When you write data, it doesn’t necessarily get written to disk right then. The kernel maintains caches of many things, and disk data is something where a lot of work is done to keep everything fast and efficient. That’s great for performance, but sometimes you want to know that data really has gotten to the disk drive. This could be because you want to test the performance of the drive, but could also be when you suspect a drive is malfunctioning: if you just write and read back, you’ll be reading from cache, not from actual disk platters.

So how can you be sure you are reading data from the disk? The answer actually gets a little complicated, particularly if you are testing for integrity, so bear with me.

Obviously the first thing you need to do is get the data in the cache sent on its way to the disk. That’s “sync”, which tells the kernel that you want the data written. But that doesn’t mean that a subsequent read comes from disk: if the requested data is still in cache, that’s where it will be fetched from. It also doesn’t necessarily mean that the kernel actually has sent the data along to the disk controller: a “sync” is a request, not a command that says “stop everything else you are doing and write your whole buffer cache to disk right now!”. No, “sync” just means that the cache will be written, as and when the kernel has time to do so.

Traditonally, the only way to be sure you were not reading back from the cache was to overwrite the cache with other data. That required two things: knowing how big the cache is at this moment, and having unrelated data of sufficient size to overwrite with.

On older Unixes with fixed sized buffer caches, the first part was easy enough, and since memory was often expensive and in shorter supply than it is now, the cache wasn’t apt to be all that large anyway. That’s changed radically: modern systems allocate cache memory dynamically and while the total cache is still small compared to disk drives, it can now be gigabytes of data that you need to overwrite. http://www.zerivista.com

Written by azerivista.com Miller.

A. The Dangers of Installing Linux on a Windows System

Many people have only one computer system, and this system is often running Windows. They still need to use Windows, but also want to learn Linux by installing it on their Windows system (rather than installing Linux on a new system or a used system). Installing Linux on an existing Windows system creates what is called a “dual-boot system”. When you boot (start up) a dual-boot system, you see a menu and you can choose to either boot to Windows, or boot to Linux.

Installing Linux on an existing Windows system can work for some, but can also be a disaster!

Linux Tech Tip: Beware that if you install Linux on a Windows system, you may damage or destroy your existing Windows software setup and lose some or all of the programs and data on the Windows system. Believe me, I know.

I had installed Linux on a Windows system and then removed Linux from the system, after I installed Linux on another system.

And I almost lost everything - data, setups, programs and all!

Fortunately I was able to boot the damaged Windows system with a “live” version of Linux, then do some steps to get the data off the system. After recovering the data, I had to re-install Windows, and all my Windows programs, but at least I got all my
data back!

B. Running Linux from a Linux “Live” CD or DVD

Another popular way to get Linux running is to boot a system, such as an existing Windows system, from CD or DVD with a “live” version of Linux. A Linux live CD or DVD isn’t used to install Linux on a system. It’s used to boot a system and then
automatically run Linux.

When you boot a system with a Linux live cd, the system boots up and then Linux starts up - and runs completely from CD or DVD - without having to be installed!

A Linux live CD or DVD gives you a fully functional version of Linux by simply booting with the live CD or DVD.

One of the reasons Linux live versions are so popular is that you don’t have to install them. Therefore, you avoid the danger of
damaging or destroying the software on an existing Windows system. This is a good solution if you already have a Windows
system, but don’t have a spare system on which to install Linux.

Written by Clyde Boom.

You can quickly and easily download Linux free as a virtual machine and run Linux in Windows!

Here’s how you can run Linux in Windows - You download and install the free Linux virtual machine player and a free Linux virtual machine. Then you run the player and “open” the virtual machine, in the same way as you run a spreadsheet program and open a spreadsheet.

Linux Tips You Need to Know - Before You Download Linux Virtual Machines

1. Some Linux virtual machines are in a small Linux download file - but get bigger! Some Linux download (virtual machine) files are very small in relation to others, and so they take much less time to download. However, once you open them in the virtual machine player, you are just starting to install them! These virtual machines are set up to start a “network installation” immediately after you open them in the virtual machine player.

When you open these files, a “Linux network installation” begins and you have to provide several configuration settings and install the Linux OS “over the internet”. This can be a very time-consuming way to get Linux in Windows.

Linux Tips: If you are a new Linux user, download Linux virtual machines that have been completely installed, as opposed to ones that need to be installed “over the network”. It takes longer to download Linux virtual machines that are completely installed, but you will save a lot of time in the long run.

2. Some Linux virtual machines have no time limit, but others only run for a limited time.

For example, Ubuntu Linux and Fedora Linux are available for free and have no time limit. However, the Red Hat Linux and SUSE Linux virtual machines have a limit to the number of days of use, such as 30 days. And you have to “sign up” for these Linux distributions by filling in a bunch of forms online.

Linux Tips: Unless you have a burning need to learn a specific Linux distribution, get one that doesn’t have a time limit. Ubuntu Linux is a very stable Linux distro that works well and is a relatively small Linux download.

Written by Clyde Boom.