Older blog entries for etbe (starting at number 78)

Hot Water

A response to a post I wrote about things to do for the environment suggested that there would be a health risk to lowering the temperature of a home hot-water system to save power.

I have just been reading about so-called tankless hot-water systems. The concept is that instead of keeping a tank of water hot (which means that you lose some energy due to the insulation not being perfect) you heat water when you need it. The down-side to this is that you need a moderate amount of power to heat water as rapidly as it’s used. The GoTankless.com products use between 11KW and 27KW of electricity. The early implementations of this idea used gas - the occasional carbon-monoxide problem with gas appliances makes me inclined to avoid them so it’s good that there’s an electric option.

One of the benefits of the tankless system is that it runs the water at a lower temperature than a regular hot water system. For a tank storing hot water you have to run it at a temperature that kills bacteria (or at least dramatically inhibits their growth) - which means greater than 50C, but for on-demand water it’s safer to have it run at something close to the desired temperature (probably not much above 40C) and not use the cold tap. Lower temperature water avoids the risk of scalding for children and the elderly and if the “hot tap” is running at a good temperature for a shower then you can just turn it on, wait 30 seconds for the pipes to warm up, and jump in! Incidentally it really sucks the way most showers have the taps under or behind the flow of water, so if the water becomes too hot before you get in then you end up getting minor burns in the process of turning on the cold tap.

I still think that solar hot water is the way to go. It apparently combines something like a tankless system on water that comes out of a tank heated by the sun. So during winter it operates like a tankless system but in summer you get more hot water than you can use.

This web site about Solar hot water systems indicates that they have a similar technology to “boost” solar hot water, so if the Sun doesn’t make the water hot enough then it can use electric or gas systems to further heat the water. It’s also interesting to note that they offer Heat Pump hot-water systems, it’s a pity that they apparently don’t support combining this with solar heating. Another interesting feature is what they call the Water Guardian that pumps cold water from the pipes back into the water tank and avoids wasting the water that you might otherwise run down the sink while waiting for it to get hot.

Share This

Syndicated 2007-07-30 21:00:08 from etbe

Porn vs Rape

Chris Samuel blogs about a plan to censor porn from the Internet in Australia. According to the ZDNet article the Fundamentalist Christian party Family First wants a national porn filter to protect children.

However there is strong evidence to suggest that the incidence of rape decreases as the availability of pornography increases. Access to porn prevents rapes!

The question is whether protecting people (mostly women and children) from rape is more important than protecting the strange and unusual (by the standards of modern Australian society) religious beliefs of a minority of people who oppose porn.

The current research on porn vs rape is based on comparing different regions of the US with different levels of Internet access. It would be more accurate if a scientific test could be performed in a controlled environment. The US has the largest number of prisoners of any country and the largest proportion of the population in prison of any first-world country and rape is common in prison. It would be easy to grant access to porn to one cell block of a prison and deny access to another and then record the incidence of reported rape (currently in the US porn is restricted in prisons and masturbation in prison is a crime). The results of such research could be used to devise government policy with the aim of protecting people from rape. Of course that would require compassion - something that’s extremely uncommon in politicians particularly the ones that claim to be Christian.

Some people are requesting the creation of Christian Porn - so Porn and Christianity don’t have to be opposed. The same site has an interesting page Masturbation: God’s Great Gift to Us. Now if all those rapists could be encouraged to watch Christian Porn and take advantage of God’s Great Gift then the world would be a better place.

Share This

Syndicated 2007-07-30 09:00:53 from etbe

Blogs and Conversation (or Lack Therof)

I recently received an email from RSA inviting me to read their blog (after having requested an evaluation copy of one of their products). They invited me to “join the conversation“. Often blogs are described as a “conversation” and I’ve been considering whether that analogy is appropriate.

The Collaborative International Dictionary of English v.0.48 has the following definition: Usage: {Conversation}, {Talk}. There is a looser sense of these words, in which they are synonymous; there is a stricter sense, in which they differ. Talk is usually broken, familiar, and versatile. Conversation is more continuous and sustained, and turns ordinarily upon topics or higher interest. Children talk to their parents or to their companions; men converse together in mixed assemblies.

There are instances of blogging that can approximate conversation when bloggers cite each other’s articles in turn and leave comments on each other’s blogs. But it’s usually not that continuous and sustained. An occasional patterin is that blogger A writes a post, blogger B writes a post disagreeing with it, blogger A comments on blogger B’s post, and that’s usually the end of it. Usually it’s just a case of multiple bloggers writing posts representing their own opinions with occasional references to other blogs and no sustained interaction. The blogging interaction seems to more closely resemble academics presenting papers with alternate solutions to a problem than a conversation.

