David Joo’s thoughts and feelings

Packet receive errors?
June 8, 2009, 10:52 am
Filed under: Linux Related... | Tags: , , , ,

“packet receive errors” usually means:
1) data is truncated, error in checksum while copying
2) udp queue is full, so it needs to be dropped
3) unable to receive udp package from encapsulated socket
4) sock_queue_rcv_skb() failed with -ENOMEM
5) it is a short packet
6) no space for header in udp packet when validating packet
7) xfrm6_policy_check() fails

many times it means the checksum is not right.


October 21, 2008, 10:16 am
Filed under: Uncategorized

Now this page has been linked from my work website 🙂

Upstart, Init SysV scripts’ replacement
October 21, 2008, 10:01 am
Filed under: Linux Related... | Tags: , , ,

I have been using F9 for awhile now, I am pretty happy with it so far.
Since the release of F9, I lost my laptop and had to install it again to a 3 years+ old laptop, X31.
But it is working great, I don’t have that much to complain about.

Yesterday, a colleague of mine asked a question about “UPSTART”.
There was an article on “Linux Format” with a small example.
Shamefully enough, I didn’t know that init has been replaced with the upstart.

For my own benefit, I have started to look up what it is, and how it is going to affect Fedora/RHEL future releases.

This is quite interesting because, it has been my jobs and interests to write SysV scripts for custom application to do “start/stop/status”. Also, I haven’t seen anything relating the upstart with “service” minds in RHEL and Fedora.

Because of that, how it is going to be integrated in F10 would be very interesting.

[LWN.Net] LPC: Upstart 1.0 plans: manifesto for a new init

Let’s make two things clear about Upstart, a proposed replacement for the Linux “init” process. First, it’s not there to speed up boot, and second, it’s not intended to parallelize startup. “Upstart is not for what most people think it is for,” said its author, Scott James Remnant, in a talk in the dbus miniconference at the Linux Plumbers Conference. What it is there for is to expand the capabilities of “init” on Linux, replace some scripts and workarounds with rules that are intended to be easier to understand and modify, and enable future improvements. Remnant is a Canonical employee, and Upstart is in Fedora as of version 9, making it a welcome example of a Canonical-sponsored project finding its way into other distributions.

While Greg Kroah-Hartman mentioned a list of core software on the Linux platform in his Plumbers Conference talk, “the one thing he never put in there was init,” Remnant said. The Linux init, originally by Miquel van Smoorenburg, has been unchanged for years, and is modeled on the System V Unix init, which is even older. Instead of updating it, Remnant says that, for too long, distributions have just worked around it. The startup process has traditionally consisted of shell scripts, started by init, but containing workarounds and extensions accumulated over the years. For example, Debian has a wrapper program called start-stop-daemon, that manages PID files, to keep track of what process ID a daemon process ends up with. Upstart handles that itself.

Current features of upstart include sending notifications for system events, for example, when a service starts; eliminating race conditions, by offering dependency tracking; and removing some service startups from the critical path for boot, again by handling dependencies. Upstart allows a distribution or sysadmin to spell out the critical path in a script, and also specify dependencies. Tracking dependencies allows distributions to eliminate “sleep” loops from the boot sequence, and instead take actions based on events.

Events are not limited to the runlevel changes familiar to sysvinit users, but can depend on other things on the system. But what other things? Future directions for Upstart could be ambitious. For 1.0, Remnant is considering adding the ability to do tasks based on cron-like criteria such as “hourly.” But should upstart really replace cron? Another possibly useful direction would be an “idle” event. The Common Unix Printing System (CUPS) is a service that makes sense to start “30 seconds before the user thinks of clicking on the print button,” he said. CUPS is not in the critical path for boot, but needs to be running to detect printers before the user needs them. Should it be possible to start non-critical services when the system becomes idle?

Even though fast boot isn’t the goal of upstart, Remnant is optimistic about being able to help. Some of the slow booting problems that Arjan van de Ven and Auke Kok identified at the conference are deep in the weeds of nested scripts, and might be smoked out by a simpler init layout. “To make boot fast we have to do a bunch of different stuff. it makes it easy for us to do the real work,” Remnant said.

[Fedora-devel-list] upstart plans for F10+

  • From: Bill Nottingham <notting redhat com>
  • To: fedora-devel-list redhat com
  • Subject: upstart plans for F10+
  • Date: Thu, 22 May 2008 16:04:45 -0400

Since I've been asked in various places what we're planning to 
do with upstart now that Fedora 9 has shipped, I figured I'd
lay out the basic plan.

To do any large-scale conversion of SysV init scripts to upstart,
we need some features that are not in the current (0.3.9) version.
Hence, the first thing is to get upstart 0.5 ready for inclusion.
For example, we need support for the following:

- Depending on multiple events

  I.e., have something start only if two separate events have
  both completed successfully. For a disturbing example of how
  we work around this now, read /etc/event.d/serial.

- Ability to enable/disable events in a way other than removing
  the file

  (The reasoning for this should be fairly obvious)

- Ability to group events into sets, or profiles

  This allows us to handle the sort of behaviors that runlevels are
  used for sanely.

- Ability to easily have upstart events depend on SysV scripts, and

  If one of a SysV scripts' dependencies is started by upstart, we
  need to be able to still handle that sanely.

This isn't meant to be an exhaustive list, but it is meant to
illustrate why we can't just move services right now.

