Monday, December 24. 2012
We don't have a regular TV set in the living room anymore, we either use our SAT receiver / recorder as a media streamer and watch TV and recordings anywhere in the house, or we watch using our projector and the big screen that nicely covers the living rooms window front when down for viewing and hides itself just above the windows when not in use.
Now the problem with the receiver / projector / screen setup is that switching it on or off requires three different remotes. It also takes about 45 seconds for the screen to come down to the desired position - time long enough to be boring but not long enough to do anything else meanwhile ...
Both the projector and the screen have infrared remotes. The receiver is a Vu+ Duo, so it runs on Linux and uses the same Enigma2 software as the DreamBox machines. So the original plan was to have the receiver send the right IR codes to the projector and screen whenever it enters or leaves standby mode via a small additional Python plugin and a minimal IR sending circuit attached to the receivers serial interface.
This turned out to be a bit too challenging due to the IR timing requirements, so "Plan B" came into action: offload the actual IR signal generation to a small Arduino board powered by USB and controlled by sending simple commands via usb2serial
The controller is an Arduino Nano, perfect for little breadboard projects like this. The extra components are:
Why not use something like a Logitech Harmony?
Thursday, September 13. 2012
So i decided to give it a little birthday present by finally writing a patch for it myself:
Wednesday, September 12. 2012
Ten years ago today the first public bug was filed on bugs.mysql.com.
As we also stumbled across an unfixed bug today that was reported almost exactly two years ago i came up with the idea of writing a little script that would report all bugs filed on exactly the same day one or more years ago that are still in an "active" state (open, verified, etc.).
The list for today can be found at
And as i was already on it i also created a RSS feed for this at
Friday, June 8. 2012
Still in love with mosh, but there is one thing that causes a lot of confusion right now:
When having something that produces more than a screen full of output, like the usual --help output of non-trivial tools or typical configure scripts i use to scroll back using the local terminals Shift-PageUp / -PageDown bindings.
This doesn't work with mosh anymore as it actually transfers screen frames and not output streams, so output that scrolls by too fast is likely to not ever reach your local screen at all.
Looks as if i need to use less in quite a few places again were i previously haven't ...
Monday, June 4. 2012
then you should definitely have a look at Mosh: the mobile shell
Mosh creates a remote terminal session on another machine similar to what ssh does, but different.
E.g. previously i was using ssh, screen and irssi to connect to our internal IRC server using the following steps:
Now with mosh i can instead use the simpler one liner (or actually a small shell script containing it):
Mosh then first uses ssh to start a session server process on the remote site, so i can still use my host entry from my ssh config file. Next it starts to communicate with this remote server using an AES encrypted UDP protocol. So the session is not bound to a TCP stream as with SSH, and it will not break if that stream is lost as there isn't one to begin with. And instead of identifying the client by IP and port number the AES session is used for this, so the mosh server does not mind if the clients IP suddenly changes.
So a mosh session survives even if you move the client device to a different network, and as UDP is connection-less the session will even survive extended amounts of client downtime.
Now when e.g. moving from my home office to the coffee-shop-with-WLAN-down-the-street i simply put my laptop to sleep (suspend), and when i open it up again there it wakes up again and even though it had been disconnected for ~10min and now has a totally different IP from a different network i can continue to work with my open mosh sessions right away.
Also when on the train with only a G3 connection that can go up and down by the minute i don't have to bother with remote session disconnects anymore.
But there's even more to it that makes it a superior ssh for interactive sessions: Mosh does not simply stream through all output, it is more like VNC for text mode as it maintains a model of the screen states on the server and client sides and tries to keep these in sync. So if you e.g. start something that produces a lot of output and then terminate it with CTRL-C you don't have to wait for all the output to stream by ... you'll get your shell prompt back almost immediately instead.
And one last goodie: if you've ever used ssh over a slow line or one with very high latencies you probably know the "can type faster than the characters appear on the screen" effect and how confusing that can be. Here mosh upon detecting a slow link will enter a mode where it tries to predict where the characters you just typed are going to end up on the screen (including a simple cursor key understanding) and will show them as underlined while waiting for the server response. Once that arrived it will replace its local prediction with the actual screen contents from the server session. It may not always guess right, but you'll at least see what you're typing right away
Now i'm desperately waiting for an Android client for this ....
Friday, April 27. 2012
I've been dealing with MySQL Cluster in one way or another since around 2005 or so (back in the MySQL 4.1 days) but it is still full of "funny" surprises. This post is a collection of different locking related issue i ran into during the previous weeks that i had not been aware of up to now (or simply may have forgotten over time)
== Unique hash indexes lock exclusively ==
This is the one that regular users are most likely to run into: in general row logs in MySQL Cluster distinguish between reads and writes so that writers can block other writers, but not readers and readers from other transactions always see the last committed row value (Cluster currently only supports the READ COMMITTED isolation level). As soon as you have a secondary unique index in addition to a primary key things are different though. Internally a unique index that is not the primary key is implemented as a unique hash index in Cluster (and optionally also as an additional ordered T-Tree), and for unique hash entries there only seems to be exclusive locking so that writers will block readers across transactions.
