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As many probably already know, PBXT is the first MySQL Storage Engine to use a log-based architecture. Log-based means that data that would normally first be written to the transaction log, and then to the database tables, is just written to the log, and the log becomes part of the database.

This result is that data is only written once, and is always written sequentially. The advantage when writing is obvious, but there is a down side (as always). The data is written to the disk in write order, which is seldom the order in which the data is retrieved. So this results in a lot of random reads to the disk when accessing the data later.

Placing the data logs on a Solid State Drive would solve this problem, because SSDs have no seek time. But the problem with this solution is that SSDs are still way to expense to base all your storage needs on such hardware.

The solution: an SSD-based 2nd Level Cache.

Using an SSD-based 2nd Level Cache you can store the most commonly accessed parts of your database on SSD for a reasonable price. For example, if you have a Terabyte database, you can cache about 15% (160 GB) of it on SSD for around $400. This can significantly affect the performance of your system.

With this thought in mind, I have just released PBXT 1.5.02 Beta, which implements a 2nd level cache for the data logs. How this works is illustrated below.

Data written to the data log is also written to the, main memory based, Data Log Cache. Once the Data Log Cache is full, pages need to be freed up when new data arrives. Pages that are freed from the Data Log Cache are written to the 2nd Level Cache.

Now, when the Data Log records are read, PBXT will read the corresponding page from the Data Log Cache. If the page is not already in the cache, it will first check to see if the page is in the 2nd Level Cache, before reading from the Data Log itself.

PBXT 1.5 is available for download from primebase.org, or you can check out lp:pbxt/1.5 from Launchpad using bazaar. The documentation has also been updated for 1.5.

Using the 2nd level cache is easy. It is controlled by 3 system variables:

• pbxt_dlog_lev2_cache_file - the name and path of the file in which the data is stored.
  
• pbxt_dlog_lev2_cache_size - the size of the 2nd level cache.
• pbxt_dlog_lev2_cache_enabled - set to 1 to enable the 2nd level cache.

It also makes sense to set a higher value for the Data Log Cache, using the pbxt_data_log_cache_size variable, which has a default value of 16MB.

Of course it will be interesting to do some benchmarks on this implementation. But that will have to wait until after my holiday! I will be away until late August, but if you decide to test the new version, be sure to let me know.
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I have just released PBXT 1.0.09 RC3. Besides bug fixes (details in the release notes), this version includes 2 Beta features:

• XA/2-Phase Commit support
• Native online backup Driver

XA support has been around MySQL for quite a while, and we all know of it usefulness, for example when sharding. So I was surprised to find a bug in the XA recovery: Bug #47134. Contrary to what is reported, the crash can also occur when using XA with just the default engines installed, so watch out for that one (the good news: the bug fix is simple).

Online backup is really cool! I have heard that it may soon be released in a coming version of 5.4, so lets hope that this is true.

In a little test, I did a backup of a 10GB database in 49.26 seconds! Admitedly this was on a system with 4 15K drives in a RAID 0 configuration. But that is still a fantastic, considering the tables are not even locked during this time!

The database itself took 19 min. 56 sec. to generate. A complete restore took only 14 min. 29 sec.

But, it gets even better....

I have been working on PBXT 1.1, where I have done a number of things to improve the I/O performance of the engine.

In the same test as above, run with PBXT 1.1, the time to generate the database was 9 min. 35 sec., and the time to restore was 6 min 18 sec! (Time to generate the backup was identical.)

PBXT 1.1 is available directly from Launchpad here: lp:~pbxt-core/pbxt/staging, if you are interested in trying it out. 1.1 also has full support for memory based tables.

The new release candidate (PBXT 1.0.09) can be downloaded from primebase.org/download. It is also available from Lauchpad as the rc3 series: lp:pbxt/rc3.

Please report bugs here.

Any feedback is welcome! You can use Launchpad questions or the PBXT mailing list for this purpose.
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