client vm
> > differences. Any why VBox for desktops: why not KVM/Xen for desktops
> > as well?
> >
> 
> With Xen, you have a minimal OS on the bottom, rather than having quite 
> a substantial Linux host as needed for VBox.  I think Xen also makes it 
> easier to attach PCI cards and other hardware to particular virtual 
> machines (though VBox has experimental support for that sort of thing). 
>   KVM is built into the kernel, and thus is probably slightly more 
> efficient.  It also has support for merging duplicate pages, which can 
> be a benefit when you have lots of virtual machines.
> 
> VirtualBox, however, beats them both on ease of use on a desktop.  And 
> because it is installed as an application, rather than part of the 
> kernel or a low-level hypervisor, it is something that can be added at 
> any time to any machine.  It is also cross-platform, so that you can run 
> it on Windows and Mac hosts too, if that is of interest.

That was quite a useful info. 

> >> You can create multiple virtual disks and play with raid in
> >> virtual machines.  I'd recommend getting a second real disk -
> >> you'll value the extra space when you make lots of experimental
> >> virtual machines, and you can also put your virtual disks on two
> >> physical disks and see that raid makes a speed difference.
> >
> > That's a very good point.
> >
> >> Another trick is to use loopback files.  Make some files of the
> >> size you want, and set them up as loopback devices - then you can
> >> pretend they are disks without having to do any virtualisation.
> >> For extra points, put the files on a tmpfs file system - then they
> >> are not even on the disk.
> >
> > I'm simply dying to play with losetup and dmsetup, just don't know
> > what *all* practical applications to use them for. It would be
> > wonderful if you/sb could share some realworld applications where you
> > have put them to good use. Here's one that I've found so far:
> > http://linuxgazette.net/114/kapil.html
> 
> I have had three practical applications for loopback devices.  One is a 
> common situation - mounting an iso file using a loopback device (using 
> "mount -o loop" rather than explicitly creating the device).  That's 
> quite a common situation.

Yep, I have used it many times myself.

> Another was to create a small file in a tmpfs directory, export it as a 
> network block device, then from a small embedded Linux system I 
> connected to it as a network block device and used it as a swap drive. 
> It was a little slow doing swap over the network, but it gave the 
> embedded system more virtual memory.

Cool! I definitely intend to try that out with my desktop as the embedded system, and a coworker's desktop as the additional virtual memory source.

(You deserve some sort of a prize for that! Also, the Unix toolbox continues to astound me.)

> Finally, and more relevantly, I used a set of loopbacks on tmpfs files 
> to learn more about md raid and mdadm.  In particular, I played around 
> with safely resizing layered raids (i.e., resizing while always having 
> redundancy).  You can read the thread here:
> 
> <http://marc.info/?l=linux-raid&m=130255832215796&w=2>

I'm not very literate with Raids just yet, so will better be able appreciate this application (of loopbacks) may be few weeks/months down the road.

Thanks again, David!

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Thread

Pointers on  LVM and Partitioning. Harry <simonsharry@gmail.com> - 2012-04-17 00:01 -0700
  Re: Pointers on  LVM and Partitioning. David Brown <david@westcontrol.removethisbit.com> - 2012-04-17 09:23 +0200
    Re: Pointers on  LVM and Partitioning. Harry <simonsharry@gmail.com> - 2012-04-17 07:38 -0700
      Re: Pointers on  LVM and Partitioning. David Brown <david.brown@removethis.hesbynett.no> - 2012-04-17 22:36 +0200
        Re: Pointers on  LVM and Partitioning. Harry <simonsharry@gmail.com> - 2012-04-19 07:14 -0700
        Re: Pointers on  LVM and Partitioning. Doug Freyburger <dfreybur@yahoo.com> - 2012-04-19 16:51 +0000
  Re: Pointers on  LVM and Partitioning. Richard Kettlewell <rjk@greenend.org.uk> - 2012-04-17 12:50 +0100
  Re: Pointers on  LVM and Partitioning. Doug Freyburger <dfreybur@yahoo.com> - 2012-04-17 17:15 +0000
  Re: Pointers on  LVM and Partitioning. GangGreene <GangGreene@invalid.com> - 2012-04-17 18:05 -0400

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