Category Archives: Scripts
Using Linux network namespaces

You don’t get to explicitly use Linux namespaces very often; one usually gets to make use of them when setting up containers or some sort of smart web hosting platform that allows hard resource limits to be put in place for customers, but even then the actual setup is hidden somewhere in the back. There are scenarios when containers simply require too much work for the particular task; I, at one time, faced the need of ensuring some network communication between 2 instances of the same service.

Communication? Same service? Have them listen on different addresses or on different ports and you’re done, you might say – but it’s not always that simple. If you have no control over the code but just want to replicate a certain behavior, there may simply not be an option to have instances listen on different ports. If the protocol also involves broadcasting, things become really complicated, as you don’t always have 2 IP addresses, on different interfaces, connected to the same network. Such scenario is easy to solve with virtualization or containers, but for this particular problem they’re overkill. The lightweight solution comes from manipulating Linux network namespaces.

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XFS corruption – repairing

Note: The “magical” tool for XFS is (obviously) xfs_repair. Having it running can sometimes be the tough issue.

Introduction

How could a filesystem corruption happen? There are a couple of likely causes to it:

  • Kernel bugs: they are infrequent but they also did happen many times in the past and will still happen in the future. Not many things to be done about them, other than applying patches / keeping the kernel up to date;

  • Memory issues, e.g. memory errors propagated to the file system in control structures: they are usually mitigated with ECC memory but they can never be ruled out;

  • Underlying storage issues: quite unlikely but nevertheless possible;

  • Using the reset button on running servers: journaling file systems are almost always able to recover from such incident;

  • RAID controller issues: this could be the leading cause and not be easy to mitigate, even if firmware upgrade is sometimes possible.

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The “Debugging” interview – a few pointers

The long interview day is nearing its end. Googamazbook got the best and the worst out of you (well, neither of those, but I’m trying to put some literature in here); the last interviewer comes in, smiles condescendently and greets you with:

Time for the easy interview, heh?

Yes, you have all the reasons to be concerned and feel you’re just one step away from failure (yes, why didn’t you spend the day by the pool in the basement of the many stars hotel they got you a room in for the interview?). But without further ado, the questions start pouring in:

Question 1

How do you figure out if a process is CPU bound or I/O bound?

Tricky! Let’s not jump to the conclusion. There are 2 variables here, this means we have 4 possibilities:

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