As I’m working through my very first ebook under the Unix Tutorial brand, I’m learning lots of very interesting things about managing software RAID devices.

Part of effective RAID management is knowing where you stand. I’ve shown you how to use /proc/mdstat already, but today’s command is a higher level one (works with all storage devices, not just RAID).

I use lsblk to report sizes of all the storage devices in my Linux system along with mount points (if known) and RAID membership info.

For each RAID device, you’ll see the following:

• type of RAID array – raid0, raid5, etc
• RAID array that any device is a member of

Check this out:

root@ubuntu18:~# lsblk -o NAME,SIZE,FSTYPE,TYPE,MOUNTPOINT
NAME SIZE FSTYPE TYPE MOUNTPOINT
loop0 86.6M squashfs loop /snap/core/4486
loop1 86.9M squashfs loop /snap/core/4917
sda 10G disk
├─sda1 1M part
└─sda2 10G ext4 part /
sdb 1G linux_raid_member disk
└─md0 2G ext4 raid0 /raid0
sdc 1G linux_raid_member disk
└─md0 2G ext4 raid0 /raid0
sdd 1G linux_raid_member disk
└─md1 1023M ext4 raid1
sde 1G linux_raid_member disk
└─md1 1023M ext4 raid1
sdf 1G linux_raid_member disk
└─md2 2G ext4 raid5
sdg 1G linux_raid_member disk
└─md2 2G ext4 raid5
sdh 1G linux_raid_member disk
└─md2 2G ext4 raid5
sdi 1G disk
sdj 1G disk
sr0 1024M rom

In this output you can see that sdb and sdc are members of RAID0 array md0. sdd and sde are forming RAID1 called md1. Finally, sdf/sdg/sdh are members of RAID5 array.

This is a post from another blog of mine, which I’m shutting down. I like the way these virtualization concepts were worded in such a relatively simple way, so I’m keeping the post 🙂

Virtual machine (VM) is a software implementation of a machine (computer) that executes programs like a real machine.

Two categories of virtual machines

Virtual machines are separated in two major categories, based on their use and degree of correspondence to any real machine.

A system virtual machine provides a complete system platform which supports the execution of a complete operating system (OS).

In contrast, a process virtual machine is designed to run a single program, which means that it supports a single process. An essential characteristic of a virtual machine is that the software running inside is limited to the resources and abstractions provided by the virtual machine — it cannot break out of its virtual world.

System Virtual Machines

System virtual machines (sometimes called hardware virtual machines) allow the sharing of the underlying physical machine resources between different virtual machines, each running its own operating system. The software layer providing the virtualization is called a virtual machine monitor or hypervisor. A hypervisor can run on bare hardware (Type 1 or native VM) or on top of an operating system (Type 2 or hosted VM).

Main advantages of system VMs

• multiple OS environments can co-exist on the same computer, in strong isolation from each other;
• the virtual machine can provide an instruction set architecture (ISA) that is somewhat different from that of the real machine.

Main disadvantages of system VMs

• there’s still an overhead of the virtualization solution which is used to run and manage a VM, so performance of a VM will be somewhat slower compared to a physical system with comparable configuration
• virtualization means decoupling from physical hardware available to the host PC, this usually means access to devices needs to go through the virtualization solution and this may not always be possible

Multiple VMs each running their own operating system (called guest operating system) are frequently used in server consolidation, where different services that used to run on individual machines in order to avoid interference are instead run in separate VMs on the same physical machine. This use is frequently called quality-of-service isolation (QoS isolation).

Process Virtual Machines

A process VM, sometimes called an application virtual machine, runs as a normal application inside an OS and supports a single process. It is created when that process is started and destroyed when it exits. Its purpose is to provide a platform-independent programming environment that abstracts away details of the underlying hardware or operating system, and allows a program to execute in the same way on any platform.

A process VM provides a high-level abstraction — that of a high-level programming language (compared to the low-level ISA abstraction of the system VM). Process VMs are implemented using an interpreter; performance comparable to compiled programming languages is achieved by the use of just-in-time compilation.

This type of VM has become popular with the Java programming language, which is implemented using the Java virtual machine. Another example is the .NET Framework, which runs on a VM called the Common Language Runtime.

See Also

• VirtualBox
• Hardware Virtualization

Lately I’ve been doing a lot of research with virtual machines created in VMware Workstation. And one of the first things you become conscious about is that disk performance in your virtual machine is far from being ideal. Don’t be too quick to blame VMware though, there’s quite a few reasons related to poor I/O, and that’s why I decided to give you a few tips to achieve maximum performance.

1. Pre-allocate virtual disk space

By default, your VMware Workstation doesn’t pre-allocate space for the disks of your virtual machine. This means that as demand for more space inside your VM grows, new storage units are being reserved by VMware Workstation and allocated from the host OS filesystem.

Naturally, such an approach is very expensive performance-wise, but it’s used by the default because it helps you save up space on your host OS filesystem – your virtual machine will only use up as much space as it needs rather than pre-allocating the whole agreed volume of the virtual disk.

Pre-allocation is one of the easiest ways to achieve best performance in VMware Workstation VMs – it creates the file before you get to run your VM, which means when the space is required, your VM will simply use portions of its virtual disk file, without having the VMware Workstation allocating new storage blocks from the host OS filesystem.

Here’s how a dialogue might look (this is a VMware Workstation 5 screenshot, but it should look similarly in VMware Workstation 6 as well):

Simply tick the “Allocate all disk space now” option.

2. Disable automatic file protection in your anti-virus software

Quite often, the reason you have a less than perfect I/O in your virtual machine is because you have an anti-virus software actively monitoring your host OS filesystem, and every time VMware Workstation access the virtual disk of your VM this file needs to be scanned by the anti-virus software.

Luckily, you can exclude certain files or directories from being scanned by your anti-virus, and in our case simply making sure .vmdk files are skipped is enough to improve the performance.

In Symantec AntiVirus, this is done using the File System Auto-Protect dialogue:

As you can see, there’s a special option there: Exclude selected files and directories, which is not active by default. Once you turn this option on, the Exclusions button to the right of it becomes active (it’s greyed out on this screenshot as you can see).

Clicking the button will give you two options: specify the extensions of the files you’d like to be excluded from the auto-protect, or simply point out the files and directories in your filesystem which should be ignored.

Speaking of VMware Workstation, it’s probably easier for you to make the .vmdk extension ignored – this will take care of all the virtual machines and their virtual disk files, wherever they are – you won’t have to specify exact directories, and if you ever move your virtual machine to another directory or disk, you won’t have to change any of these exclusion options again.

The extensions dialogue looks like this:

Simply specify the extensions you want ignored, and you’re done!

3. De-fragment the host OS filesystem with files for your VMs

This is another very important step: you have to regularly de-fragment the host OS filesystem, because virtual disk files for your VMs are quite large by their nature, and therefore are likely to be defragmented.

With virtual disk space pre-allocation, your will be in a slightly better position, but de-fragmenting the host OS filesystem is still a good idea.

To make it even better, you can create a separate filesystem to store VMs, this way the amount of created/deleted files there will be minimal, thus greatly reducing the filesystem fragmentation.