I had an interesting discussion with 2 colleagues about the possible interest of putting the swap space of a system on a SSD.

If I consider the gain in latency that an SSD brings versus a capacity disk - in the region of 100x - the solution seems obvious. Swapping - or more precisely paging - must be much faster/ with an SSD. Since RAM is expensive versus SSD, I could even be tempted to design a system with a small amount of RAM and a large amount of swap space on SSDs. In other words, I can ask myself if trying to prevent my system to page is still a good fight?

Let's try to shed some light on these questions.

Paging takes place when my system runs out of RAM because more processes are created or because existing processes requires more memory (check this article for details about how to …

Today I had to swap 2 columns in one of my MySQL tables. The task, which seems easily accomplishable by a temp variable, proved to be a bit harder to complete. But only just a bit.

Here are my findings:

1. The

   UPDATE swap_test SET x=y, y=x;

   approach doesn't work, as it'll just set both values to y.

   PostgreSQL seems to handle this query differently, as it apparently uses the old values throughout the whole query. [Reference]

2. Here's a method that uses a temporary variable. Thanks to Antony from the comments for the "IS NOT NULL" tweak. Without it, the query works unpredictably. See the table schema at the end of the post. This method doesn't swap the values if one of them is NULL. Use method #3 that doesn't have this limitation. …

Right now there is a discussion on Planet MySQL regarding MySQL / Linux swap problem. Peter Zaitsev originally brought the problem of MySQL swapping to light. Recently, Dathan Pattishall also wrote about it in his post Linux 64-bit, MySQL, Swap and Memory. Don McAskill followed up with his post, MySQL and the Linux Swap problem, and an interesting way to get around the issue: "make swap partitions out of RAM disks." Don also points to another article by Kevin regarding …

Ever since Peter over at Percona wrote about MySQL and swap, I’ve been meaning to write this post. But after I saw Dathan Pattishall’s post on the subject, I knew I’d better actually do it.

There’s a nasty problem with Linux 2.6 even when you have a ton of RAM. No matter what you do, including setting /proc/sys/vm/swappiness = 0, your OS is going to prefer swapping stuff out rather than freeing up system cache. On a single-use machine, where the application is better at utilizing RAM than the system is, this is incredibly stupid. Our MySQL boxes are a perfect example – they run only MySQL and we want InnoDB to have a lot of RAM (32-64GB …

All production servers are normally installed using kickstart files. Unfortunately, the ks file had a bug that it didnt add a swap partition if there was only one logical or physical disk.

Once the database was setup without swap, the configuration was sized such that all processes fit into memory. But we started seeing OOM killing processes due to lack of memory. Though under normal conditions we had 1-2 GB of free memory.

Unfortunately, OOM at times picked to kill mysql. We were running 500GB+ database having myisam tables. We ended up having to repair the tables every time OOM felt like killing mysqld . Adding a swap device might help us fix the issue, but for that we might have to resize the logical/physical partitions, which wasn’t cool. Then this following idea popped up, create a file, format it as swap and add it as swap device dynamically and also add it /etc/fstab to survive reboot.

dd …