Once we get upstart 0.5 in, we can then look at potentially moving
some subset of things that are now handled elsewhere to upstart.
Examples would be:

- Always-on services such as dbus, syslog, and audit
- Reworking things like netfs to be more sane, when
  it comes to networks coming and going, network block devices being
  attached and detached, and so on
- Potentially splitting some of rc.sysinit into multiple events.
  Not sure this buys us much, as right now the flow is *extremely*
  sequential (start_udev -> fsck -> remount r/w -> clean out /tmp)
- Coming up with a sane network notification strategy
  Right now, we have events kicked off on network changes:
  - via netreport
  - via NetworkManagerDispatcher
  - conceivably, via upstart (after all, spawning commands/etc based
    on events is its raison d'etre)
  Having a coherent strategy for apps to only need to worry about
  dropping things in one place would make sense.
- (possibly) having either upstart 'handle' sysv services more natively
  or wrap tools such as chkconfig, /sbin/service so they handle both
  SysV and upstart.

All clear as mud?


NFS and IPv6….
July 21, 2008, 3:40 pm
Filed under: Linux Related... | Tags: , , ,

Since I work closely with telecommunication company, I have seen few companies who have asked the status of RHEL’s IPv6 support.
With RHEL5 U2, I believe, it will be certified under TAHI.
However, there is a small downfall…. WHAT COULD THIS BE????

Short Answer?
NO, it doesn’t work.
There are couple of points where it can be pointed to;
* Red Hat’s Bugzilla
* Kernel Patch submitted on 18th of June 2008

[Link] Linux Kernel’s Interactive Map
July 21, 2008, 10:04 am
Filed under: Linux Related... | Tags: , , ,

Due to the fact that I work with Linux very closely, I always had a interests in the linux kernel. The only thing is that whenever I try to learn of it or even try to dig into it, I found myself disappointing at myself on lack of abilities to understand it.

Few years ago, there was a work conference that I attended and this was introduced as a nice tool to use when we do a kernel debugging.

Until now, it didn’t click, but during last week of discussions between Daniel, I realised that this would be a great tool to have when I do the debuggings.


[Link] 25 Great Photography Tutorials and Links From Around the Web
July 17, 2008, 11:29 am
Filed under: Photography | Tags: , ,

I am so fortunate to have around 3 friends who has the same passion as me on “Photography”.

One person is already in Professional Level and the other two friends are intermediate level.
I am not even close to their visions and skills, but I expect to learn a lot from them.
Also, I am so confident to say that they have taught me enormous amount.

Again, this morning, Norm, who to me, is in the professional level of photography skills, directed me to this link to learn. (Thanks Norm)

Following is extracts from the article;

17 Hot Photography Links From Around the Web


  1. Make Your own BikeCAM – a fun DIY with a video
  2. 5 Ways to Hold Your Viewer’s Attention – 5 simple composition techniques
  3. Transforming a Daylight Image into a Wild NightPhotoshop tutorial
  4. Nine Reasons to Manually Focus When Taking Pictures – sometimes manual focusing is your best option
  5. The Importance of focus and quick tips on how to get it right – another focusing related tutorial
  6. My Photography Workflow – Photographer Thomas Hawk talks us through his workflow
  7. The 15 Second DIY Adjustable Snoot! – a fast DIY
  8. The Nuts and Bolts of Off-Camera Flash – first part of a four part series on off camera flash (part 2 here).
  9. 8 Photography Myths Debunked – just what the title says 🙂
  10. Getting Rid of Dust Spots – Photoshopping tips
  11. Flicker Flash – How to capture movement with a series of strobe flashes
  12. Introduction to Digital Photography – 10 Ways to Improve your skills
  13. On Assignment: Night Chopper Pt 2 – 2nd in a series of shots taking you on a shoot of a chopper at night
  14. Your Guide to Buying Old Film Cameras – Feeling Retro? This one’s for you!
  15. Seeing the Possibilities – a great post on how to see possibilities in different locations
  16. Understanding Camera Exposure Modes – talking you through spot, evaluative and center-weighted metering.
  17. Curves color enhancement tutorial – a video screencast of GIMP in action

5 Tutorials from our Archives that You Might have Missed!

As usual in these posts I thought I’d throw in a handful of articles from our own archives – from July 2007 – for those of you who are new to Digital Photography School and might have missed some of what we’ve already covered:

  1. 18 Exceptionally Useful Photoshop Shortcuts – the most popular post from July last year
  2. 7 Strategies for Avoiding Flash Blow Out – we’ve all suffered from it – but how do you overcome flash blowout?
  3. 4 Rues of Composition for Landscape Photography – some basic ideas on composition
  4. Using Flash in Action Photography – tips and some cool photos
  5. How to Photograph Pets – always a popular photographic subject – Pets!

Start of New age….
July 15, 2008, 2:37 pm
Filed under: Daily Whines... | Tags:

So far I have used around 4-5 different blog site to blog.
Well, I am not a great writer but interested in trying things out and also trying to find the right flavour for my like.

Only different thing in this case is that for the first time, I am going to write articles/blogs in English rather than Korean that I have used till now.

My ultimate goal would be to host my own site somewhere and be able to find a permanant home for my ideas and photo, till then, I will be keep on moving around places to find a place for me.

Main things that I will be posting here are;

  • Linux Related articles
  • Photos that I take and things that I have learnt from other sites