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My SQL Interview Questions and Answers


MySQL - System/Compile Time and Startup Parameter Tuning
We start with the system level things since some of these decisions have to be made very early. In other cases a fast look at this part may suffice because it not that important for the big gains. However, it is always nice to have a feeling about how much one could gain by changing things at this level.

The default OS to use is really important! To get the most use of multiple CPU machines one should use Solaris (because the threads works really nice) or Linux (because the 2.2 kernel has really good SMP support). Also on 32-bit machines Linux has a 2G file size limit by default. Hopefully this will be fixed soon when new filesystems are released (XFS/Reiserfs). If you have a desperate need for files bigger than 2G on Linux-Intel 32 bit, you should get the LFS patch for the ext2 file system.

Because we have not run MySQL in production on that many platforms, we advice you to test your intended platform before choosing it, if possible.

Other tips:

If you have enough RAM, you could remove all swap devices. Some operating systems will use a swap device in some contexts even if you have free memory.
Use the --skip-locking MySQL option to avoid external locking. Note that this will not impact MySQL's functionality as long as you only run one server. Just remember to take down the server (or lock relevant parts) before you run myisamchk. On some system this switch is mandatory because the external locking does not work in any case. The --skip-locking option is on by default when compiling with MIT-pthreads, because flock() isn't fully supported by MIT-pthreads on all platforms. It's also on default for Linux as Linux file locking are not yet safe. The only case when you can't use --skip-locking is if you run multiple MySQL servers (not clients) on the same data, or run myisamchk on the table without first flushing and locking the mysqld server tables first. You can still use LOCK TABLES/UNLOCK TABLES even if you are using --skip-locking 12.2.1 How Compiling and Linking Affects the Speed of MySQL
Most of the following tests are done on Linux with the MySQL benchmarks, but they should give some indication for other operating systems and workloads.

You get the fastest executable when you link with -static.

On Linux, you will get the fastest code when compiling with pgcc and -O6. To compile `sql_yacc.cc' with these options, you need about 200M memory because gcc/pgcc needs a lot of memory to make all functions inline. You should also set CXX=gcc when configuring MySQL to avoid inclusion of the libstdc++ library (it is not needed). Note that with some versions of pgcc, the resulting code will only run on true Pentium processors, even if you use the compiler option that you want the resulting code to be working on all x586 type processors (like AMD).

By just using a better compiler and/or better compiler options you can get a 10-30 % speed increase in your application. This is particularly important if you compile the SQL server yourself!

We have tested both the Cygnus CodeFusion and Fujitsu compilers, but when we tested them, neither was sufficiently bug free to allow MySQL to be compiled with optimizations on.

When you compile MySQL you should only include support for the character sets that you are going to use. (Option --with-charset=xxx). The standard MySQL binary distributions are compiled with support for all character sets.

Here is a list of some measurements that we have done:

If you use pgcc and compile everything with -O6, the mysqld server is 1% faster than with gcc 2.95.2. If you link dynamically (without -static), the result is 13% slower on Linux. Note that you still can use a dynamic linked MySQL library. It is only the server that is critical for performance.
If you connect using TCP/IP rather than Unix sockets, the result is 7.5% slower on the same computer. (If you are connection to localhost, MySQL will, by default, use sockets).
If you compile with --with-debug=full, then you will loose 20 % for most queries, but some queries may take substantially longer (The MySQL benchmarks ran 35 % slower) If you use --with-debug, then you will only loose 15 %. On a Sun SPARCstation 20, SunPro C++ 4.2 is 5 % faster than gcc 2.95.2.
Compiling with gcc 2.95.2 for ultrasparc with the option -mcpu=v8 -Wa,-xarch=v8plusa gives 4 % more performance. On Solaris 2.5.1, MIT-pthreads is 8-12% slower than Solaris native threads on a single processor. With more load/CPUs the difference should get bigger.
Running with --log-bin makes [MySQL 1 % slower.
Compiling without frame pointers -fomit-frame-pointer with gcc makes MySQL 1 % faster.
The MySQL-Linux distribution provided by MySQL AB used to be compiled with pgcc, but we had to go back to regular gcc because of a bug in pgcc that would generate the code that does not run on AMD. We will continue using gcc until that bug is resolved. In the meantime, if you have a non-AMD machine, you can get a faster binary by compiling with pgcc. The standard MySqL Linux binary is linked statically to get it faster and more portable.

MySQL - Disk Issues
As mentioned before, disks seeks are a big performance bottleneck. This problems gets more and more apparent when the data starts to grow so large that effective caching becomes impossible. For large databases, where you access data more or less randomly, you can be sure that you will need at least one disk seek to read and a couple of disk seeks to write things. To minimize this problem, use disks with low seek times.
Increase the number of available disk spindles (and thereby reduce the seek overhead) by either symlink files to different disks or striping the disks.
Using symbolic links
This means that you symlink the index and/or data file(s) from the normal data directory to another disk (that may also be striped). This makes both the seek and read times better (if the disks are not used for other things).

Striping
Striping means that you have many disks and put the first block on the first disk, the second block on the second disk, and the Nth on the (N mod number_of_disks) disk, and so on. This means if your normal data size is less than the stripe size (or perfectly aligned) you will get much better performance. Note that striping is very dependent on the OS and stripe-size. So benchmark your application with different stripe-sizes.

Note that the speed
difference for striping is very dependent on the parameters. Depending on how you set the striping parameters and number of disks you may get a difference in orders of magnitude. Note that you have to choose to optimize for random or sequential access.
For reliability you may want to use RAID 0+1 (striping + mirroring), but in this case you will need 2*N drives to hold N drives of data. This is probably the best option if you have the money for it! You may, however, also have to invest in some volume-management software to handle it efficiently.
A good option is to have semi-important data (that can be regenerated) on RAID 0 disk while storing really important data (like host information and logs) on a RAID 0+1 or RAID N disk. RAID N can be a problem if you have many writes because of the time to update the parity bits.
You may also set the parameters for the file system that the database uses. One easy change is to mount the file system with the noatime option. That makes it skip the updating of the last access time in the inode and by this will avoid some disk seeks.
On Linux, you can get much more performance (up to 100 % under load is not uncommon) by using hdpram to configure your disk's interface! The following should be quite good hdparm options for MySQL (and probably many other applications): hdparm -m 16 -d 1

Note that the performance/reliability when using the above depends on your hardware, so we strongly suggest that you test your system throughly after using hdparm! Please consult the hdparm man page for more information! If hdparm is not used wisely, filesystem corruption may result. Backup everything before experimenting!
On many operating systems you can mount the disks with the 'async' flag to set the file system to be updated asynchronously. If your computer is reasonable stable, this should give you more performance without sacrificing too much reliability. (This flag is on by default on Linux.)
If you don't need to know when a file was last accessed (which is not really useful on a database server), you can mount your file systems with the noatime flag.

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