Monthly Archives: February 2010

Arduino, LED-s, LM35 and DS18B20 or temperature sensors. 2

Checked out my local electronics store yesterday and got some stuff. First, a breadboard and some wires. Also got me a soldering iron. Those are just for testing. At least I do not have to twist wires together now. Then I thought, LED-s are cool for something, right? There is a nice site So I did get 10 1/4W 1K ohm resistors, and some leds, specifically 2 red, 2 blue and 2 green. 3mm and somewhere near 2000-4000 mcd.

First Steps

Looking around, I decided to get one plain Arduino Duemilanove. Luckily a local dealer is near so I did place an order on it at 15:48 and made a bank transfer. At 17:03, less than 1.5 hours later I got an e-mail stating that I can get my hands on it around 19 o'clock. After a quick call, I got confirmation, that my transfer had gone through and couple of hours and a small trip later I was a enthusiastic owner of an Arduino Duemilanove. Great thanks for and Zazou in particular.

Why Arduino

Why start playing with Arduino? For a long time I was looking for a way to get temperature readings from outside of computer case and draw graphs of them. Specially with linux. There are a lot of ready to use temperature readers, but all of them have some flaws.

Fast way to create or assemble mdadm devices

Booting from cd and no /dev/md devices? First, correct kernel modules are needed. modprobe raid1 If configuration uses different raid, then a different raid module should be loaded. Be it a raid 0,5,6 or whatever. Devices can be made then mknod /dev/md0 b 9 0 mknod /dev/md1 b 9 1 and so on. As many of them as needed. From this point on door is open for mdadm create,assemble or whatever is needed. Creating a device goes like this: mdadm --create --verbose /dev/md0 --level=1 --raid-devices=2 /dev/sda2 /dev/sdc2

Postfix, MySQL lookups and Temporary lookup failure 1

After setting up postfix to use mysql as lookup backend for alias or virtual or whatever table, everything seems ok. Mails are delivered. But sometimes strange "Postfix SMTP server: errors from " e-mails are received. Usually more than one in the same second, then there can be no errors for days or weeks. Then they appear again. There is a "Temporary lookup failure" or "queue file write error" cited in those error e-mails but everything seems to work ok again. Even those e-mails that get error on first try get delivered on second try some minutes later. One thing to look out for is

Squid: WARNING! Your cache is running out of filedescriptors 2

Sometimes squid stops working and "WARNING! Your cache is running out of filedescriptors" is logged in cache.log. Restart usually resolves that problem. Temporarily though. This article is based on adding some Gentoo specific bits. Squid, as a web cache should, holds a lot of files from internet on a local hard disk. It means that a lot of files are open at the same time. On busy servers this amount grows till the limit imposed by the operating system. Then no more files can be opened and squid seemingly dies.