See http://bugs.mysql.com/65086 for actual examples.
== Long running transactions can block starting nodes ==
Less likely to happen, but very annoying if it happens and you don't know about it:
A starting node needs to lock all rows for a very short period of time near the end of start phase 5. At this point it needs to wait for all current transaction to free the locks they are holding (by either COMMIT or ROLLBACK). This is usually not a problem as cluster transaction are not supposed to last very long anyway, but if there happens to be a long running transaction it can potentially block a node start forever. A stopping node will wait for pending transactions for 5 seconds and will then terminate these transactions the hard way, a starting node on the other hand will gracefully wait forever.
This is bad in two ways:
1) it is obviously bad availability wise as you may end up with a non-redundant configuration for extended periods of time and without any way to automatically identify and terminate the offending transaction(s)
2) currently the blocked starting node does not even tell what it is waiting for, it just silently sits and waits. So unless you know what is going on you are faced with a node that is simply stuck without doing anything (CPU, disc and network load next to zero)
The related bug report is http://bugs.mysql.com/65037 , proposed solutions are meaningful log messages in the short term and killing active transactions after a grace period in the long run.
== Weird error message on lock timeout ==
This last item is related to the non-standard INSERT ... ON DUPLICATE KEY UPDATE construct. What can happen here is that the INSERT part fails with a lock wait timeout as the key is write locked by another transaction. The expected error message in this case would simply be "Lock wait timeout exceed" but what you'll actually get is "Transaction already aborted". What seems to happen here is that the ON DUPLICATE KEY UPDATE part is triggered by any error in the INSERT phase and not only by duplicate key violation. So the INSERT part is tried tried, fails with "Lock wait timeout" (which implicitly rolls back the current transaction), next the UPDATE part is tried, fails with "Transaction already aborted" and overwrites the previous error ...
The related bug report is http://bugs.mysql.com/65130
Friday, April 6. 2012
After the fun with InnoDB and MyISAM and massive partitioning it's time to move on to some other storage engines. So for the next round of fun i chose ARCHIVE:
PS: MariaDB does not seem to be affected ...
Monday, March 12. 2012
While actually trying to check for something completely different (more on that later maybe)
So my /usr/local/mysql-5.5.21/bin directory now uses 130MB instead of the 17MB that
Wednesday, March 7. 2012
So this should not only affect ALTER TABLE but all operations on MyISAM tables with large number of partitions should eventually run into this. Lets try and see what happens ...
Continue reading "Fun with partitions and MyISAM, part #2"
Wednesday, February 29. 2012
Looks as if MyISAM tries to open all the 1000 partition .MYD/.MYI files at once and runs out of file handles during this operation ...
Creating or dropping a partitioned table on InnoDB can become a quite expensive operation, on my laptop i'm seeing the following times for a simple table with 100 or 1000 partitions (using 5.1.58 right now as i'm testing on stock Ubuntu 11.10):
So the time to create a partitioned InnoDB table grows linearly with the number of partitions at a 'speed' of about 20 partitions per second, and during that time the hard disk LED is always on. The rate for creating regular InnoDB tables on this machine is about 10 tables per second by the way.
MyIsam on the other had can create around 17 tables per second, and creating a single partitioned MyISAM table only takes about a 10th of a second.
So what is going on with InnoDB here? First of all from the engines point of view each partition of a partitioned table is an actual table, all the partitioning magic happens on a layer above the storage engine one, and the storage engine only receives handler requests for the actual partition tables involved. On the storage engine layer the actual engines are not aware that these tables are part of a larger partitioned table at all (with the exception of ndbcluster tables, but MySQL Cluster is a different story in this respect anyway).
So when creating an InnoDB table with 1000 partitions the InnoDB storage engine actually receives 1000 individual requests to create a table. Now what seems to happen is that for each
So this may be something to keep in mind when planning to use large numbers of partitions on InnoDB ...
Thursday, February 23. 2012
It is probably about time to re-read the MySQL manual end-to-end as i more and more find myself discovering features i either completely forgot about or which i never was aware of in the first place ...
Todays guest is slave_compressed_protocol, an option that has probably been there ever since MySQL 3.23 at least (so that i can't claim that i seem to have missed the ChangeLog entry as i usually do
With slave_compressed_protocol enabled the communication between slave and master uses the MYSQL_OPT_COMPRESS option to compress the protocol stream if both sides support it (and it's very unlikely to find an installation that does *not* support it these days), so this can be a big savior if your master and slave are at different sites with only limited bandwidth between the two.
