Older blog entries for logic (starting at number 173)

Quick initrd note

This is mainly a note for myself, but someone else might find it handy. Working with modern initrds requires monkeying with cpio, which I can never remember the correct command line parameters for off the top of my head. So, here's a quick workflow (borrowed from Chip Shabazian) for manipulating an initrd image.

mkdir initrd
cd initrd
gzip -dc path/to/unzipped/initrd.img | cpio -id
# Make the changes you need
find . | cpio -c -o | gzip -9 > /path/to/new/initrd.img

Chip Shabazian's presentation at LinuxWorld

Syndicated 2007-07-18 12:11:00 from esm

Are you on...the INTERNET?

This is too cool for words:

God, I feel old.

Syndicated 2007-07-12 21:42:00 from esm

Observations on Geek Behavior

I came across a del.icio.us link to this article on Coding Horror this morning, and it interested me on several levels.

First, there's the obvious: the article talks about the difficulty in locating programmer candidates that can perform basic programming assignments in an interview setting. Several commenters hit on some of the things that can contribute to this problem, including the psychology of an interview environment, the ability of the candidate to manage their stress, and the obvious problem that there's a lot of CS graduates who haven't spent enough time behind a keyboard to be useful in a production programming shop. That's what his article is about, and I'll leave that discussion there.

I'm much more interested in the response to the article. Responses range from senior developers to folks still in college or high school, and the resounding pattern is: they want to solve the problem. That's not what the author was shooting for, obviously, but readers were falling all over themselves (myself included) to post up the first response in their favorite language.

My speculation is that programmers who love what they do - the kind of folks who hang out on programming-related websites, who read the latest O'Reilly books when they come out, who participate in open source projects and "scratch itches" regularly - can't ignore an opportunity for practice, much like what Dave Thomas talked about when he introduced his idea of "Code Kata". Throw out the accusation that most programmers can't solve a simple problem, and you've practically demanded that they take a swipe at it.

There's more to it, though; lower the barrier enough, and more and more people get attracted to the idea. You see this kind of behavior on topical mailing lists: ask a complex question, and you may get an answer, but not a lot of people will jump up and offer their help. Ask an easier question, perhaps something that a lot of people know the answer to (or can easily derive the answer to), and suddenly, you get an overwhelming response. There's an aspect of ego at play here that's interesting; it's an opportunity to present yourself as a subject matter expert, or at least as someone knowledgable of issues surrounding the question.

Interesting stuff, to be sure. I suspect there's a good group dynamics case study or paper to be had here for someone with the time.

Syndicated 2007-02-27 10:15:00 from esm

Ruby ActiveRecord and lazy instantiation

Let's say I have a data set in a SQLite database. And, let's say I need to iterate over every row in that database, performing some action based on it. There's a few ways I can do this. First, the old, direct-to-driver version:

require 'sqlite3'

db = SQLite3::Database.new("test.db")
db.execute("select * from test_entries") do |row|

SQLite example

That works, but of course, it's bound forever to SQLite, and we'd like to abstract that a bit. The "new hotness" appears to be ActiveRecord. So, let's reproduce that using AR:

require 'active_record'

class TestEntry < ActiveRecord::Base; end

  :adapter => 'sqlite3', :dbfile => 'test.db'

TestEntry.find(:all) do |entry|

ActiveRecord example

Here's a tip: don't do that. ;-) That find call actually instantiates an ActiveRecord::Base for every single row in the database, then returns the result set, which you're now iterating over. So, for anything but a trivially-small result set, you're going to use a ton of memory, and create an enormous workload for very little good reason.

So, I suppose this blog entry is really just a whine: why isn't there a lazy-instantiation iterator for result sets in ActiveRecord? In this (common, I'd think) case where you want to do some sort of trivial filter or action across the entire table on a per-row basis, but don't care about the row after you've worked with it, that would be a huge win. The interesting thing is that this is exactly how the auto-generated association methods work, and you actually have to go out of your way to do eager loading of associations (since just automatically doing joins under the hood is an obviously bad idea).

My specific use case is an application log stored in a table that I'd like to iterate over for some basic statistical analysis, and a single day of logs is a 600M SQLite database (approximately 3.6M rows). This won't fly for doing a single day of processing, and I'd like to be able to do something for monthly and quarterly reporting.

