At Wed, 24 Jun 2015 14:06:30 -0400 CentOS mailing list <centos@xxxxxxxxxx> wrote:
>
> Gordon Messmer wrote:
> > On 06/23/2015 08:10 PM, Marko Vojinovic wrote:
> >> Ok, you made me curious. Just how dramatic can it be? From where I'm
> >> sitting, a read/write to a disk takes the amount of time it takes, the
> >> hardware has a certain physical speed, regardless of the presence of
> >> LVM. What am I missing?
> >
> > Well, there's best and worst case scenarios. Best case for file-backed
> > VMs is pre-allocated files. It takes up more space, and takes a while
> > to set up initially, but it skips block allocation and probably some
> > fragmentation performance hits later.
> >
> > Worst case, though, is sparse files. In such a setup, when you write a
> > new file in a guest, the kernel writes the metadata to the journal, then
> <MVNCH>
>
> Here's a question: all of the arguments you're giving have to do with VMs.
> Do you have some for straight-on-the-server, non-VM cases?
In the most *common* case the straight-on-the-server, non-VM case are the VM
themselves. Basically, in the vast number of servers you most commonly have
a host with a number of VMs. The VMs are the publicly visible servers and the
host is pretty much invisible. The VMs themselves won't be using LVM, but the
host server will be.
Otherwise...
I recently upgraded to a newer laptop and put a 128G SSD disk in it. My
previous laptop had a 60gig IDE disk. Since I didn't have any need for more
files (at this time!) I set the laptop with LVM. Because of how I do backups
and because of the kinds of things I have on my laptop, I have multiple
logical volumes:
newgollum.deepsoft.com% df -h
Filesystem Size Used Avail Use% Mounted on
/dev/mapper/vg_newgollum-lv_root
9.8G 5.7G 3.6G 62% /
tmpfs 1.9G 8.2M 1.9G 1% /dev/shm
/dev/sda1 477M 86M 367M 19% /boot
/dev/mapper/vg_newgollum-lv_home
4.8G 4.0G 602M 88% /home
/dev/mapper/vg_newgollum-scratch
30G 10G 18G 36% /scratch
/dev/mapper/vg_newgollum-mp3s
9.8G 5.1G 4.2G 55% /mp3s
I only have about 60gig presently allocated (there is about 60gig 'free').
And yes, this is a laptop with a single physical disk. Some day I might
create additional LVs and/or grow the existing LVs. I *might* even install a
VM or two on this laptop.
My disktop machine is also a host to a number of VMs (mostly used for build
environments for different versions / flavors of Linux). Here LVM is pretty
much a requirement, esp. since its disks are RAID'ed.
I also manage a server for the local public library. The host runs CentOS 6 on
the bare metal. It also provides DHCP, DNS, Firewall, and IP routing. The
library's workstations (for staff and patrons) are diskless and boot using
tftp, but they actually run Ubuntu 14.04 (since it is more 'user friendly'),
so I have a Ubuntu 14.04 (server) VM providing tftp boot for Ubuntu 14.04's
kernel and NFS for Ubuntu 14.04's root and /usr file systems. (The CentOS host
provides the /home file system.) And just as an extra 'benefit' (?) I have a
VM running a 32-bit version of MS-Windows 8 (this is needed to talk to the
library's heating system). This is a basic server, but uses virtualization for
selected services. Except for 'appliance' servers, I see things being more and
more common that pure 'bare metal' servers becoming the exception rather than
the rule. For all sorts of reasons (including security), servers will commonly
be using virtualization for many purposes. And LVM makes things really easy to
deal with disk space for VMs.
>
> mark
>
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--
Robert Heller -- 978-544-6933
Deepwoods Software -- Custom Software Services
http://www.deepsoft.com/ -- Linux Administration Services
heller@xxxxxxxxxxxx -- Webhosting Services
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