One factor that I believe defines conversation but which isn’t mentioned in the dictionary is that the parties involved have an equal standing. The comments section of a blog is not such a forum, the blogger has a significantly more powerful voice (at least in connection with their own blog) than the people who comment. Someone who posts a comment may have it deleted, and it if is left then a rebuttal in another post will be seen by a significantly larger number of people (the number of people who read comments on a post is a tiny minority of the people who read the post). This doesn’t preclude conversation between bloggers who are of equal popularity in a community, for example most readers of my blog come from Planet Linux Australia and Planet Debian so any blogger who is also syndicated on those sites has an equal voice to me.

Another factor in conversation is whether responses are even read. Many blogs don’t accept comments and it’s never certain that a blogger will see a track-back from a blog that references one of their posts. When one party ignores the other (or appears to do so) then there is no conversation.

I’m not aware of whether a conversation with RSA people would be possible. While their blog refuses to send content to my Planet installation I guess I’m not going to find out…

Is the lack of conversation a bad thing? One problem with conversation is that it often degenerates into what GCIDE defines as talk, while that is good for a friendly mailing list (EG your local LUG) it isn’t so good for the exchange of technical information. A compounding problem for mailing lists is the number of posts that can not be interpreted without the context of a thread of discussion. This often makes mailing lists unreasonably difficult to use in the search for answers to technical problems. When reading google search results I will usually read blog URLs before mailing list posts as a blog post will usually stand alone and either give me an answer or be obviously not related to my problem - sometimes I have to read a dozen messages in a list discussion to determine that it’s not going to help me!

To improve things in this regard I plan to increase the number of posts I write with solutions to random technical problems that I encounter with the aim of providing a resource for google searches and to randomly inform people who read my blog. I find such posts by other people quite useful, I often get inspired to implement a technology after reading a blog post about it - there are many things that have a low priority in my todo list because they seem difficult, a blog post that reveals them to be easier than expected and advises how to avoid common problems can really make a difference!

Share This

Syndicated 2007-07-29 21:00:22 from etbe

A Worse Visual Migraine

Last night I had a Visual Migraine that was significantly worse than the previous ones. It started in the usual manner (flickering lights in my central vision and an inability to read text on a computer screen in a font that is normally quite readable) but then developed a new feature. Grey clouds appeared in the periphery of my vision which left me with only central vision which was obscured by flickering lights.

About an hour later it was gone. I’m glad that I don’t drive much, such limited vision is enough to allow me to use public transport without serious problems (although crossing a road could be interesting). Spending an hour in a parked car at the side of a road waiting for a migraine to go away is not something I’m looking forward to.

Share This

Syndicated 2007-07-29 08:48:55 from etbe

Bonnie++ and Postal shirts

Dear lazyweb, I want to design T-Shirts for my Bonnie++ and Postal projects. But representing those projects in a picture seems more difficult than SE Linux (see one of my SE Linux T-Shirt designs below). If you have any conceptual design ideas then please let me know.

Share This

Syndicated 2007-07-28 21:00:39 from etbe

Outsourcing

It’s interesting to see an eWeek article about outsourcing to Canada, apparently the US immigration laws are restrictive enough that US companies (such as Microsoft) are establishing offices there. It really makes sense, Canada seems like a much nicer place to live. It’s also interesting to see an eWeek blog post about reductions in outsourcing to India that claims that Indian salaries have increased so much that there are few benefits in outsourcing the work.

I have previously blogged about my approval of outsourcing as a form of charity to developing countries which received this response in Spanish. An English translation of a section of that post is “Conclusion, outsourcing hurts and the damage to many individuals is permanent. In countries in the third world like the one where I live, the damage affects the majority of the population (80% of the population is very poor). It’s ridiculous that less than 10% of the Mexican population earn 8 to 10 thousand pesos per month which only allows them to survive and save a little to buy a car or a house“.

I agree that the inequality in Mexico is a bad thing, but I think that outsourcing is more of a cure than a cause for such inequality. The competition for employees who are capable of doing such work will increase salaries for workers and create a middle-class. Also the net access which is needed for outsourced work will drive some political changes. When discussing these issues with Indians I never hear any complaints about outsourcing!

Share This

Syndicated 2007-07-28 09:00:05 from etbe

Red Hat virt-install and Partitions

I have recently been using virt-install on CentOS 5 to install some virtual machines (I previously posted a summary of the experience). One problem I had is the inability to do an install of Fedora to /dev/hda - it insisted on partitioning the disk and installing it on a partition. I have filed Red Hat Bugzilla #249837 about this problem.