I'll probably resort to DBI to accomplish this in a database-agnostic manner, but that feels like an incredible step backwards. I just can't understand why the find* methods weren't written as iterators that do lazy instantiation of the result set; the interface would be effectively the same, but the potential performance gains could be tremendous in certain cases.

Syndicated 2007-01-18 12:01:00 from esm

Linux on a BladeRunner system

So, you bought one of those shiny new Penguin Computing BladeRunner systems, and were thinking to yourself: "I wish there were a good guide to getting Linux to do what it should on these things". Well, I'm going to try and cover two of the basics here: serial console configuration, and interface bonding. Everything else is pretty much stock stuff, but I had a heck of a time figuring out configurations that worked here. The discussion below assumes you're running RHEL or Fedora, but the idea should be fairly clear.

First step: learn about some of the remote management features of the BladeRunner. You have a couple of configuration commands that are useful here, related to console management and power. Log into the chassis, and type conf; for some reason, they put this under "configuration" rather than "management" or some other similar tree of the command set. Now, you can control power to individual blades with server-blade power <num> [cycle|forced-off|off|on] (where <num> is the number of the blade you're managing). on means exactly what it sounds like; power on the blade if it's currently off, just like it would if you hit the power button on the front. off sends an ACPI power vevent to the blade, advising it to shut down; it doesn't actually force power to be pulled; that's what forced-off does. cycle removes power to the blade, and adds it again; off the top of my head, I don't recall if it does so gracefully or not. Check the documentation, it might be there (but probably not).

The next command that's interesting here is server-blade console-redirect <num>: that grabs the serial console for that particular blade, and displays it to your current session. The serial console configuration appears to be set in stone, from what I can see: 57600 bps, 8 data bits, no parity, 1 stop bit, with hardware (RTS/CTS) flow control. If you power up a blade fresh from the factory, you'll notice that the BIOS is already set up to redirect output to both the serial port and the built-in KVM in the chassis.

That's about all I'll say about the chassis management side of things, since they actually do cover this stuff fairly well in the documentation. The problem I found with the documentation was that it didn't cover host-side configuration of certain things, specifically interface bonding and serial console configuration. Maybe they thought it was out-of-scope for their documentation; I hate to be the bearer of bad news, but the chassis is pretty useless without systems to run on it. ;-) (Here endeth the snide remarks, hopefully.)

Next up is host-side serial console configuration. This is fairly standard stuff, once you know what the settings need to be, but I'll spell it out here too. First is the bootloader; GRUB, in my case. You'll want to add a couple of lines to the GRUB configuration file (in the main stanza):

  serial --unit=0 --speed=57600 --word=8 --parity=no --stop=1
  terminal --timeout=10 serial console

Optionally, you can also add hiddenmenu to the options; that reduces the amount of text being displayed at boot time to a minumum, unless you need it. Set the timeout on the terminal line to whatever is appropriate for you; 10 seconds before you're kicked to the primary display was good enough for my needs, since I didn't want the boot process to take much longer than it already did.

Next up, also in the grub configuration file (on the command line of each kernel you want to boot), are the kernel parameters that get your boot output sent to the right place. Adding console=tty0 console=ttyS0,57600n8r to the kernel command line seemed to do the job for me (display on /dev/ttyS0 at 57000 bps, no parity, 8 data bits, hardware flow control). If this is a stock Red Hat/Fedora installation, I suggest getting rid of the rhgb and quiet entries on that line as well; for server use, you really want to see that console output, so that when you inevitably get a kernel that has a bad day, you don't have to monkey with grub configuration via a 57600bps serial line to capture the panic.

The final step is making it possible to log in on the console. Adding the following line to /etc/inittab worked great for my needs:

  co:2345:respawn:/sbin/agetty -h ttyS0 57600 vt100

The co is an name for the entry, but it's become a defacto standard for the inittab name for the serial console entry. 2345 should be obvious (runlevels to operate in), respawn means run it again after you log out, not just once. The agetty command line just specifies hardware flow control (-h), linux device (ttyS0), speed (57600), and terminal type (vt100); change terminal type to anything appropriate for your environment, but vt100 is pretty safe for most folks.

