Since I wasn’t able to get to this year’s MySQL UC (employer change caused problems with US visa obtaining and I didn’t get visa in time) I’m really interested in all presentations people are posting after their sessions. I decided to collect them all in one place and would like to share with others – maybe someone will find it interesting to read what people have to say about many interesting aspects of MySQL usage.
So, I’ve created a folder in my Scribd.com account which you could use (and track using RSS readers) to find out what interesting presentations were published. You can use either my account or mysqluc08 folder there. One more possible option to track mysqluc presentations/documents is using our tagging (I tag all my docs with mysqluc08 tag).
Even though I didn’t go to MySQL conf this year (really sad about this), this week is gonna be most active in the community so I decided to do some community stuff too 🙂 Today I’ve released version 0.3 of our innodb recovery toolkit. Now it became much faster, stable and accurate. At this moment it is possible to recover almost any table from corrupted/deleted tablespace without so much effort as it was before. Here is a short changes list (since 0.1 announced here):
- More MySQL data types added: DECIMAL (both old and new), DATE, TIME
- CHAR data type handling improved in table definitions generator
- Indexes filtering added to page_parser
- 64-bit stat() support added to all tools
- Linux has no isnumber() function so we define our own implementation (pretty simple)
- Lots of fixes in create_defs.pl script – now it generates definitions which could recover your data in 80% cases w/o any changes.
- Min/max record size calculation fixed in constraints-based parser.
- Nullable fixed-size columns support is fixed.
- Debug logging is much cleaner now.
As always, if you need any help with your recovery, we would love to help.
We were using memcache in our application for a long time and it helped a lot to reduce DB servers load on some huge queries. But there was a problem (sometimes called a “dog-pile effect”) – when some cached value was expired and we had a huge traffic, sometimes too many threads in our application were trying to calculate new value to cache it.
For example, if you have some simple but really bad query like
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| SELECT COUNT(*) FROM some_table WHERE some_flag = X |
which could be really slow on a huge tables, and your cache expires, then ALL your clients calling a page with this counter will end up waiting for this counter to be updated. Sometimes there could be tens or even hundreds of such a queries running on your DB killing your server and breaking an entire application (number of application instances is constant, but more and more instances are locked waiting for a counter).
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How often do we think about our http sessions implementation? I mean, do you know, how your currently used sessions-related code will behave when sessions number in your database will grow up to millions (or, even, hundreds of millions) of records? This is one of the things we do not think about. But if you’ll think about it, you’ll notice, that 99% of your session-related operations are read-only and 99% of your sessions writes are not needed. Almost all your sessions table records have the same information: session_id and serialized empty session in the data field.
Looking at this sessions-related situation we have created really simple (and, at the same time, really useful for large Rails projects) plugin, which replaces ActiveRecord-based session store and makes sessions much more effective. Below you can find some information about implementation details and decisions we’ve made in this plugin, but if you just want to try it, then check out our project site.
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Last few days one of our customers (one of the largest Ruby on Rails sites on the Net) was struggling to solve some really strange problem – once upon a time they were getting an error from ActiveRecord on their site:
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| (ActiveRecord::StatementInvalid) "Mysql::Error: Lock wait timeout exceeded; try restarting transaction: UPDATE some_table..... |
They have innodb_lock_wait_timeout set to 20 seconds. After a few hours of looking for strange transactions we were decided to create s script to dump SHOW INNODB STATUS and SHOW FULL PROCESSLIST commands output to a file every 10 seconds to catch one of those moments when this error occurred.
Today we’ve got next error and started digging in our logs…
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