# n=1
# while true ; do dd if=/dev/vg0/orig of=/dev/vg0/dest bs=512 skip=$n count=1000 ; echo $n ; file -sL /dev/vg0/dest ; n=$(($n+1)) ; done

After doing a CentOS or Fedora install I then have to fix things so that my desired option (of using a single unpartitioned virtual disk) is used. To do this I first needed to determine how many sectors at the start of the filesystem were used for the partition table. I used the above shell commands to test skipping different numbers of sectors and using file -sL to determine whether the result was recognised as an Ext2/3 filesystem. Below is part of the output:

62
/dev/vg0/dest: data
1000+0 records in
1000+0 records out
512000 bytes (512 kB) copied, 0.018812 seconds, 27.2 MB/s
63
/dev/vg0/dest: Linux rev 1.0 ext3 filesystem data (large files)
1000+0 records in
1000+0 records out
512000 bytes (512 kB) copied, 0.115528 seconds, 4.4 MB/s
64
/dev/vg0/dest: data
1000+0 records in
1000+0 records out
512000 bytes (512 kB) copied, 0.018623 seconds, 27.5 MB/s

As you can see 63 sectors are used for the partition table and other boot blocks at the start of the disk. So I used the below command to fix it:
dd if=/dev/vg0/orig of=/dev/vg0/dest bs=512 skip=63

The next problem after this was booting. The pygrub program that is used by the CentOS Xen installation to boot DomU’s only works with partitioned block devices. This however is quite easy to solve, I merely had to copy the kernel and initrd from the Xen image to the Dom0 filesystem and use the files for booting. Of course every time I upgrade the kernel I will need to copy the files, but that’s not such a great inconvenience. I usually end up running multiple DomU’s with the same distribution so I can copy the kernel and initrd once and use it for all instances.

Share This

Syndicated 2007-07-27 21:00:52 from etbe

Childhood

Recently there has been some discussion and controversy about a 15yo boy being allowed to perform a Caesarian operation on a woman (without her consent). Don Marti seems to think it’s OK and gives some examples of what 15 year old people used to do in past times. However he misses a couple of significant points, one is that the knowledge of surgery was much lower a few hundred years ago and the expectations of the patient were a lot lower, the other is that the patient in this case did not consent to being operated on by a 15yo. If the boy’s mother thought that he was capable of doing the job correctly then she could have allowed him to deliver a younger sibling…

Given a choice I would rather have someone like Doogie Howser operate on me than a random surgeon - but I would be extremely unhappy to discover after an operation that it had been performed by a different surgeon than planned who didn’t have a medical license and who’s motivation was to invade my privacy by making a movie of the event!

Don cites the interesting essay Against School by John Taylor Gatto which makes some really good points about the state of the education system in most first-world countries. An interesting point that John Gatto doesn’t mention is that in Japan school-boys wear uniforms that are based on Prussian military uniforms. From all the evidence that I have seen the Japanese school system is more Prussian (IE worse by many objective measures) than most countries.

The Sydney Morning Herald has an interesting article comparing education systems which claims that a major cause in lack of academic performance is unjustified praise. I first became aware of the extent of this problem when discussing education with a misguided university lecturer. He told me that he felt that his purpose was to make the students believe that they had achieved something and that this was much more important than actually teaching them. My response was to point out that heroin and cocaine are both good options for people who would rather feel successful than succeed and to enquire as to whether he thought that they should be advocated for children. The conversation ended soon after that and he requested that I not name him or his university when blogging about it. I take this as an admission of guilt, if you act decently in public then you should not be afraid of appearing on a blog - decent actions will either be too boring for a blog entry or things that you are proud of.

Share This

Syndicated 2007-07-27 09:00:44 from etbe

A Support Guide for Xen

Here’s a guide to supporting Xen servers for people who are not Linux experts. If your job means that you have root access to a Xen server that someone else installed for the purpose of fixing problems when they are not available then this will help you solve some common problems.

Xen is a virtualization system that is primarily used for running Linux virtual machines under a Linux host. It is mostly used as a Paravirtualization system in that the virtual machine knows that it is running in a virtual environment - this allows some performance benefits.

The host environment is known as Dom0 and root in that domain has the ability to control the other domains (which are known as DomU domains). If you perform an orderly shutdown of the Dom0 (via the shutdown or reboot commands or notification from the UPS of an impending power failure) then when the machine is booted again the DomU’s will be automatically restarted (if the on_reboot setting has the value restart - a common configuration). If you run the command shutdown in a DomU then the domain will be destroyed, and the command reboot will restart the DomU with the same settings - if you want to change the settings for a DomU you need to shut it down and create a new instance.

