IO speed in Linux

Did I feel oh so stupid.

I purchased HP ML110 server to our company to work as a workgroup-server, threw in three disks, installed Centos 5.2 and configured necessary software for our small team. Everything seemed to work just fine, but from time to time users complained that Alfresco was slow. When I tested, I did not notice anything and thought that it was nothing.

However yesterday I noticed that the server was extrenely slow and disk was running all the time, even though there was nothing particular actually going on in the server. This gave me the reason to actually test and figure out what is going on. As a launched hdparm and started to diagnose the server, I almost fell down from my chair as hdparm said that read/write-speed of my disk was no more than 2 megabytes / sec – when my expected value was supposed to be somewhere between 50 – 80.

Lots of googling and trying to figure out what is wrong and why DMA is not enabled on the drive led me try to edit grub configuration and frustrated search around the options in BIOS-configuration. DMA seemed to be enabled on all the disks, but still Linux identified disks wrongly and mapped them to /dev/hda instead of /dev/sda. Finally I did a lucky guess and tried to change very innocent configuration of Serial-ATA mode from Automatic to Seriat-ATA and lo and behold – it worked!

After that io speeds jumped through the roof all the way to 70 megabytes / sec – and I was finally ready head back home.

Yeah. Felt stupid that I hand’t tested it propely when I installed the software and that I did not notice that io was over 20 times slower than it was supposed to be.

Works now, and hopedully this posting helps others that have similar problems in the future.

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