Information Technology Reference
In-Depth Information
Aligning Virtual Machine File systems
In Chapter 6, we introduced the concept of aligning VMFS, and we suggested that the VM's fi le sys-
tem should also be aligned. If you construct VMs with separate virtual hard drives for the operating
system and data, then you are most concerned with the alignment of the fi le system for the data
drive because the greatest amount of I/O occurs on that drive. For example, a VM with Disk 0 (that
holds the operating system) and a blank disk called Disk 1 (that holds data that will incur signifi cant
I/O) should have Disk 1 aligned. h e need to align the guest fi le system is applicable to almost all
distributions of Linux and all but the most recent versions of Windows. For example, Windows 7
and Windows Server 2008 align themselves properly during installation, but earlier versions do not.
Perform the following steps to align Disk 1 of a VM running a version of Windows earlier than
Windows Server 2008:
1.
Log into the VM using an account with administrative credentials.
2.
Open a command prompt, and type
Diskpart
.
3.
Ty pe
list disk
, and press Enter.
4.
Ty pe
select disk 1
, and press Enter.
5.
Ty pe
create partition primary align = 64
, and press Enter.
6.
Ty pe
assign letter =
X
, where
X
is an open letter that can be assigned.
7.
Ty pe
list part
to verify the 64 KB off set for the new partition.
8.
Format the partition.
Perhaps you are thinking that this seems like a tedious task to perform for all your VMs. It
is
a
tedious task; however, the benefi t of doing this is realized most when there is a signifi cant I/O
requirement.
As you can see, running your operating system in a VM offers advantages when it comes time
to reconi gure hardware, even enabling such innovative features as CPU hot-plug. There are
other advantages to using VMs too; one of these advantages is a vSphere feature called snapshots.
Using Virtual Machine Snapshots
VM snapshots provide administrators with the ability to create point-in-time checkpoints of a
VM. The snapshot captures the state of the VM at a specii c point in time. VMware administra-
tors can then revert to their pre-snapshot state in the event the changes made since the snapshot
should be discarded. Or, if the changes should be preserved, the administrator can commit the
changes and delete the snapshot.
This functionality can be used in a variety of ways. Suppose you'd like to install the lat-
est vendor-supplied patch for the guest OS instance running in a VM but you want to be able
to recover in case the patch installation runs amok. By taking a snapshot before installing the
patch, you can revert to the snapshot in the event the patch installation doesn't go well. You've
just created a safety net for yourself.
Other Features Leverage Snapshots Too
Snapshots are leveraged by vSphere Update Manager and are also used by various VM backup
frameworks.
