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catching top waits

Modern systems are complicated beasts with lots of interdependent activities between threads, programs and kernels. Figuring out some problems is nearly impossible without building some time machine and crystal ball mix that tells exactly what happened. Did your cgroups CPU limit inject a sleep in the middle of mmap_sem acquisition by ‘ps’? Is everyone...

my talk on facebook infrastructure

MySQL does not need SQL

The “DBMS” part of MySQL is fine – storage engines (especially with new kids on the block), replication, etc, but it really sucks at executing SQL. I won’t even get into complex SQL that has complex data dependencies, will start with very basic tasks. SELECT * FROM table WHERE indexed = "A" LIMIT 1 If multiple indexes can satisfy the query, MySQL...

linux memory management for servers

We’ve been learning for many years how to run Linux for databases, but over time we realized that many of our lessons learned apply to many other server workloads. Generally, server process will have to interact with network clients, access memory, do some storage operations and do some processing work – all under supervision of the kernel. Unfortunately,...

on removing files

If you remove a file, file system generally just marks in its metadata that previously occupied blocks can now be used for other files – that operation is usually cheap, unless the file has millions of segments (that is such a rare case, only seen in experimental InnoDB features that Oracle thought was a good idea). This changes a bit with SSDs – if...

on political correctness

Wikipedia administrators received this letter (“Midom” is my username on Wikipedias): Hi, I’ve nothing to do with any of this but passing through oc.wikipedia.org I have noticed someone who I presume to be some kind of admin, one Midom who seems to be rather lacking in social skills, judging by what’s going on here:https://oc.wikipedia.org/wiki/Discussion_Utilizaire:Midom...

on ORDER BY optimization

Generally in MySQL we send queries massaged to a point where optimizer doesn’t have to think about anything. In our major user database environment 99.9% of queries don’t have alternative query plans available (either because of forced indexes or just straightforward Primary Key read). We have various other systems and from time to time we have to do...

on time in mysql processlist

There was one MySQL feature that I was always very afraid to ask for. I sure needed it for past 15 years of my MySQL practice, and yet I never got to it. I wanted MySQL processlist (‘SHOW PROCESSLIST’, later – information_schema.processlist) to have more accurate query execution time. I submitted a patch for that to MongoDB (and it got merged and released...

how innodb lost its advantage

For years it was very easy to defend InnoDB’s advantage over competition – covering index reads were saving I/O operations and CPU everywhere, table space and I/O management allowed to focus on database and not on file systems or virtual memory behaviors, and for past few years InnoDB compression was the way to have highly efficient OLTP (or in our...

on io scheduling again

Most of database engines have to deal with underlying layers – operating systems, device drivers, firmware and physical devices, albeit different camps choose different methods. In MySQL world people believe that InnoDB should be handling all the memory management and physical storage operations – maximized buffer pool space, adaptive/fuzzy flushing,...

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