That covers serial console configuration. For more details on that piece of the puzzle, I strongly recommend reading over the Remote Serial Console HOWTO; it discusses some of the ins-and-outs of other boot loaders, some other configurations you might be interested in trying, and some general advice for this kind of setup.

Next up is interface bonding; every blade has two Broadcom NetXtreme BCM5780S Gigabit Ethernet NICs built in, each connected to one of the two management blades through an internal backplane. I've been using the tg3 drivers for these quite successfully; the bcm5820 module is also available as a third-party add-on, but appears to be deprecated at this point. So, your /etc/modules.conf or /etc/modprobe.conf (depending on version) will likely have a couple of lines in it like alias eth0 tg3 for each interface. Next up is bonding configuration: add the following lines:

  alias bond0 bonding
  options bonding mode=2 arp_interval=500 arp_ip_target=

Change to whatever your default gateway is. You might be asking yourself why you shouldn't use MII instead of ARP on these systems, and it's a good question. The failure mode of the chassis internal network precludes it; you can end up with one management switch down, but the MII status of the ethernet interface still shows it being up. You can test this out by setting your bonding configuration to use MII instead, then yanking out one of the chassis management blades.

The next piece is very specific to RHEL or Fedora; please refer to your Linux vendor's documentation for details on how to do it elsewhere. In /etc/sysconfig/network-scripts, create a file called ifcfg-bond0 that looks exactly like a normal ifcfg-* file (in fact, you might want to just move your existing ifcfg-eth0 or similar file there); specify how you want this combined interface to behave. Then, for each of ifcfg-eth0 and ifcfg-eth1, add a file that looks like:


(Change DEVICE= to the appropriate line for each device.) More information on interface bonding under linux is available from the OSDL Linux networking site.

That's the main stuff. Obviously, test all of these changes with a complete boot to make sure console redirection is happening the way you want it, and to be sure that your bonded interfaces are showing up as a single bond0 interface (and that you have connectivity to and from them). Good luck. :-)

Syndicated 2006-08-26 10:23:00 from esm

Production vs. Development

I've been doing UNIX administration for quite a few years now, and I know a few other people who would call this their profession. I've been noticing, though, a rather drastic difference in attitude toward systems and users in some of them, and I think I've finally nailed down where that difference might be coming from. Fair warning: as usual, this is written basically as a stream-of-consciousness kind of thing, so I may or may not actually get to the point.

You have a few different priorities as a systems administrator, and your job is to juggle them using your best judgement (weighing business needs, available resources, personal time, etc). One of these priorities is production support; keeping the systems that run your business online and performing whatever task it is that they are supposed to be doing (in the case of an Internet-facing business, for instance, this could mean keeping the web, app, mail, and other servers running; for a development shop, it could mean keeping the build systems working smoothly). Inevitably, though, there is another audience that systems administrators are asked to deal with: users. In some cases, that means developers; in others, it means the sales team. In either case, as a technical lead, your job is to help them get their job done. Some administrators get lucky: they only have to deal with one environment or the other. In most shops, however, the administrative staff shares responsibilities. This sounds fine, until you consider the psychology of the two situations.

Production support generally involves saying no a lot. Change is bad; slow, methodical improvements are fine over time, but radical departures from what is known to work are frowned upon. Taken to the logical extreme, you end up with a heavy-weight change control process where even small system changes are scrutinized heavily. And to those who hate such things (and I'm right there with you), this is a good thing. Production systems are differentiated from others by the fact that the business lives and dies by them functioning correctly. By extension, things that effect those systems ought to be tightly controlled and monitored.

But development (or, in some environments, user) systems are a completely different thing. Joel Spolsky has talked about this kind of thing at length: the systems are a tool to help the users do useful work. If a development group needs a machine or two for testing, then find a couple somewhere. If a user needs access to the Internet to research a sales pitch they're working on, give it to them. This doesn't mean giving them everything they ask for; this means finding out what they need to do their jobs, and working toward that in a collaborative manner. The systems administrator provides the role of technology expert and facilitator in this arena.

Take someone who has done production support for years, and give them a development group to support, and Bad Things Happen. The admin can't understand why on earth they want to do things like arbitrarily shuffle 50GB files around the network, use various outdated tools that only support older insecure protocols, or talk on IRC or an instant messenger with people outside the company. They lack perspective: they don't see how that style of systems management affects real people trying to get their job done.

