Month: February 2011

Upgrading passwords from old_passwords to “new passwords”

MySQL

You have old_passwords=1 in your my.cnf. I’m guessing this is because you used one of the my-small.cnf, my-large.cnf etc. templates provided with your MySQL distribution.

These files can easily win the “most outdated sample configuration file contest”.

Usually it’s no big deal: if some parameter isn’t right, you just go and change it. Some variables, though, have a long-lasting effect, and are not easily reversed.

What’s the deal with old_passwords?

No one should be using these anymore. This variable makes the password hashing algorithm compatible with that of MySQL 4.0. I’m pretty sure 4.0 was released 9 years ago. I don’t know of anyone still using it (or 4.0 client libraries).

The deal is this: with old_passwords you get a 16 hexadecimal digits (64 bit) hashing of your passwords. With so called “new passwords” you get 40 hexadecimal digits (plus extra “*“). So this is about better encryption of your password. Read more on the manual.

How do I upgrade to new password format?

You can’t just put a comment on the “old_passwords=1” entry in the configuration file. If you do so, the next client to connect will attempt to match a 41 characters hashed password to your existing 16 characters entry in the mysql.users table. So you need to make a simultaneous change: both remove the old_passwords entry and set a new password. You must know all accounts’ passwords before you begin.

Continue reading » “Upgrading passwords from old_passwords to “new passwords””

Upgrading to Barracuda & getting rid of huge ibdata1 file

MySQL

Some of this is old stuff, but more people are now converting to InnoDB plugin, so as to enjoy table compression, performance boosts. Same holds for people converting to Percona’s XtraDB. InnoDB plugin requires innodb_file_per_table. No more shared tablespace file.

So your ibdata1 file is some 150GB, and it won’t reduce. Really, it won’t reduce. You set innodb_file_per_table=1, do ALTER TABLE t ENGINE=InnoDB (optionally ROW_FORMAT=COMPRESSED KEY_BLOCK_SIZE=8), and you get all your tables in file-per-table .ibd files.

But the original ibdata1 file is still there. It has to be there, don’t delete it! It contains more than your old data.

InnoDB tablespace files never reduce in size, it’s an old-time annoyance. The only way to go round it, if you need the space, is to completely drop them and start afresh. That’s one of the things so nice about file-per-table: an ALTER TABLE actually creates a new tablespace file and drops the original one.

The procedure

The procedure is somewhat painful:

On generating unique IDs using LAST_INSERT_ID() and other tools

MySQL

There’s a trick for using LAST_INSERT_ID() to generate sequences in MySQL. Quoting from the Manual:

  1. Create a table to hold the sequence counter and initialize it: 
    mysql> CREATE TABLE sequence (id INT NOT NULL);
mysql> INSERT INTO sequence VALUES (0);
  2. Use the table to generate sequence numbers like this: 
    mysql> UPDATE sequence SET id=LAST_INSERT_ID(id+1);
mysql> SELECT LAST_INSERT_ID();

This trick calls for trouble.

Contention

A customer was using this trick to generate unique session IDs for his JBoss sessions. These IDs would eventually be written back to the database in the form of log events. Business go well, and one day the customer adds three new JBoss servers (doubling the amount of webapps). All of a sudden, nothing works quite as it used to. All kinds of queries take long seconds to complete; load average becomes very high. Continue reading » “On generating unique IDs using LAST_INSERT_ID() and other tools”

Generating Google line charts with SQL, part I

MySQL

In this series of posts I wish to show how Google Charts can be generated via SQL. We discuss the Google Charts limitations which must be challenged, and work towards a simple chart.

I’m going to present the algorithm I use in mycheckpoint, a MySQL monitoring utility, which generates Google charts by raw data using views. An example of such chart follows:


http://chart.apis.google.com/chart?cht=lc&chs=370x180&chts=303030,12&chtt=Latest+24+hours:+Nov+9,+05:50++-++Nov+10,+05:50&chf=c,s,ffffff&chdl=Rentals+rate:+custom_1_psec&chdlp=b&chco=ff8c00&chd=s:GDGKGFLFGMJHRLMPPNULJRPLTOPRUMYPPVRNbQUSUSbSNWUOfSWTObVSUVWSVYVPbTPjfTbRTdXReUWhcTQRQZbTWYVYPaVZXdYYWPTabYUTbW99QLgLNIOIRNNMIKRJEHGFHGJGGFIFDFGDK&chxt=x,y&chxr=1,0,8.720000&chxl=0:|+||08:00||+||12:00||+||16:00||+||20:00||+||00:00||+||04:00||&chxs=0,505050,10,0,lt&chg=4.17,25,1,2,0.69,0&chxp=0,0.69,4.86,9.03,13.20,17.37,21.54,25.71,29.88,34.05,38.22,42.39,46.56,50.73,54.90,59.07,63.24,67.41,71.58,75.75,79.92,84.09,88.26,92.43,96.60&tsstart=2010-11-09+05:50:00&tsstep=600

mycheckpoint does not actually call on Google to do the chart rendering, but invokes its own JavaScript code to visualize the URL locally.

Here are some downsides for using Google charts:

  • The URL cannot be as long as you like. 2048 characters is an upper bound you’ll want to keep behind. [Google charts POST method calls are available, which leads to 16K equivalent of URL length — this is still not too helpful due to the nature of POST calls]
  • Features are inconsistent. To specify label or tick positions, one must specify exact positions. To specify grid positions, one must supply with step, offset, etc. There are more such inconsistencies.
  • Google charts are not too friendly. Taking the ticks and grids example from above, there really shouldn’t be a reason why grids would not be automatically generated according to ticks definitions. But we are required to specify positions for the ticks as well as for the grids.
  • There is no support for time-series. One must translate time as x-axis values.
  • Perhaps most intimidating to many people: to generate a Google chart, once must send data to Google. Which is the main reason I used local JavaScript rendering.

Anyway, let’s build a very simple chart. Since I will not cover everything in this post, we make for some relaxed conditions. Continue reading » “Generating Google line charts with SQL, part I”