Unfortunately this is a global server option though, IMHO this should be part of the options provided by the CHANGE MASTER command, similar to all the SSL encryption related stuff that is part of CHANGE MASTER.
Looks as if it is about time for YAFR (Yet Another Feature Request) ...
Monday, October 10. 2011
Looks as if we have the next member in the "I want to become Caliph instead of the Caliph" club: PHPreboot
This is just a braindump of thoughts on the various bullet points and examples on the projects home page (but i don't think it's worth any more time to analyze it and comment on it either):
Not sure whether this is really an improvement, while $ and ; are not really necessary from a parsers (or lazy typers) point of view they do carry some context information ... so this is breaking the "The burden shall be on the writer, not the readers" principle IMHO
Yes, that's all that needs to be disabled to guarantee secure code ... NOT
Magic quotes were a bad idea, but for slightly different reasons. eval() by itself is not a bad idea either if used properly, same as with backticks, system(), popen(), ...
And just by disabling magic quotes and eval() you do not make code secure by default, XSS and SQL injections can still happen without these, and i'd bet that most malicious code injections in vulnerable PHP apps were not using eval() but include/require as attack vector to make PHP execute their own code ... want to forbid these, too?
That's a good thing, we may just have different ideas of "full" ...
I do like the "to" part of it ... not really sure about the "from" part ...
Which SQL dialect? And looking at the code: how would i handle queries against multiple database connections?
I personally prefer to write preg_match() literally ... but that may be just me and my PERL allergy ...
I don't see that much gain in that one over explicit fopen(), ... calls either
There's room for improvement in PHP optimizers and op code caches for sure ...
In my personal use cases a JVM based PHP would actually be a loss though. Most stuff i'm using PHP for these days is medium complex command line stuff (like my code generators), and for most of this my PHP scripts are already done with their job in a time that would not even be sufficient for a JVM to initialize and execute the very first byte code op of my user code
There's also the more general issue that when going the way of the JVM you either go bytecode all the way and have to say goodbye to all the PHP extensions that are actually just thin wrappers around native C libraries, re-implement those libraries functionality in a language supported by the JVM or go the way of JNI ...
... instead of SQLite and ?forgot-the-name-of-the-upcoming-integrated-webserver-thingy? ...?
Sorry, can't see how that's "native XML syntax", PHP and its use of XML processing instruction syntax is as native as you can get, hiding executable code in DATA not so much
Sorry but even that simple example does not really look like PHP to me as all. Maybe part of that is due to the use of "elseif" instead of "else if". I haven't seen any PHP code using "elseif" in years, i even had to double check that it works in this context and not just in the IF: ... ELSEIF: ... ENDIF; form.
Again i'm not sure whether i want to have that on the Syntax level ...
So to summarize:
=> so why should it have the letters PHP in its name at all?
Tuesday, October 4. 2011
Debian and Ubuntu make use of the !includedir directive at the end of the packaged my.cnf to allow for extending and overriding their default configuration without having to modify the distribution config file.
This is especially nice in combination with configuration tools like puppet as it is usually much easier to add a file to the system than to modify an existing file.
I ran into a small gotcha with this though: the default my.cnf binds mysqld to the localhost TCP interface so that it listens IP 127.0.0.1 only. To make the mysql server reachable from other hosts the bind=127.0.0.1 setting needs to be overwritten.
The most obvious would be to simply say
, this does not have any effect though. What needs to be done instead is
to make mysqld listen on all interfaces again.
This is obviously a solution for IPv4 only, for allowing incoming connections from all v4 and v6 interfaces allowing bind= without any argument to reset any existing bindings would probably be more clever?
Saturday, September 17. 2011
For testing some stuff related to MySQL Cluster, port binding and having two network interfaces on the same subnet i needed a bit more than my usual "all nodes on localhost" setup. Looking for a reusable solution i this time did not just simply plug several of my old laptops into the same hup but tried to create a Vagrant setup for this.
This quickly lead to the question how to define multiple network adapters per Vagrant VM. The manual is not too clear about this yet, and the key piece of information is not yet to be seen in the network section but is hiding in the section on port forwarding: the :adapter option.
So to define multiple adapters you need to add :adapter => $value options on each of the vm.network lines, with $value counting up from one, e.g.:
config.vm.network("18.104.22.168", :adapter => 1);
In my case i need both adapters to be on the same IP subnet. Vagrant (or VirtualBox?) is not clever enough for this yet and will create two distinct host-only networks for this, routing and binding works as expected though as the kernels inside the VMs do not care about (and are not even aware of) this.
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