I won't even get into the mess that can happen if you try the reverse (a development-only systems administrator in a production role).

I'm finding that there are very few administrators who can successfully balance both mindsets. As geeks, we seek out uniformity because it regularizes our workloads, but you simply can't reconcile the two: to be successful at both, you have to treat both as the unique environments that they are. I've talked before about administrative geeks needing to step beyond their job title, and this is part of it: working with a mindset of "value to company" rather than the micro-view of what you happen to be doing today.

Anyone can say "no" all the time.

Syndicated 2006-05-23 09:10:00 from esm

Systems Administrators != Programmers

I had a good reminder today of what Erik Naggum was talking about when speaking with a co-worker. He was trying to come up with a "good" solution to a Perl problem, so I had him describe it to me. It turns out that he has an object, which may or may not contain the attribute he's looking for. If it doesn't have it, the object has an attribute referring to another object. This other object, in turn, may or may not have the attribute he's looking for, and also has a reference to another object; this repeats until the "next object" is null.

Astute readers will have recognized this as a basic linked list.

I figure, okay, he hasn't been programming in a long time, so we'll mentally walk him through it. First, a quick nudge to see if he can remember his basic CS from a long time ago: "Why don't you write something that will just recurse through that list?" He nods, but the eyes, they don't have it. He goes away to think about it for a while, and then we wind up on a conference call in a meeting room. While the person we're calling is talking, he's quickly jotting down on the board:

  O -> O -> O -> ...

I think, okay, he has the mental idea now, because he's diagramming it correctly. We talk a bit more, and he's obviously trying to come up with a way to iterate over the list. Once again, I suggest a simple recursive function (because that's natural to me, goddamnit, I don't care what you iterative programmers say), and quickly psuedocode something up for him:

  function blah(o)
    if !o:
      return null
    if o->whatImLookingFor:
      return o->whatImLookingFor
    return blah(o->nextObject)

  found = blah(myList)

He looks at it, and appears puzzled. This appears to be a new construct to him, and he asks a few questions that confirm this. "But it'll just keep calling itself and looking at the same object all the time, won't it?" My first psuedocode example unfortunately used similar variable names in both the "mainline" and the function itself, but after renaming things, he still didn't seem to understand. Ah! Local vs. global scoping rules, maybe? So I suggest that, in Perl, you'd have something like my ($o) = @_; as the first line of that function. He still doesn't get it, and grunts something about just dumping the objects and grepping for what he's looking for. He just couldn't make himself look at the "CS" solution, because it didn't fit what he was expecting from Perl: a quick hack that did what he wanted without having to think too hard about it.

Systems administrators who spend to much time with "qwiky" languages like Perl are doomed to forget everything they ever knew about Computer Science. I'm convinced of this now. You might think I had this conversation with a newbie programmer or someone who whipped up scripts now and then, but you'd be wrong; this is a fellow who did software development professionally, and moved into systems administration later in his career. He's no idiot.

(I liken the problem to "l337 sp33k". I've known colleagues who could carry on a perfectly professional conversation face-to-face, but the moment they sit down at a keyboard, they immediately regress to "OMG r u 4 r34l?!" and, setting aside perceptions, they actually behave dumber while doing this. Perl has a similar effect on programmers; in the end, you end up with something that's a tangled web of spaghetti code and system() calls, getting the job done but disgusting anyone who has to look at it. Sort of like reading, "omg u pwn3d that bug, yo!". I wonder when someone will spec a programming language called "l337"...nevermind, Google to the rescue. Ye ghods.)

Postscript: yeah, I probably should have given him an iterative version of the pseudocode. Something like this:

  function blah(o):
    while o:
      if o->whatImLookingFor:
        return o->whatImLookingFor
      o = o->nextObject
    return null

Just as simple, conceptually. Maybe some people have trouble wrapping their heads around recursion, but it's always seemed like a very straightforward idea to me.

Syndicated 2006-04-29 02:46:00 from esm


Go away. :-) The funny thing? I'd use IMolatr.

Syndicated 2006-04-28 23:49:00 from esm

Code Monkey

Code Monkey think maybe manager want to write goddamn login page himself.

Pure geeky comic gold from Jonathan Coulton's "thing-a-week". :-)

Syndicated 2006-04-24 15:22:00 from esm

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