The main sys-admin command related to Xen is xm. Here are the main xm options that are useful in support:

1. xm list provides a list of running domains. For each domain it gives the name of the domain, the ID number, the memory allocated to it, the number of virtual CPUs allocated to it, the state, and the amount of CPU time used in execution. The ID numbers are allocated sequentially, if you reboot a DomU by running the command “reboot” inside it then it will get a new ID number when it re-starts. Many xm operations that may take the name of a domain will also take a Domain ID number. Generally you never use an ID number and ignore it - the only relevant thing about an ID is whether it is 0.

   Here is a sample of the output of xm list:
   # xm list
   Name        ID Mem(MiB) VCPUs State  Time(s)
   Domain-0    0    1236    4 r---–  14116.3
   wind        13    2999    3 -b----  60114.1
   wind-f7    52      519    1 -b----  3329.9

   You can see from this output that the domain named wind has 2999M of RAM, 3 virtual CPUs (out of 4 physical CPUs in the machine) and has 60,114 seconds of CPU time used (that is 114 minutes of CPU use - the equivalent of almost two hours for a single CPU). Here are the values you might see in the state field (from the man page xm(1)):
   

   
   
   • r - running
     The domain is currently running on a CPU - note that Dom0 will always appear to be running because you are running the xm utility!
     
   • b - blocked
     The domain is blocked, and not running or runnable. This can be caused because the domain is waiting on IO (a traditional wait state) or has gone to sleep because there was nothing else for it to do.
   
   
   • p - paused
     The domain has been paused, usually occurring through the administrator running xm pause. When in a paused state the domain will still consume allocated resources like memory, but will not be eligible for scheduling by the Xen hypervisor.
   
   
   • c - crashed
     The domain has crashed, which is always a violent ending. Usu‐ ally this state can only occur if the domain has been config‐ ured not to restart on crash. See xmdomain.cfg for more info.
   
   
   • d - dying
     The domain is in process of dying, but hasn’t completely shut‐ down or crashed.
   
   
   
   If you see domains that are running which normally aren’t busy then make a note of this. If you see domains that are paused, crashed, or dying then contact the sys-admin.

   Also know which domains are expected to be running so that if a domain is missing then you will recognise it as a problem!
   



2. xm top is similar to the top command in Unix but displays Xen data, by default it displays the same information as xm list but also includes the amount of data read and written from network devices and disks. If your terminal is less than about 145 columns wide the lines will wrap and it will be confusing - stretch the width of your xterm before running it.

   If you have multiple network interfaces then you can see the transfer counts for each of them separately by pressing the N key. If you have multiple network interfaces in DomU’s then this can help diagnose some network problems (although you may find that tcpdump is more useful).

   If you have multiple disk devices in a DomU then you can see their transfer counts separately by pressing the B key. One problem that can be partially diagnosed through this is excessively poor performance. If a DomU is running extremely slowly then it may be impossible to login to diagnose and/or fix the problem (it could take tens of minutes to login), in that case seeing where the disk access is going from outside the DomU can shed some light on the problem.

   VBD  768 [ 3: 0]
   VBD  832 [ 3:40]
   VBD 5632 [16: 0]
   VBD 5696 [16:40]

   Above is the identification of the virtual devices /dev/hda and /dev/hdb in a DomU. The numbers inside the brackets are the device node numbers in hexadecimal, so 16:40 means the device 22,64 as a pair of decimal numbers (22*256+64=5696).

   # ls -l /dev/hd?
   brw-r---– 1 root disk  3,  0 Jul 23 17:24 /dev/hda
   brw-r---– 1 root disk  3, 64 Jul 23 17:24 /dev/hdb
   brw-r---– 1 root disk 22,  0 Jul 23 17:24 /dev/hdc
   brw-r---– 1 root disk 22, 64 Jul 23 17:24 /dev/hdd

   Above is the result of a ls -l on the devices in question from inside the DomU.

   When I set up a Xen DomU I generally use /dev/hda for the root filesystem and /dev/hdb for swap. So if the machine is performing poorly and /dev/hdb ([3:40]) is being accessed excessively then it indicates that the machine has some memory hungry programs running and is paging heavily.



3. xm list –long [domain] gives detailed information on all domains, or it can be run with the name of a domain such as xm list –long wind to give the detailed information on only one domain. Generally this is something that you will log to a disk file before restarting domains, in the short-term there is little use for this.


