16 April 2012

Increase of Linux IO scheduler queue size might lower performance

The blog post Linux I/O scheduler queue size and MyISAM performance explains a good way to increase performance when using Mysql and the Myisam database table engine. I have tried this an can confirm that it will increase performance for this typical application.

I have unfortunately discovered that it may not always increase the overall performance if you have other IO intensive operations besides an Mysql database running on your server. I have a system that runs both Mysql with Myisam and also a custom written database application that uses a lot (like 10000) small files to save data.

The system handles a stream of "messages". I measure performance by looking at graphs of how many messages are buffered (0 to 100000) and how many messages that has been discarded because the buffer is full.

For nr_requests = 128

For nr_requests = 10000

As seen from the graphs the configuration with default nr_requests (128) performs better. My advice is thus to benchmark if your specific application benefits an increased queue size.

09 April 2012

A free ssh client for Ipad

There are a bunch of ssh clients in Apple App Store that costs money. If your looking for a free one for Ipad the number is reduced: a search for the "ssh" and Price = "Free" yields 9 hits. Out of these 9 only 2 is and actual ssh client: zatelnet and Rove Mobile Admin Client.

Out of curiosity I tried Rove Mobile Admin Client. It's main purpose is actually not to be an ssh client, but to be a remote management tool for their commercial server software. But that doesn't matter, since the ssh client part of the app is free to use standalone. I could successfully connect to my home Linux computer:

Don't expect miracles out of this app, since it is free it is probably a lot worse than some "real" non-free ssh clients. But you can do basic things.

  1. Can use private key for authentication
  2. Can edit files in nano (requires rather good terminal emulation and the abilities to send Ctrl sequences).
  3. Does maintain connection when you switch to another app
  1. Does not support UTF-8 (since Ubuntu uses this as default it can be a bit frustrating)
  2. Does not support select, copy and paste in the terminal emulation.
  3. Very slow (the time for a key press to "echo" is in the magnitude of 1 s).
  4. Can only have one open connection at a time

A side note: the app also includes an RDP client, but forget about using it, it is painfully slow.