4. xm console <domain> gives you the console of a domain. If a domain is not working correctly and it is impossible to login via ssh (either due to a network problem or a problem with ssh) then you can access the console (equivalent to a serial-port login on physical hardware) to diagnose the problem. Often the kernel will log messages to the console, such messages will be stored by the Xen system until they are read. If you suspect that there may be many such messages then use script(1) to log the output to disk, if you are unsure then use script to make sure that you don’t miss any data. Even if you don’t understand it the sys-admin probably will!

   If the system is half-working then you can login as root to investigate problems. You can escape from the console by pressing CTRL-].



5. xm dmesg gives Xen logging data comparable to the dmesg command in Linux. If you ever have to reboot the machine (run reboot from Dom0) due to a problem with Xen then you MUST save the output of xm dmesg to a file for later review by the sys-admin.
   


6. xm destroy <domain> will kill a specified domain. It’s a last resort for stopping a domain that is not working correctly - it is greatly preferrable to login to the domain via ssh or xm console and give an orderly shutdown.


7. xm create [-c] <domain> creates a new domain. The configuration for the domain will be taken from a file of the same name in the current directory or in the /etc/xen directory - if /etc/xen is not the current directory when you run xm create then make sure that there is no file-name conflict. You can use this command after destroying a domain or to start a domain that was not previously run.

   If you want to change a configuration option of a domain (such as the amount of RAM used) then the usual procedure is to edit the configuration file, run halt or shutdown from within the domain, and then create the domain again with xm create. Note that the -c option is used to attach to the console after starting the domain (you usually want to do this).

I will probably update this post when I get some feedback. I may write more posts of a similar nature if there are requests.

Share This

Syndicated 2007-07-26 21:00:46 from etbe

Train Routing

Last year I spent several months living on one side of Melbourne and working on the other and travelling by train to work. Every day I had to catch two trains each way with an average wait of 5 to 10 minutes for each train to arrive giving a total of at least half an hour a day spent waiting for trains.

The obvious solution to this problem is to have trains not go back and forth on one line but instead go from one side of the city to the other. This map shows that there are 14 train lines out of the city with about 5 major lines. The smart thing to do would be to have every major line have one train every 5 minutes during peak hours and to have every train go back out on a different line. Then if you are on one of the major lines and want to travel out on one of the other major lines then every 20 minutes there would have a train that would take you straight there without changing trains!

Currently we don’t even have such frequent trains, during peak hour the trains on most lines run no more often than 5 per hour, the Sydenham line has trains 4 times per hour during peak hours and the trains are crammed full before they get half-way to the city.

If the trains ran more frequently and were routed through the city then commuters who travel through the city would save 20+ minutes per day without going to any effort and 30+ minutes a day if they chose to start their journey at a time to avoid changing trains. This would be a significant incentive for catching the train instead of driving!

For the commuters who travel to work via a single journey then having trains run every 5 minutes at peak times would mean that an average of 2.5 minutes was spent waiting for a train each way (an average of 5 minutes per day) instead of the current situation of 15 minutes per day or more. This would mean triple the number of trains on the Sydenham line which may sound excessive. However the trains are currently so crowded that there could be twice as many trains and all seats would still be full. If there were three times as many trains then I expect that more people would catch the train (surely some people would be convinced to drive to work by the idea of spending 20 minutes with barely room to stand), it’s not inconceivable that there could be three times as many trains and all seats could still be full!

The next issue I have been considering is the time taken for a tram ride to/from the central city areas in peak hours. Peak hour trams stop at every stop because there are always people getting on and off. If a tram could stop less frequently then it could make a slightly higher average speed. One way of achieving this would be for the peak hour trams to stop at every second stop outside the center of the city. On the way in half the trams would accept passengers at each stop (each tram would be designated as either odd or even and labelled as such - the tram stops are already numbered). But if you have twice as many trams then the average wait would be the same while the duration of the trip would be reduced. On the way out of the city the tram driver would announce that after stop 10 (to pick a random number that might work) the tram would only allow passengers to get on or off at even/odd stops. If you knew that your stop was on an even number and the tram was an odd-numbered tram then you would change trams to an even tram. The small delay in changing trams would be made up by a faster trip overall.

Politicians are always talking about ways to alleviate the water shortage caused by climate change and to improve the economy. Having people spend an extra 10 minutes a day working because of saved time on the trains would help the economy. Encouraging people to catch the trains via more frequent and efficient service as well as less overcrowding would help reduce climate change - which is the best way of improving our water supply and the only way of helping the farmers long-term!

Share This

Syndicated 2007-07-26 09:00:18 from etbe